UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 CROP SEQUENCES AT DAVIS 
 
 JOHN W. GILMORE 
 
 BULLETIN 393 
 
 October, 1925 
 
 UNIVERSITY OF CALIFORNIA PRINTING OFFICE 
 
 BERKELEY, CALIFORNIA 
 
 1925 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of California, Davis Libraries 
 
 http://www.archive.org/details/cropsequencesatd393gilm 
 
CROP SEQUENCES AT DAVIS 
 
 JOHN W. GILMORE* 
 
 The purpose of this bulletin is to set forth the trends of the yields 
 of some cereal crops under several rotation practices in relation to 
 certain soil and climatic conditions and the use of green manures. 
 The data cover the ten-year period from 1914 to 1923 inclusive ; and 
 for convenience of interpretation the results of the first and second 
 
 (jsiWEflsrry of i:auh»ma 
 
 fORNSA I ■ tOUEGE OF Al.B!ClSLTt;*£ 
 
 -Twt mm ■ '■"».; *\,.w 
 
 » BCf ^ J. ' j&B-f 
 
 Ik ? — ^L BI , ' -^ k w i4 
 
 Fig. 1. — Continuous production of the same crop depletes the yield. 
 
 1. Wheat, fallow, barley, peas.. ..average yield 42.3 bushels per acre 
 
 2. Wheat, alternate fallow average yield 40.0 bushels per acre 
 
 3. Wheat alternate vetch average yield 38.7 bushels per acre 
 
 4. Wheat continuously average yield 22.6 bushels per acre 
 
 five-year periods, where possible, are shown in comparison. Obviously 
 a ten-year period for data of this kind is not sufficient for definite 
 conclusions to be drawn in all instances. As time goes on the data 
 will become more comprehensive and have increased significance. It 
 is believed, however, that the data herewith presented show clearly 
 the general trends of certain practices under the conditions existing 
 at Davis. 
 
 Division of agronomy. 
 
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 TABLE 1 
 
 Arrangement of Plots, Their Treatment and Yields 
 Bu. Per Acre 
 
 Plot 
 No. 
 
 Wheat continuously.. 
 
 Wheat, cultivated- 
 fallow 
 
 Wheat, cloddy — 
 fallow 
 
 Check 
 
 Wheat, weedy — 
 fallow 
 
 Barley continuously. 
 
 Check 
 
 Rye continuously 
 
 Oats continuously 
 
 Check 
 
 Wheat— manure con- 
 tinuously 
 
 Milo continuously — 
 
 Check 
 
 Saccari ne— sorgh um 
 continuously 1 
 
 Wheat, wheat, fallow 
 
 Check 
 
 Wheat, fallow, wheat 
 
 Fallow, wheat, wheat 
 
 Check 
 
 Wheat, wheat, wheat, 
 fallow 
 
 Wheat, barley, fallow 
 
 Check 
 
 Oats, wheat, barley, 
 fallow 
 
 Barley, milo, wheat, 
 fallow 
 
 Check 
 
 Milo, wheat, fallow, 
 barley 
 
 Wheat, fallow, bar- 
 ley, milo 
 
 Check 
 
 Fallow, barley, milo, 
 wheat 
 
 Barley, peas, wheat, 
 fallow 
 
 Check.. 
 
 Peas, wheat, fallow, 
 barley 
 
 Wheat, fallow, bar- 
 ley, peas 
 
 Check 
 
 Fallow, barley, peas, 
 wheat 
 
 Wheat, peas 
 
 13-14 
 
 14.5 
 
 19.3 
 11.3 
 
 13.6 
 50.4 
 
 61.2 
 14.1 
 
 79. 4€ 
 16.3 
 
 7. 
 9.3 
 11.3 
 8.3 
 
 9.1 
 
 11.6 
 11.3 
 10.6 
 
 59.3 
 
 63.6 
 
 12.3 
 
 64.64 
 
 17.8 
 17.6 
 
 58 
 21.3 
 
 3.34' 
 22.3 
 
 26. 
 
 30 
 
 14-15 
 
 23.67 
 
 24.83 
 
 40.80 
 
 26. 
 
 21.5 
 
 51.56 
 
 22.33 
 
 23 
 
 86.62 
 
 20.83 
 
 12.6 
 
 17.33 
 
 20.33 
 
 18.17 
 14.83 
 
 13. 
 
 36.4 
 
 14.83 
 
 18. 
 
 78.21 
 8.50 
 
 13.50 
 
 11.17 
 37 40 
 
 5.36 
 
 7. 
 
 27.33 
 
 7.67 
 
 15-16 
 
 38.33 
 
 51.67 
 
 50.67 
 38.33 
 
 43.00 
 
 60.60 
 
 36 
 
 12.16 
 
 80 
 
 39.33 
 
 42.33 
 61.67 
 38.67 
 
 7.73 
 
 41. 
 60.67 
 48.33 
 44 
 
 54.67 
 
 52.67 
 51.67 
 
 tO 4 
 36.67 
 
 20 20 
 
 58.33 
 32.33 
 
 70.40 
 25.67 
 
 3.63 
 44 67 
 
 16-17 
 
 41.50 
 
 46.00 
 
 74.2 
 54 
 
 38.6 
 122.5 
 
 45.33 
 65.6 
 48.5 
 
 9.6 
 57.16 
 47.16 
 51.33 
 
 42 
 
 53.66 
 40.33 
 
 43.66 
 72.80 
 
 35 
 
 45.33 
 43.6 
 
 10.51 
 32.16 
 
 65 
 
 7.25 
 
 17-18 
 
 25.8 
 46.8 
 
 44.3 
 
 42 
 
 42.3 
 67.7 
 38.8 
 33.5 
 84.3 
 33.3 
 
 39.3 
 7.11' 
 37.5 
 
 5.62 
 39. 
 24.1 
 
 49.3 
 37. 
 
 42.3 
 71.2 
 35 1 
 
 79.6 
 
 62.5 
 38.8 
 
 7 
 32.6 
 
 70.8 
 25.3 
 
 13.7 = 
 
 49.3 
 25.5 
 
 52.3 
 
 18-19 
 
 28.7 
 
 27.5 
 
 27.4 
 
 22 
 
 14 
 
 27.3 
 
 32.2 
 
 21.3 
 26.6 
 27.8 
 
 3.12 
 
 17.4 
 17.4 
 19.6 
 12.5 
 
 13.7 
 
 13.7 
 
 2.75 
 12.1 
 
 24.5 
 
 13 5 
 
 44 2 
 3.75 
 
 19-20 
 
 3.7 
 
 24.1 
 
 20.3 
 4.8 
 
 33.9 
 16.4 
 
 3 3 
 
 6.6- 
 
 4.8 
 
 2.4 
 
 2.2 
 2.3 
 3. 
 
 6. 
 
 38.4 
 
 1.8 
 
 3.2 
 
 2.2 
 
 4. 
 
 25 
 
 2 
 
 11.6 
 
 3. 
 2.3 
 
 43.6 
 4 6 
 
 3 5 
 
 31.1 
 
 3.2 
 
 41. 
 3.8 
 
 1.25 
 30 5 
 
 20-21 
 20.6 
 
 20 3 
 
 25.2 
 16.3 
 20.8 
 18.8 
 18.6 
 
 22.3 
 43.1 
 21.5 
 
 7.6 
 17.3 
 14.3 
 
 24 6 
 21 
 
 25.2 
 20.7 
 
 17.6 
 
 49.6 
 
 46.7 
 11.6 
 
 18.6 
 
 13.9 
 
 49.6 
 
 .56 
 12. 
 
 17.6 
 
 45 
 
 21-22 
 
 38. 
 
 13.3 
 
 63.3 
 
 
 66.3 
 
 
 26.7 
 
 7.7 
 
 57 7 
 
 
 41. 
 28 2 
 30.7 
 69.7 
 30.7 
 
 41 7 
 46.3 
 41. 
 
 4.15 
 
 29 8 
 58.3 
 45.7 
 36.7 
 
 69.7 
 
 36.8 
 
 85.6 
 
 71. 
 
 35.8 
 
 52.2 
 21.3 
 
 65.6 
 
 28.2 
 
 4 22 
 
 61.8 
 29.3 
 
 54 3 
 
 22-23 
 
 22.1 
 9.5 
 15. 
 61.9 
 10. P 
 
 3? 
 
 3b. 9 
 24.6 
 
 3.92 
 71.3 
 14.5 
 23.2 
 
 17 
 
 24 8 
 58.3 
 13.5 
 
 47.2 
 
 39.3 
 22.8 
 
 68.3 
 
 11.6 
 74.4 
 5.311 
 
 Tons green matter per acre. 
 
Bull. 393 
 
 CROP SEQUENCES AT DAVIS 
 
 TABLE 1— (Continued) 
 
 Plot 
 No. 
 
 50 
 
 Check 
 
 Peas, wheat 
 
 Wheat, vetch 
 
 Check 
 
 Vetch, wheat 
 
 Wheat, milo, rye — 
 
 rape 
 
 Check 
 
 Milo, rye — rape, 
 
 wheat 
 
 Rye — rape, wheat, 
 
 milo 
 
 Check 
 
 Wheat, milo, rye — 
 
 vetch 
 
 Milo, rye — vetch, 
 
 wheat 
 
 Check 
 
 Rye — vetch, wheat, 
 
 milo 
 
 Wheat-clover con- 
 tinuously 
 
 Check 
 
 13-14 
 
 35 
 
 3.34 
 24 
 22 
 
 3.31 
 
 24 
 25.3 
 
 56.78 
 
 20.5 
 
 33.3 
 
 29.46 
 24.6 
 
 28.5 
 24.6 
 
 14-15 
 
 15. 
 18.60 
 6.1i 
 12.16 
 15.92 
 
 64.12 
 10.50 
 
 4.50! 
 
 19.83 
 13.58 
 
 47.28 
 
 11.33 
 20.66 
 
 23.66 
 
 17.33 
 21.66 
 
 15- 
 
 21 67 
 
 8.47 
 
 54.33 
 
 29.35 
 
 6.05 
 
 28.33 
 42 
 
 22.19 
 35 
 
 42.67 
 32. 
 
 35.66 
 
 5-17 
 
 30.60 
 39.33 
 10. 25 1 
 27.33 
 36. 
 
 35.66 
 
 28 
 
 90.3 
 
 34 
 36.5 
 
 97.3 
 39 
 
 24.35 
 28. 
 
 17-18 
 
 25.5 
 11.91 
 
 51.6 
 25 5 
 10' 
 
 
 
 23.8 
 
 13.81 
 
 40.1 
 31.6 
 
 35.1 
 31.6 
 
 38 
 
 26.3 
 31.1 
 
 18-19 
 
 16.9 
 21.2 
 1.75 
 14.5 
 21.2 
 
 8.1 
 16.1 
 
 25.8 
 
 60 6 
 18.8 
 
 6.37 
 
 35 
 21.4 
 
 56.6 
 
 15.8 
 13.8 
 
 19-20 
 
 3.4 
 1.5" 
 
 30.9 
 3. 
 1.25 
 
 28 
 2 5 
 
 6.3 
 
 2.25 
 4. 
 
 34.6 
 
 3.5 
 
 2.12 
 
 2.1 
 3. 
 
 20-21 
 
 14.2 
 20.5 
 25 
 12.8 
 22.8 
 
 40.9 
 14.2 
 
 3.69 
 
 20.1 
 16.4 
 
 43.2 
 
 2.75 
 14.7 
 
 18.1 
 
 17.8 
 12.7 
 
 21-22 
 
 28. 
 
 5.12 
 47.7 
 25.7 
 
 5 25 
 
 4.37 
 26.2 
 
 55. 
 
 78 8 
 21.3 
 
 7.34 
 
 60.8 
 29 
 
 82.3 
 
 36.7 
 27.3 
 
 22-23 
 
 5.56 
 15 5 
 
 67 
 
 42.9 
 10.5 
 
 15.7 
 16.7 
 
 1 Tons green matter per acre. 
 
 2 Pasture. 
 
 The importance of this field of investigation is emphasized by the 
 fact that the major portion of the land area in farms in California is 
 not irrigated and receives a rainfall of less than 20 inches, which falls 
 mainly during the winter months. Under such conditions the eco- 
 nomic diversification of crops is restricted, and the maintainance of 
 the crop producing power of the soil and the conservation of the mois- 
 ture supply present additional difficulties. 
 
 Plan and Procedure. — A series of plots was laid out in 1913 on 
 land that had been fallowed the previous year but which had grown 
 grain continuously for thirty years or more. It comprises 52 plots 
 each 121 feet long, and 18 feet wide, %o of an acre. There is an 
 open cultivated space of three feet between the plots. The arrange- 
 ment is such that a check plot occurs adjacent to each treatment 
 plot, each third plot being a check and these checks, 18 in all, are 
 planted with wheat continuously. 
 
 The plan comprises continuous cropping with wheat, oats, barley, 
 rye and milo ; continuous wheat with a legume ; continuous wheat 
 with manure ; wheat and alternate green manure ; rotation including 
 fallow, a four course rotation with and without green manure and 
 several minor trials. Table 1 shows the arrangement of treatments 
 
b UNIVERSITY OP CALIFORNIA EXPERIMENT STATION 
 
 and checks. The yields of grain are expressed in bnshels per acre, 
 and the yield of legumes, forage, etc., in tons of green matter per acre. 
 The sequence of the plots in the table is that of their arrangement in 
 the field 
 
 This table covers a period of ten years from 1914 to 1923. It is 
 presented for general reference and no comment upon it is necessary 
 here except to point out that the yields from year to year have 
 varied widely, and that very low yields occurred on most plots in 
 1920. This was a very dry season during Avhich the rainfall (8.94 
 inches) was only a little more than half the normal. 
 
 m 
 
 Fig. 2. — Kough and smooth fallow. The yield of these two plots over a 
 ten-year period has been practically the same, though the rough fallow has 
 dropped a little more rapidly during the second half of the period. See 
 table 9, page 18. 
 
 The grain crops, including the vetch and peas, were planted 
 in the fall, usually in December, though with occasional variation 
 because of rainfall conditions. The plots summer fallowed were plowed 
 in December and again in March and given two or three cultivations 
 during the early summer (except plot 5, which was left rough or cloddy 
 each year. See fig. 2). The alley-ways between the plots were cultivated 
 during the summer. Sometimes the plots have shown an increased 
 growth along these open ways, but the yield has not been deducted 
 for this because all the plots showed practically the same effect and 
 the results are comparable. 
 
Bull. 393] CROP SEQUENCES AT DAVIS 7 
 
 The mile- and saccharine sorghum crops were planted in the 
 spring usually about April 15th, thinned to 8 inches in the row 
 and cultivated three to four times during April and May. The rows 
 were spaced three feet which gave six rows to the plot. 
 
 CLIMATE AND LOCATION 
 
 Davis is located near the geographical center of the state, 13 miles 
 from Sacramento, and is properly a part of the Sacramento Valley. 
 Its climate, however, with special reference to rainfall, humidity and 
 temperature, is somewhat modified by the conditions of these factors 
 prevailing over the Bay region of San Francisco. This is especially 
 noticeable in regard to temperatures, for northern points in the 
 Sacramento Valley have a higher mean temperature during the 
 growing months than does Davis, and a lower mean temperature 
 during the winter months. Thus, for example, the mean temperature 
 for Davis and Chico are for the months of April to September in- 
 clusive 70.6 and 73.5 degrees, and for the months October to March 
 inclusive, 54 and 52.8 degrees respectively. 
 
 Occasional northern winds occur at all points in the Sacramento 
 Valley from April to September, and these are sometimes a serious 
 factor in crop production. They are both dry and warm and hence 
 during the early summer months extract a good deal of moisture from 
 the soil, and if the grain is in bloom or filling they effect a deleterious 
 influence on pollination and consequent yields. The relative dryness 
 of these winds is shown by the evaporation from a free water surface, 
 and a single instance may suffice as an illustration. 
 
 June 14-16, 1917, average temperature, 105; evaporation, .48 inches; pre- 
 vailing wind S.W. 
 
 June 18-20, 1917, average temperature, 107; evaporation, 1.20 inches; pre- 
 vailing wind N. 
 
 Rainfall. — Both temperature and rainfall vary widely at Davis, 
 Temperature, however does not exert so great an influence on 
 cereal yields as does rainfall, because during the season of 
 crop growth the temperature is at no time above or below that 
 which is favorable for cereals. The rainfall, however, is a very im- 
 portant factor in crop growth, and its effectiveness is measured not 
 only by the annual amount, but also by its distribution over the 
 season. Since practically all of the rainfall of the valley areas of 
 California falls during the winter months, October to March, the 
 farmer must meet a special set of problems, in moisture conservation 
 and in the maintenance of the optimum physical condition of the 
 
8 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 soil. Because of this peculiarity of the rainfall it semes best to pre- 
 sent the rainfall data according to season rather than by calendar 
 years. (See table 2.) 
 
 TABLE 2 
 
 Eainfall at Davis, 1913-23 
 
 
 July 
 
 Aug. 
 
 Sept. 
 
 Oct. 
 
 Nov. 
 
 Dec. 
 
 Jan. 
 
 Feb. 
 
 Mar. 
 
 Apr. 
 
 May 
 
 June 
 
 Seasonal 
 
 12-13 
 
 
 
 
 
 
 
 3.43 
 9.17 
 4.62 
 11.01 
 1.28 
 .95 
 
 .15 
 4.35 
 5.01 
 1.93 
 5.90 
 3.59 
 
 1.08 
 .85 
 
 1 69 
 
 1.14 
 .61 
 
 3.10 
 
 .77 
 
 1.00 
 
 91 
 
 .11 
 
 .51 
 .82 
 
 .36 
 
 .60 
 
 2.20 
 
 .10 
 
 .07 
 
 .66 
 
 
 13-14 
 
 
 
 
 
 4.63 
 .16 
 .53 
 .35 
 .12 
 
 7.44 
 
 4.75 
 
 6.03 
 
 4.81 
 
 .59 
 
 28.70" 
 20.05 
 20.88 
 14.11 
 9.66 
 
 
 14-15 
 
 
 
 
 .71 
 .03 
 
 .46 
 
 
 15-16 
 
 
 
 
 
 ► 93 . 40 Total 
 
 16-17 
 
 
 .02 
 
 1.7 
 
 .49 
 
 
 18.68 Av. 
 
 17-13 
 
 
 
 
 
 
 
 
 
 18-19 
 
 
 
 4.07 
 .55 
 .03 
 
 .28 
 
 2.19 
 .31 
 4.02 
 1.62 
 3 2) 
 .53 
 
 1.69 
 2.61 
 4.39 
 4.39 
 7.37 
 .88 
 
 2.49 
 .37 
 5.11 
 2.29 
 2 62 
 
 7.12 
 .76 
 .32 
 
 5.85 
 .70 
 
 1.54 
 3.47 
 1 24 
 1.47 
 
 .02 
 .83 
 .30 
 .40 
 
 2.22 
 
 
 
 19. 40^ 
 8.94 
 17.17 
 16.63 
 17.86 
 
 
 19-20 
 
 
 
 
 .04 
 
 
 20-21 
 
 
 .04 
 
 1.46 
 .21 
 
 1.56 
 .40 
 
 .26 
 .40 
 .10 
 
 \ 80 Total 
 
 21-22 
 
 
 16 Av. 
 
 22-23 
 
 
 
 
 
 
 23-24 
 
 
 
 .35 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Normal rainfall for Davis is 17.23 inches. 
 
 During the first period of these five years there fell 93.4 inches, average 18.68, 
 
 1.45 above normal. 
 During the second period of these five years there fell 80 inches, average 16, 
 
 1.23 below normal. 
 
 From this record it may be seen that the rainfall has varied during 
 the ten years mentioned from more than 28 inches for the season 1913- 
 14 to less than 9 inches for the season 1919-20. This variation in 
 seasonal rainfall has had a marked influence on the crops produced on 
 all the plots of this experiment, and accounts to some extent for the 
 wide variation in yields shown in table 1. During seasons of very 
 heavy rainfall the winter grain and legume crops are injuriously 
 affected by an excess of water in and on the soil, and during seasons 
 of deficient rainfall, spring growth of the grain crops is retarded and 
 the summer growing crops suffer for a lack of moisture, especially on 
 the continuously cropped plots. 
 
 Seasonal and Spring Rainfall. — Annual winter growing crops are 
 also affected by the amount of precipitation during certain months. 
 Thus, yields in 1914, after an abundant rainfall, were not, for most 
 of the winter grown crops, so heavy as the yields of 1918 after a defi- 
 cient rainfall, and this despite the fact that the 1914 crop had the 
 advantage of early planting (Dec. 1) as compared with late planting 
 (Jan. 24) for the crop of 1918. The figures of table 3 tend to illus- 
 trate this point. 
 
 These years were chosen because they represent about equal devia- 
 tions from the normal rainfall (17.23). The years 1914 and 1918 
 
Bull. 393" 
 
 CROP SEQUENCES AT DAVIS 
 
 illustrate the same tendency, but their rainfall is further from the 
 normal, both above and below. By referring to table 3 it will be 
 noted that in 1916, a year of above normal rainfall, more than 17 
 inches, or 84 per cent of the total, fell before February 1st. The 
 earlier part of the year had more moisture than was needed because 
 of the nature of the crops and their culture, but this abundance of 
 moisture was largely lost because it was in excess of what could be 
 absorbed and retained. In 1917, a year below seasonal rainfall, 
 about 7 inches or a little less than half of the total precipitation fell 
 after the first of February, while in 1916, a year above normal seasonal 
 rainfall, only 3.28 inches fell after that date. These data with ex- 
 tensive observations tend to emphasize the fact that, granted a 
 reasonable amount and distribution of moisture during the winter 
 months, it is the rainfall of the spring months that makes abundant 
 grain crops. Moreover, late spring rains are nearly always accom- 
 panied and followed by cool, humid weather. This not only retards 
 evaporation and consequently, loss of moisture from the soil, but also 
 prolongs the ripening season of grain crops, and both of these influ- 
 ences enhance the yield of these crops. Thus in 1916 the mean tem- 
 perature for the months March, April and May was 59.6, while that 
 for the same months in 1917 was 56.3, the normal being 58 for the 
 same months. 
 
 It is not supposed, however, that these yields are consequent upon 
 rainfall alone. There are other factors involved, such as date of 
 planting, shattering, lodging and prevalence of disease, the influence 
 of which it is impossible to evaluate; but it is strongly indicated, at 
 this stage of these experiments, that on these soils and between reason- 
 able dates of planting, the rainfall is the most influential seasonal 
 factor involved in yield of the crops under study. 
 
 TABLE 3 
 
 The Influence of Seasonal Distribution of Eainfall on Yield of Crops 
 
 Grown Continuously 
 
 Seasonal rainfall 
 
 Spring rainfall Feb. -April, inc 
 
 Wheat— average, all checks (18 plots) 
 
 Oats continuously, plot 11 
 
 Rye continuously, plot 10 
 
 Barley continuously, plot 8 
 
 Milo continuously, plot 14 
 
 Wheat and manure, plot 13 
 
 Wheat and legumes, plot 53....... 
 
 Date of planting winter crops 
 
 1916 
 
 1917 
 
 20.88 
 
 14.11 
 
 3.28 
 
 ,7.02 
 
 35.35 
 
 39.58 
 
 80.00 
 
 122.50 
 
 12 16 
 
 38.60 
 
 60.60 
 
 74.20 
 
 61.67 
 
 65.60 
 
 42.33 
 
 45.33 
 
 21.00 
 
 24.35 
 
 March 5 
 
 Dec. 26 
 
10 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 SOILS 
 
 #//////////, 
 
 "Hi 
 
 
 * '/////&///, 
 
 wmw< 
 
 2 
 
 The soils over which these plots are distributed are of two types — 
 fig. 3 — Yolo loam and Yolo silt loam. These soils are described 
 by the Bureau of Soils 1 as follows : ' ' The Yolo loams 
 as mapped in this area comprise the Yolo loam and 
 Yolo silt loam. The Yolo loam consists of a light- 
 brown or brown loam of medium to heavy texture. 
 Gravel is usually absent and the soil as a rule is 
 friable and easily tilled. Considerable variation in 
 the subsoil is noted, but below a depth of 24 inches 
 it often grades into a silty loam or sandy loam of a 
 buff or brownish-yellow color. Clay loam or clay at 
 times constitutes the deeper subsoil and gravelly beds 
 may occur at a depth of 4 to 6 feet long along the site 
 of old stream channels. Altogether the type shows 
 decided variations in the texture of both soil and 
 subsoil, as indicated by borings taken at close range. 
 
 ' ' The Yolo silt loam is friable in texture. Typically 
 it consists of 36 inches of light-brown or dark grayish- 
 brown silt loam of heavy texture. The soil frequently 
 grades into yellowish-brown substrata of silty clay, 
 clay loam or similar material. 
 
 "Extreme ranges from layers of sandy loam or 
 sand to clayey beds are encountered in the subsoil of 
 a single boring, but on the whole the group is friable 
 when cultivated and retentive of moisture. On the 
 other hand, the soils are sufficiently open and porous 
 to drain quickly." 
 
 * ' These soils usually lie a little more elevated than 
 surrounding soil areas and are usually well drained. 
 Alkali seldom occurs in them. They are adapted to 
 a variety of crops, but their yields of grain are notice- 
 ably high during seasons of ample rainfall. ' ' 
 
 Mechanical Analysis. — The following table gives 
 the results of mechanical analyses of samples of the 
 soil and subsoils of the Yolo loams : 
 
 Fig. 3_ — The 52 plots in this rotation cover two types of 
 soil, the Yolo loam (L) and the Yolo silt loam (Sil). Each is 
 underlain by a light and heavy phase of subsoil marked L and 
 PI respectively below the line. 
 
 ' //////// , 
 
 '■WMM 
 
 » ///////////, 
 
 ■■'//////////, 
 
 *4\W///A 
 
 <fNKW/: 
 
 
 
Bull. 393] 
 
 CROP SEQUENCES AT DAVIS 
 
 11 
 
 TABLE 4 
 Mechanical Analysis of Yolo Loams 
 
 
 Fine 
 gravel 
 
 Coarse 
 sand 
 
 Medium 
 sand 
 
 Fine 
 sand 
 
 Very fine 
 sand 
 
 Silt 
 
 Clay 
 
 Yolo loam — Soil 
 
 0.1 
 
 0.4 
 
 0.9 
 
 9.5 
 
 27.3 
 
 43.3 
 
 18.2 
 
 Subsoil 
 
 0.0 
 
 0.4 
 
 1.4 
 
 14.3 
 
 16.5 
 
 49.3 
 
 18.1 
 
 Yolo gilt loam — Soil 
 
 0.0 
 
 0.1 
 
 0.1 
 
 1.2 
 
 5.0 
 
 69.1 
 
 24.4 
 
 Subsoil 
 
 0.0 
 
 0.0 
 
 0.0 
 
 1.7 
 
 9.1 
 
 61.3 
 
 27 9 
 
 The Yolo silt loam predominates over the particular area under 
 consideration; in fact it occupies the entire area except plots 3 to 6 
 inclusive and small areas in other parts of the strip. Reference to 
 figure 1 will indicate the distribution of these types over the area 
 occupied by the plots. 
 
 Yields of Wheat. — It would seem that on the area under consid- 
 eration the Yolo loam is somewhat more productive than the Yolo 
 silt loam. The average yield of two check plots, 3 and 6, which are 
 Yolo loam with a light subsoil, is a little higher over a period of ten 
 consecutive years than is the average on six check plots covering the 
 silt loam with light subsoil. The following table illustrates this 
 point : 
 
 TABLE 5 
 
 Average Yield of Wheat Grown Continuously Over a Period of Ten Years 
 
 on Yolo Loam and Yolo Silt Loam — Bushels Per Acre 
 
 Plot treatment 
 
 Average 
 
 yield 
 10 crops 
 
 First 
 
 half 
 
 period 
 
 Second 
 
 half 
 period 
 
 Soil type 
 
 Average 
 yields 
 
 
 25.26 
 24.95 
 
 29.66 
 32.49 
 
 20.87 
 17.40 
 
 
 25 10 
 
 
 
 
 
 
 
 
 22.17 
 23.70 
 24.06 
 
 28.79 
 29.52 
 30 78 
 
 15.50 
 17.90 
 17 30 
 
 
 
 
 
 23 46 
 
 
 Yolo silt loam 
 
 
 
 
 
 
 20 07 
 20.55 
 19 26 
 
 27 23 
 26.26 
 23.50 
 
 12.80 
 14 80 
 15.00 
 
 Yolo gilt loam. 
 
 
 33 check — wheat continuously 
 
 36 check — wheat continuously 
 
 Yolo silt loam 
 
 Yolo silt loam 
 
 19.96 
 
 
 
 
 It is true that the average of six plots covering Yolo silt loam 
 (in two blocks) is compared with that of two plots (3 and 6), cover- 
 ing Yolo loam. If the two groups of silt loam plots are averaged there 
 would seem to be a significant difference in productivity in favor of 
 the Yolo loam. 
 
12 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Moisture Relationships. — There are several variations in the sub- 
 soils of this area, each type being underlaid in places by light and 
 heavy subsoils. (See fig. 2.) But in no cases are these soils imper- 
 vious to moisture, hence the subsoils do not influence yields to any 
 appreciable extent. The moisture holding capacity of these soils is 
 good, and except during periods of heavy rainfall their absorptive 
 capacity is sufficient. In general the rain permeates the soil readily 
 and standing water does not occur on the surface except when the 
 very heavy rains occur. The season of 1915-16 is typical of the rela- 
 tionship of soil moisture, rainfall and yield. 
 
 TABLE 6 
 
 Average Percentage of Moisture by Feet in the Soil of 18 Check Plots on 
 Six Different Dates for the Season 1915-16 
 
 Depth 
 
 Sept. 1, 1915 
 
 Nov. 1, 1915 
 
 Apr. 1, 1916 
 
 June 1, 1916 
 
 Sept. 1, 1916 
 
 Nov. 1, 1916 
 
 1 foot 
 
 2 feet 
 
 3 feet 
 
 4 feet 
 
 5 feet. 
 
 6 feet 
 
 6.32±118 
 11 92±.453 
 18.05±.819 
 21.51±.896 
 22.24±705 
 26.64±.731 
 
 6.25±.410 
 12.35±577 
 17.04±.812 
 21.22±.490 
 24.98± 841 
 25.10±.752 
 
 19.25±217 
 22.73±.300 
 26.65±473 
 30.25± 370 
 29.90=b 319 
 32.71±346 
 
 10 66±.377 
 14.38±.211 
 18.31±.471 
 23.13±625 
 28.03±.587 
 29.07±.400 
 
 8.47±221 
 12.60±.304 
 18.73±701 
 23.76±670 
 24.32±.536 
 27.43±591 
 
 8.67±.175 
 13.96±.331 
 19.36±.730 
 22.11±.702 
 25.94±.603 
 28.00±.688 
 
 The total rainfall for the season was 20.88 inches (normal 17.23) 
 and the average yield of wheat on the 18 check plots was for the har- 
 vest of 1916, 35.23 bushels per acre (average for 10 years 22.63). 
 
 Volume Weight. — The volume weight of these soils shows an ap- 
 proximation to the normal for these types of soil under cultivation. 
 The average volume weight of the first foot from the 18 check plots 
 determined from samples taken about November 1, 1924, was 1.5174± 
 .0479. In cases where organic matter has been added, such as plot 
 13, where manure has been applied (V.W. 1.294) ; plots 38 and 40 
 (V.W. av. 1.392) where peas have been plowed under and plots 1, 41 
 and 43 (V.W. av. 1.447), where vetch has been plowed under, the 
 volume weight has been increased and there is a corresponding 
 increase in yield of wheat over that of the check plots. 
 
 These results are corroborated by observation. In plowing the 
 plots, it has been noticed during recent years that in general the check 
 plots plow with greater difficulty than do those fallowed or those on 
 which organic matter has been applied. This observation does not 
 hold, however, for the sorghum plots, for these have also broken up 
 with greater difficulty than the fallowed or the manured plots. This 
 is probably due to certain physical conditions brought about in the soil 
 
BULL. 393] C ROP SEQUENCES AT DAVIS 13 
 
 by the influence of the growth and decay of the sorghum roots and 
 stubble. That this is true is indicated by recent investigations by 
 Breazle 2 and Hawkins, 3 who have found that a sorghum crop 
 leaves the soil in poor physical condition as compared with corn and 
 some other crops. 
 
 CROP SEQUENCE 
 
 One of the oldest observations in agricultural practice is the influ- 
 ence of a preceding on a succeeding crop. It has long been known that 
 certain legumes have a beneficial influence on crops succeeding them ; 
 and sometimes on crops accompanying them. That intertilled crops 
 are beneficial to broadcasted crops following them, and that poor lands 
 remaining in sod for a few to many years are improved in physical 
 condition and producing power, have also long been noted. 
 
 These observations can generally be explained in the light of 
 present-day knowledge by one or more of the following reasons: 
 Legumes are beneficial to crops following them because of the available 
 nitrogen which the legumes fix in the soil and because of their ame- 
 liorating effects relating to organic matter. Cultivated crops are 
 beneficial because through tillage the physical condition and moisture 
 relationships of the soil are improved for the succeeding crops. Where 
 poor lands are improved by remaining in sod continuously for several 
 years the benefit is due to an increase of organic matter in the soil 
 which improves its physical condition. In these experiments some 
 evidence has been obtained of the influence of crops in sequence, es- 
 pecially in respect to yield. 
 
 Effect of MiJo on Succeeding Crops. — During recent years it has 
 been very definitely shown 4 that in some instances one crop leaves 
 substances, the products of decay of roots and plant parts, which are 
 toxic to plants or crops following. One of the most recent investi- 
 gations 2 on this subject shows that decaying sorghum stubble has a 
 poisonous effect on wheat, oats or barley following. After presenting 
 significant experimental data the author of this investigation states : 
 "It appears that a toxic compound is developed in sorghum stubble 
 during the process of decomposition'' and "it would seem that the 
 toxic effect might be more pronounced upon heavy soils or upon soils 
 with heavy subsoils and upon soils with poor drainage than upon 
 sandy soils or soils with good drainage. ' ' 
 
 Wheat after Milo, on the plots under study, has shown some dimi- 
 nution of yields, and the diminution would probably have been even 
 greater had the soils been heavier or the drainage poorer. 
 
14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 The average yield of nine crops of wheat following milo was 30.27 
 bushels, whereas wheat continuously on the 18 checks, produced an 
 average yield of 22.63 bushels and wheat after peas, as an average of 
 18 crops, produced 40.31 bushels per acre. There is much evidence to 
 indicate that a cultivated crop preceding a wheat crop should not 
 have a tendency to reduce the wheat yield so greatly as in this case. 
 
 
 Fig. 4. — For various reasons wheat after milo does not do so well as wheat 
 following some other crops. Plot on left (28) is wheat after milo, yield (1919) 
 20 bushels per acre. Plot on the right (27) wheat after wheat (check) 18 
 bushels per acre. See table 20. 
 
 Although the beneficial or baneful influence of one crop on an- 
 other has long been recognized ; yet because of many influences, such 
 as climate, soil, topography and the like, no fixed code of facts bearing 
 upon crop sequence has been drawn up. The following data and dis- 
 cussion are presented with the belief that they indicate general trends 
 rather than definite conclusions. 
 
 Effect of Continuous Cropping on Yield. — Perhaps no fact in 
 agricultural practice is better established than the injurious effects 
 that ultimately result from the practice of growing the same crop on 
 the soil continuously. This is a general fact that may be observed in 
 all agricultural regions and over all periods of time. The rapidity of 
 the deterioration depends on many conditions and these vary under 
 different circumstances, such as crop grown, inherent quality of the 
 soil, and climatic factors. The reasons for the deterioration are many, 
 
Bull. 393] CROP SEQUENCES AT DAVIS 15 
 
 and much remains to be learned about the factors involved. It must 
 be remembered too, that, in the great valleys of Central California, 
 seasonal rainfall has a great influence on crop yields, hence inter- 
 pretation of the results shown is complicated and it is impossible to 
 determine the exact measure of emphasis that should be placed on 
 seasonal rainfall and on soil conditions. 
 
 Fig. 5. — Crop of 1920. Continuous wheat on the left, wheat plus 5 tons of 
 manure per acre annually on the right. Yields 2.4 and 2.2 bushels per acre 
 respectively. This amount of manure seems to have a depressing effect on growth 
 and yields of wheat during dry years. 
 
 In 1916 Madson 6 reported on the yield of wheat produced continu- 
 ously on the same land for a period of four years. The average of 
 eight crops thus produced (plots duplicated) was 11.38 bushels per 
 acre. This yield, however, might have been greater had it not been for 
 a subnormal rainfall during three years of the period. The present 
 experiments are a continuation and an enlargement of those reported, 
 and cover a longer period. 
 
 The influence of continuous cropping on the yield of wheat can not 
 be fully determined by the data at hand, for the period covered by 
 the experiment has not been long enough and there are other factors, 
 such as rainfall, prevalence of wild oats, time of planting and the like, 
 the exact bearing of which cannot be measured. 
 
16 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Enough data is at hand, however, to indicate the trend of this 
 practice. Thus table 7 shows the yields of the 18 check plots over 
 the ten-year period, making in all 180 trials. 
 
 TABLE 7 
 
 Summary of Yields of Check Plots, Wheat Continuously, for 10 Years, 
 1914-23 Inclusive, Bushels Per Acre 
 
 Plot 
 
 1914 
 
 1915 
 
 1916 
 
 1917 
 
 1918 
 
 1919 
 
 1920 
 
 1921 
 
 1922 
 
 1923 
 
 Aver- 
 age 
 
 1st 
 
 half 
 
 of 
 
 period 
 
 2nd 
 
 half 
 
 of 
 
 period 
 
 3 
 
 6 
 
 9 
 
 12 
 
 15 
 
 18 
 
 21 
 
 24 
 
 27 
 
 30 
 
 33 
 
 36 
 
 39 
 
 42 
 
 45 
 
 48 .... 
 
 51 
 
 54 
 
 19. 
 11.3 
 19 
 
 14 1 
 16.3 
 11.3 
 9.1 
 10.6 
 12.3 
 17.6 
 21.3 
 26.6 
 35 
 22 
 
 25.3 
 20.5 
 24.6 
 24.6 
 
 23.67 
 
 24.83 
 
 26. 
 
 22 33 
 
 20.83 
 
 20.33 
 
 14.83 
 
 14.83 
 
 8.50 
 11.17 
 
 7. 
 
 7.67 
 15. 
 
 12.16 
 10.56 
 13.58 
 20.66 
 21.66 
 
 38.33 
 
 38.33 
 
 36. 
 
 39.33 
 
 38.67 
 
 41. 
 
 44. 
 
 53. 
 
 51.67 
 
 36.67 
 
 32.33 
 
 25.57 
 
 21.67 
 
 20.35 
 
 28.33 
 
 35 
 
 32 
 
 24 
 
 41 5 
 
 46. 
 
 54. 
 
 46.16 
 
 48.5 
 
 47 16 
 
 42.66 
 
 40.33 
 
 43.66 
 
 38. 
 
 45 33 
 
 32.16 
 
 30.6 
 
 27.33 
 
 28. 
 
 34 
 
 39 
 
 28 
 
 25.83 
 
 42 
 
 38 83 
 
 33.33 
 
 37.5 
 
 24.16 
 
 37 
 
 35.16 
 
 38.83 
 
 32.7 
 
 25.33 
 
 25.5 
 
 25.5 
 
 25.5 
 
 23.83 
 
 31.66 
 
 31.66 
 
 31.66 
 
 28 7 
 
 27.5 
 
 22. 
 
 32.2 
 
 27.8 
 
 17.4 
 
 12.5 
 
 13.7 
 
 18. 
 
 15. 
 
 12.1 
 
 13 5 
 
 16.9 
 
 14.5 
 
 16.1 
 
 18.8 
 
 21 4 
 
 13.8 
 
 3.7 
 
 4.8 
 
 3 3 
 
 2.4 
 
 3. 
 
 1.8 
 
 2.2 
 
 2. 
 
 2.3 
 
 4.6 
 
 3.2 
 
 3.8 
 
 3.4 
 
 3. 
 
 2.5 
 
 4. 
 
 4. 
 
 3. 
 
 20.6 
 
 20.3 
 
 16.3 
 
 18.6 
 
 21.5 
 
 14.3 
 
 21. 
 
 20.7 
 
 17.6 
 
 11.6 
 
 13.9 
 
 12. 
 
 14.2 
 
 12.8 
 
 14.2 
 
 16 4 
 
 14.7 
 
 12.7 
 
 38. 
 
 26.7 
 
 28 .2 
 
 30.7 
 
 41. 
 
 29.8 
 
 36.7 
 
 36.8 
 
 35.8 
 
 21.3 
 
 28.2 
 
 29.3 
 
 28 
 
 25.7 
 
 26 2 
 
 21.3 
 
 29 
 
 27.3 
 
 13 38 
 
 7 7 
 
 9.5 
 
 10.6 
 
 24.6 
 
 14 5 
 17. 
 13.5 
 22.8 
 11.6 
 16.8 
 16.5 
 15.3 
 17. 
 25. 
 15.5 
 10 5 
 16.7 
 
 25.26 
 24.95 
 25.51 
 24.98 
 27 97 
 22.17 
 23.70 
 24.06 
 25.15 
 20.07 
 
 20 55 
 19.26 
 20.56 
 18.93 
 20. 
 
 21 07 
 22.75 
 20.34 
 
 29.66 
 
 32 49 
 34.76 
 31.05 
 32 36 
 28.79 
 29.52 
 30 78 
 30 99 
 27.23 
 26.26 
 23.50 
 25 55 
 21 46 
 23 20 
 26.95 
 29.58 
 25.98 
 
 20.87 
 
 17.4 
 
 15.86 
 
 18.92 
 
 23 58 
 
 15.56 
 
 17.88 
 
 17.34 
 
 19.3 
 
 12.82 
 
 14.84 
 
 15.02 
 
 15.56 
 
 14.60 
 
 16.80 
 
 15.2 
 
 15.92 
 
 14.70 
 
 Ave. 
 
 18.92 
 
 ±.992 
 
 16.42 
 ±.946 
 
 35.35 
 ±1.43 
 
 39.58 
 ±1.18 
 
 31.43 
 
 ±921 
 
 19. 
 
 ±965 
 
 3.2 
 ±.426 
 
 16.3 
 ±.559 
 
 30. 
 ±804 
 
 15 47 
 
 ±755 
 
 22 63 
 
 ±469 
 
 28.34 
 
 16.79 
 
 It will be observed that there are wide variations in yield both as 
 between years, and between plots. However, the general trend has 
 been downward with the passing of time. The grand average yield 
 for all the check plots for the entire period is 22.63 bushels per acre. 
 This is a little higher than the average for the state for the ten-year 
 period covering the same years, which was 17.14 bushels per acre. 
 When it is remembered that a large part of the wheat in California 
 is grown on fallow land, which should enhance the crop, it will be 
 seen that the effects of continuous cropping on the area under study 
 are not yet in the state of unprofitable production. It requires, one 
 year with another and under a wide range of prices and labor condi- 
 tions, about 14 bushels of wheat per acre to pay the cost of production. 
 
 Comparing the first half of the period with the last half, however, 
 the trend of production does not appear so favorable. Thus, during 
 the first five years of the period the average yield on the 18 check 
 plots, representing 90 trials, was 28.34 bushels per acre, while these 
 
Bull. 393] 
 
 CROP SEQUENCES AT DAVIS 
 
 17 
 
 same plots produced during the last 5 years of the period only 16.79 
 bushels per acre. 
 
 Rainfall in Relation to Decreasing Yield Under Continuous Crop- 
 ping. — These figures must not be taken as expressing the exact produc- 
 ing power of these plots, for a rainfall factor is involved. Between the 
 two halves of the period the rainfall varied widely. Thus during the 
 first half of the ten-year period the total rainfall for each season 
 averaged 18.68 inches, which is above normal by 1.45 inches, while 
 that of the latter half of the period amounted to only 16 inches, which 
 is 1.23 inches below normal. Each half of the ten-year period, how- 
 ever, had a year of very low rainfall (1917-18, 9.66 in.) and (1919-20, 
 8.94 inches). The rainfall difference is even more striking when that 
 falling during the spring months of February-May inclusive is taken 
 into account. During these months for the first five-year period the 
 average rainfall was 8.21 inches, while for the same months during the 
 second five-j^ear period the average rainfall was only 3.60 inches. ■ ' 
 
 The yields of these check plots have also been modified by the pres- 
 ence of wild oats, the amount of which does not show in the figures for 
 yields. Wild oats mature ten days to two weeks earlier than the wheat. 
 Consequently the yield of oats does not appear in the figures given. 
 
 Exactly what significance should be placed upon the influence of 
 this subnormal seasonal and spring rainfall upon the yield of wheat 
 during the second five-year period and upon the presence of wild 
 oats is difficult to determine. But the data at hand point clearly to 
 the declining producing power of the soil under continuous cropping. 
 
 The same general trends are apparent under continuous cropping 
 to oats, barley, rye and milo. The following table illustrates this point. 
 In the experiment under consideration one plot was continuously 
 planted with each of these four grains, and the yields in bushels per 
 acre are tabulated as follows : 
 
 TABLE 8 
 Influence of Continuous Cropping on Barley, Eye, Oats and Milo 
 
 Average 
 10 crops 
 
 Average - 
 lat-5 crops 
 
 Average - 
 2nd-5 crops 
 
 Plot 8, continuous barley. 
 
 Plot 10, continuous rye 
 
 Plot 11, continuous oats... 
 Plot 14, continuous milo. 
 
 42.58 
 24.09 
 79 45 
 52.37 
 
 58.74 
 30.75 
 79.91 
 73 34 
 
 26.42 
 17.42 
 78.98 
 35 60 
 
 It is probable that the rapid decline in the second period is due 
 largely to the lower seasonal and spring rainfall during that period 
 and to the fact that the second of two closely following dry years fell in 
 
18 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 that period. For some reason which is not fully apparent oats have 
 not shown the decline in yield shown by the other crops. The yield 
 of oats from year to year, however, has varied over a wide range, 
 being above 100 bushels per acre two years and below 30 one year. 
 
 Influence of the Summer Fallow. — Three plots in this experiment 
 are devoted to alternate wheat and fallow, namely, Plot 4, wheat 
 alternate with cultivated fallow; Plot 5, wheat alternate with cloddy 
 
 Fig. 6. — Crop of 1920. Wheat continuously on the left. Wheat after fallow 
 on the right. Yields, 4 and 25 bushels per acre respectively. The fallow has 
 a very marked effect on wheat growth and yield during dry years. 
 
 fallow, and Plot 7, wheat alternate with weedy fallow. The yields 
 on these plots compared with their adjacent check plots bring out very 
 strikingly the influence of the summer fallow on the yield of wheat. 
 
 TABLE 9 
 The Yield of Wheat on Alternate Fallow, Bushels Per Acre 
 
 Plot: Treatment 
 
 Yield 
 10-year 
 period 
 
 Yield 
 
 lst-5 year 
 
 period 
 
 Yield 
 
 2nd-5 year 
 
 period 
 
 3 wheat continuously, 10 crops 
 
 25.26 
 40.07 
 40 17 
 24.95 
 22.63 
 40 12 
 25.10 
 15.02 
 
 29.66 
 37.66 
 38.09 
 32.49 
 28.34 
 37.87 
 31.07 
 6.7 
 
 20.87 
 
 
 43.7 
 
 
 42.3 
 
 6 wheat continuously, 10 crop? 
 
 17.4 
 
 
 16.79 
 
 Average of fallow plots 
 
 43.00 
 
 Average adjacent checks 
 
 19.13 
 
 Difference in favor of fallow 
 
 15.02 
 
 
 
BULL. 393] CROP SEQUENCES AT DAVIS 19 
 
 In this comparison it is interesting- to note that the yields on the 
 fallowed plots are greater during the second five-year period than 
 during the first, a condition in reverse of that on the check plots ; and 
 this notwithstanding the fact that seasonal rainfall was less during 
 the second half of the period than during the first half. There is not 
 sufficient data at hand to enable us to give a full explanation of this. 
 The drying of the soil with the activities and changes that go on 
 during that process may partially account for this trend of results. 
 Klein 7 and Kelly and McGeorge 8 have shown that both drying and 
 heat have a modifying influence on the productivity of soils. The 
 former author shows that under humid conditions and with the soils 
 studied the drying of the soil increases productivity mainly ' ' through 
 changes in the organic content, the physical nature of the soil and the 
 formation of nitrates. Without doubt some or all of these changes 
 take place in our soils during the process of summer fallowing and the 
 consequent alternate drying and wetting tends to keep them produc- 
 tive as long as the organic matter content remains within suitable 
 limits. 
 
 It should be noted also that the yield of the check plots during 
 the second period are much reduced because of the very low yields 
 obtained from the harvest of 1920 (see table 7). The average of all 
 check plots (18) during this year was 3.2 bushels per acre, and that 
 of these two adjacent check plots (3 and 6) was 4.25, yet the average 
 yield of wheat on plots that had been fallowed the previous year was 
 26.1 bushels per acre. The indications from this as well as from other 
 evidence, is that continuous cropping, even for a short period results 
 in a serious depletion of yields during dry years. The rainfall for 
 the season 1919-20 was only 8.94 inches — 8.29 inches below normal. 
 The factor that probably prevented complete failure that year was the 
 3.47 inches of rainfall that came in March. 
 
 The teaching of these data is also borne out by the yields of 
 wheat on plot 23, where a fallow preceded the wheat crop of 1920, 
 which yielded 25 bushels per acre ; whereas the adjacent check plot 24, 
 yielded only 2 bushels per acre. Likewise on plots 29 and 35 on 
 which barley was preceded by a fallow the preceding year, the barley 
 crop in 1920 was 43.6 bushels and 41 bushels per acre respectively; 
 whereas the barley crop on plot 25, which was preceded by a crop 
 of wheat, was that year only 11.6 bushels per acre. 
 
 The influence of the summer fallow when properly made is further 
 shown by the yields on plots 19, 20 and 22, wheat 2 years-fallow and 
 wheat 3 years-fallow, with their adjacent checks 18 and 21 as follows : 
 
20 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 TABLE 10 
 
 Influence of Frequency of Fallow on the Succeeding Crops of Wheat, 
 
 Yields in Bushels Per Acre 
 
 Plot 
 18 
 
 19 and 20 
 
 21 
 
 22 
 
 Treatment 
 
 Adjacent check 
 
 Fallow-wheat-wheat 
 
 Adjacent check 
 
 Fallow-wheat-wheat-wheat 
 
 Average yield 
 full period 
 
 22. 17 (10 crops) 
 32.97(13 crops) 
 23.70 (10 crops) 
 29.22 (8 crops) 
 
 Average yield 
 1st — 5 years 
 
 28. 79 (5 crops) 
 39.44 (6 crops) 
 29.52 (5 crops) 
 30.39 (4 crops) 
 
 Average yield 
 2nd— 5 years 
 
 15.56 (5 crops) 
 
 27.4 (7 crops) 
 17.88 (5 crops) 
 
 28.05 (4 crops) 
 
 On plots 19 and 20 the average yield of the six crops of wheat 
 following the fallow was 38.03 bushels per acre, while the average 
 yield of the seven crops preceding the fallow was 28.52 bushels per 
 acre. In the case of plot 22, where three crops of wheat occurred 
 in succession a similar comparison may be made. The average of the 
 three crops following the fallow was 41.2 bushels per acre. The yield 
 of the three crops preceding the fallow (third crop after) was 29.29 
 bushels and the intervening crop averaged for the three years 17.2. 
 The fact that the intervening crop was less than that of the third year 
 removed from the fallow is not easily explained but may have been 
 due to seasonal conditions; the other two figures, however, are 
 significant. While the influence of a fallow every third or every 
 fourth year is not so marked as the fallow every alternate year, yet 
 even at these intervals it shows a favorable influence on yields over 
 the adjacent check plots. Here again also is the influence of a dry 
 year shown, for plot 19 yielded in 1920, when preceded by wheat, 
 only 3.2 bushels per acre, and plot 22 yielded under the same circum- 
 stances 4 bushels per acre. 
 
 Barley is affected in a similar manner by a well-made fallow. 
 Thus plots 26, 28, 29 and 31 in which barley follows fallow in a 
 rotation of barley-milo-wheat-fallow gave an average yield from 9 
 crops of 60.03 bushels per acre ; whereas barley preceding fallow on 
 plots 23 and 25 gave an average yield from 6 crops of 36.91 bushels 
 per acre. On most of these plots the barley crop was preceded by a 
 wheat crop. On plot 23 it was one year removed from the previous 
 fallow and on 25 it was 2 years removed. When these figures are 
 compared with the ten crops from plot 8, barley continuously, where 
 the average yield was 42.08 bushels the influence of a fallow on barley 
 yields is at once seen. Also in another rotation (plots 32-37) in 
 which barley follows fallow, and peas turned under takes the place of 
 milo, the average yield from nine crops of barley was 57.36 bushels 
 per acre. 
 
BULL. 393] CROP SEQUENCES AT DAVIS 21 
 
 The influence of fallow on the yield of wheat and barley is striking, 
 and it should be further noted that on all plots receiving" fallow treat- 
 ment every second, third or fourth year, the soil is in better physical 
 condition as noted by the ease of plowing and the readiness with 
 which the soil pulverizes. There is no measure at hand for this effect, 
 but in the observation of all who have come in contact with these 
 plots, the fact is evident. Moreover, the presence of a summer fallow 
 in the rotation helps materially in keeping the land clean of weeds. 
 The continuously cropped plots are noticeably foul with weeds and 
 wild oats. 
 
 These data coupled with extensive observations suggest three 
 primary benefits to be derived from the employment of a properly 
 prepared summer fallow in grain production under the climatic and 
 soil conditions under study. First, the cultivated summer fallow 
 maintains a good physical condition in the soil, which aids in the 
 preparation of the soil and the control of weeds. Second, the summer 
 fallow conserves moisture which is not only a benefit to the succeeding 
 crop but favorably influences chemical and bacterial action. Third, 
 fallow land is well prepared for planting when the season and mois- 
 ture conditions are most suitable. Timeliness of planting is an 
 important factor in grain production in California; for if the plant- 
 ing is delayed because of time required in preparing the land after 
 the fall rains begin, the crop may be subjected to the hazards of 
 unfavorable weather conditions and disease infestation. 
 
 Does Summer Fallowing Pay. — The question arises, however, does 
 the increased yield of wheat or barley on the fallowed land bring 
 sufficient compensation for the loss of the land to crop during the 
 period of the fallow. Upon this question sufficient data for a precise 
 answer has not been obtained; but it is believed that when all the 
 benefits of fallow, including the mainteance of producing power of 
 the land, freedom from weeds, improved physical condition of the 
 soil and increased yield are taken into consideration, a summer fallow 
 occurring every second or third year is in the code of economic prac- 
 tices. If $20 per acre may be considered a reasonable cost for growing 
 wheat and the product is worth one dollar per bushel, then the value 
 of the wheat above the cost of production on all the check plots for the 
 period of ten years has been $2.63 per acre per year. The value of 
 the wheat on the three plots alternately fallowed has been $7.22 per 
 acre per year above the cost of production, when five dollars per acre 
 additional is allowed for making the summer fallow. It is realized 
 that this is not a complete analysis of the problem but it serves to 
 
22 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 indicate that if the producing power of the land can be maintained 
 at a high level a fallow every other year may not be unprofitable. 
 
 Residual Effect of Fallow. — How often should a summer fallow 
 occur in a cropping scheme with small grains? This is a pertinent 
 question when it is desired to secure as much from the land as pos- 
 sible consistent with the maintenance of its producing power. Sugges- 
 tive data may be secured on this point from several plots in the 
 experiments, which are set forth in the following table: 
 
 TABLE 11 
 
 Eesidual Effect cf Fallow on Wheat Yields 
 
 Yield of wheat 1st year after fallow 43.39 bushels per acre, average 11 crops 
 Yield of wheat 2nd year after fallow 30.18 bushels per acre,' average 9 crops 
 Yield of wheat 3rd year after fallow 26.06 bushels per acre, average 10 crops 
 
 The figure for the third year after fallow includes the yields of 
 wheat after milo in a four-year sequence (plots 26-31) : fallow, barley, 
 milo, wheat. The yields of wheat in the second year after fallow 
 includes the yields of wheat after barley in a three-year sequence 
 (plot 23), fallow, barley, wheat. The indications are that a fallow 
 every fifth year will not be of any particular advantage, so far as 
 yields are concerned, over continuous cropping. It is interesting to 
 note that in a four-year sequence (plots 32-37), where wheat three 
 years after fallow and immediately following peas turned under, 
 the average yield of wheat for six crops is 40.97 bushels per acre. 
 If these six crops should be included in the 10 crops averaged above 
 (3rd year), the yield is brought up from 26.06 to 31.65, thus indicat- 
 ing the benefits of a green manure crop. 
 
 The Influence of Manure. — It has often been observed that the 
 use of barnyard manure in grain production under dry farming 
 conditions does not produce economic returns ; and it has been observed 
 in some instances to actually deplete yields (fig. 7). Similar results 
 do not take place at all or are not so marked under irrigated condi- 
 tions. There are a number of reasons for this, some of which are 
 obscure. But it is quite evident that under conditions of a limited 
 moisture supply, too much carbon, which is a large part of the content 
 of barnyard manure, may accumulate in the soil, and thus upset the 
 ratio of this element to nitrogen that is required for optimum plant 
 development. Sievers and Holtz s have shown that straw or strawy 
 material, low in nitrogen content, turned into the soil under conditions 
 of restricted summer rainfall has a depressing effect on yields of 
 grain. This effect continues until the ratio of carbon to nitrogen 
 
Bull. 393 
 
 CROP SEQUENCES AT DAVIS 
 
 23 
 
 is brought into limits suitable to the crop's demands as related to 
 kind of soil and moisture, and seasonal conditions. 
 
 In this experiment one plot (13) has been planted with wheat 
 continuously and 500 pounds (5 tons per acre) of barnyard manure 
 added each year. (1000 lbs. were applied in years 1919-22.) The 
 yields of this plot are set forth in the following table : 
 
 TABLE 12 
 Effect of Manure on the Yield of Wheat — Bushels Per Acre 
 
 Plot 
 
 Treatment 
 
 Average 
 10 crops 
 
 Average yield 
 1st — 5 years 
 
 Average yield 
 2nd— 5 years 
 
 13 
 
 
 28.55 
 24.98 
 
 33.19 
 31 05 
 
 23 9 
 
 12 
 
 
 18.92 
 
 
 
 
 
 Fig. 7. — Influence of green manure during a dry year (1920, 8.94 ins.). 
 Plot on left (38) is wheat after peas, yield 30.5 bushels per acre. Plot on right 
 (39) is wheat continuously (check), yield 3.4 bushels per acre. In the right 
 rear may be seen plot 41, wheat after vetch, yield 30.9 bushels per acre. 
 
 These results indicate a slight advantage in favor of the manured 
 plot over its adjacent check, but it would seem that the increase from 
 the use of manure is not so great as might have been expected if there 
 had been an abundance of moisture from irrigation or rainfall. 
 
 Influence of Seasonal Rainfall. — Under climatic conditions such 
 as prevail at Davis, there is a relation between the use of manure 
 and available moisture. It would seem, moreover, that the available 
 moisture during the spring or growing months has a greater influence 
 on yield than does the total rainfall or seasonal rainfall. This is 
 illustrated by the following figures where the yields of wheat and 
 rainfall in seasons of above and below rainfall are compared : 
 
24 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 TABLE 13 
 
 Eelation of Seasonal and Speing Rainfall (Feb. -April) to Yields of Wheat 
 With and Without Manure — Bushels Per Acre 
 
 Five years above normal seasonal rainfall 
 
 Five years below normal seasonal rainfall 
 
 Seasonal 
 rainfall 
 
 Spring 
 rainfall 
 
 Average 
 
 yield 
 
 wheat and 
 
 manure 
 
 plot 13 
 
 Average 
 
 yield 
 
 plot 12 
 
 (check) 
 
 Average 
 yield 
 
 18 
 checks 
 
 Seasonal 
 rainfall 
 
 Spring 
 rainfall 
 
 Average 
 
 yield 
 
 wheat and 
 
 manure 
 
 plot 13 
 
 Average 
 
 yield 
 
 plot 12 
 
 (check) 
 
 Average 
 yield 
 
 18 
 checks 
 
 21.38 
 
 6.41 
 
 26.92 
 
 23.73 
 
 21.03 
 
 13.30 
 
 5.99 
 
 32.76 
 
 26.23 
 
 24 10 
 
 Six years above normal spring rainfall 
 
 Four years below normal spring rainfall 
 
 18.26 
 
 8.10 
 
 31.10 
 
 29.82 
 
 25.89 
 
 15.90 
 
 3.38 
 
 24.71 
 
 22.76 
 
 21.21 
 
 Thus it will be seen that the average yields on both manured and 
 check plots were higher for the five seasons of below normal seasonal 
 rainfall than they were for the years of above normal seasonal rainfall ; 
 but when considered on the basis of spring rainfall the six years of 
 above normal precipitation gave better yields than the four seasons 
 of below normal rainfall during the growing months. 
 
 These figures would indicate that in a dry climate, such as that at 
 Davis, there is a possibility of getting too much organic matter of low 
 nitrogen content and possibly other materials such as nitrates as 
 well. Under such conditions even though the seasonal rainfall be 
 normal the yields are materially reduced if there is a shortage of 
 precipitation during the spring months. 
 
 A similar rotation has been carried on at Kearney Vineyard, near 
 Fresno, for a ten-year period covering the same years as at Davis. 
 At Kearney, however, the yield of wheat does not seem to be influenced 
 by a variation of seasonal or spring rainfall so much as it is at Davis. 
 This is probably due to the fact that the land at Kearney receives a 
 supply of moisture from the subsoil, especially during the summer 
 months, that supplements the rainfall during periods of deficiency. 
 
 TABLE 14 
 Relation of Manure, Fallow, Continuous Cropping and Rainfall to Yields 
 of Wheat at Kearney, 1913-23 
 
 (Wheat yields bushels per acre.) (Rainfall, seasonal normal 9.68 inches, spring normal 4.53 inches.) 
 
 
 Wheat in 
 rotation 
 
 with 
 manure 
 
 Wheat in 
 
 rotation 
 
 no manure 
 
 Wheat 
 after 
 fallow 
 
 Wheat 
 continu- 
 ously 
 
 Seasonal 
 rainfall 
 
 Spring 
 
 rai nfall 
 
 Feb.-April 
 
 
 48.29 
 40 14 
 56.45 
 
 35.92 
 26.50 
 51.35 
 
 45.27 
 34.81 
 55.74 
 
 25.02 
 25 62 
 24.43 
 
 9.68 
 9.84 
 8.65 
 
 4 53 
 
 
 4 45 
 
 
 4.10 
 
 
 
Bull. 393 
 
 CROP SEQUENCES AT DAVIS 
 
 25 
 
 The rotation employed was corn, wheat, beans ; and the plots were 
 so arranged that each crop was grown each year, though not on the 
 same plot. (In 1916, however, the wheat crop was cut for hay.) 
 Manure was applied to one series at the rate of about 10 tons per acre. 
 The plots were of *4 acre. 
 
 During the eleven years period (10 crops) the yield of wheat on 
 the manure rotation was 12.37 bushels per acre greater than on the 
 unmanured rotation. During two years, however (1919-1921) the 
 unmanured plots gave higher yields than the manured plots, but the 
 difference was small. During these two years both the seasonal and 
 spring rainfall was below normal ; the spring rainfall in fact was but 
 little above half the normal for these years, being 2.59 and 2.51 inches 
 respectively. 
 
 The results for these two years, with the influence of fallow are 
 set forth in the following table : 
 
 TABLE 15 
 
 Yield of Wheat on Manured and Unmanured Plots and After Fallow at 
 
 Kearney During Dry Years — Bushels Per Acre 
 
 Year 
 
 Wheat 
 manured 
 
 Wheat 
 unmanured 
 
 Wheat 
 after fallow 
 
 Seasonal 
 rainfall 
 
 Spring 
 rainfall 
 
 1919 
 
 41.93 
 37.73 
 
 43.20 
 38.73 
 
 51.43 
 50.47 
 
 6.90 
 8.19 
 
 2.59 
 
 1921 
 
 2.51 
 
 
 
 Both of these years fall in the second period and their yields are 
 lower than the average for that period. However, the wheat yields 
 after fallow both years are higher than on the manured or unmanured 
 plots. These results corroborate the results generally obtained ; 
 namely, that a well prepared summer fallow induces or maintains a 
 high yield of wheat during dry years. While there are considerable 
 variations in yield during different years of the entire period, it is 
 of interest to note that the yields under all treatments, except con- 
 tinuous cropping, have been greater during the second half of the 
 ten-year period and this in spite of the fact that the rainfall for both 
 the seasons and the spring has been slightly lower during the second 
 than the first half. It is believed that the rotation employed and the 
 better physical condition of the soil have been the contributing factors. 
 
 The comparison of manured plots and unmanured plots is, how- 
 ever, of striking interest. In general the application of manure has 
 been of benefit, but evidently the increased yield has not been sufficient 
 to pay for the application. The difference in yield of wheat between 
 
26 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 the manured and unmanured plots was much greater during the first 
 half of the period (13.64 bushels per acre) than during the second 
 half (5.10 bushels). 
 
 From all the evidence it would seem that with the heavy application 
 of manure each season, the manure is being applied more rapidly than 
 it can decay under the light rainfall received. The accumulation of 
 undecayed organic matter is beginning to exert a retarding influence 
 on the crop. The application of manure to grain land under a deficient 
 
 Fig. 8. — The plot on the right (23) is continuously cropped to wheat, aver- 
 age yield for 10 crops 20.50 bushels per acre. The plot on the left is wheat 
 after peas turned under in a four-year rotation. Average yield for ten crops 42.34 
 bushels per acre. Crop of 1919. 
 
 moisture supply raises some interesting problems. The amount to 
 apply which will bring the best results depends on several factors, 
 including stage of decomposition, coarseness and presence of straw, 
 moisture supply in the soil, culture practices, nitrogen content, and 
 the like ; but in all events manure under our climatic conditions must 
 be applied with greater understanding and care than under humid 
 conditions. 
 
 It should be noted further that the decline in yield of wheat under 
 continuous cropping (table 14) is not so marked between the two 
 periods as in the case at Davis, but that continuous cropping results 
 in a lower yield than wheat in rotation with or without manure. 
 
Bull. 393 
 
 CROP SEQUENCES AT DAVIS 
 
 27 
 
 Effect of Green Manure on Yield of Wheat. — In this experiment 
 green manure crops turned under have had a pronounced effect at 
 Davis on the yield of wheat (fig. 8). This is brought out in two 
 rotations — one of wheat alternated with peas turned under, and the 
 other of wheat alternated with vetch turned under. Both of these 
 rotations consisted of two plots arranged in such a way that a crop, 
 both of wheat and legume was grown each year. 
 
 The results are set forth in the following table : 
 
 TABLE 16 
 Effect of Peas and Vetch on Yield of Wheat 
 
 
 Harvest for the season 
 
 Aver- 
 age 
 
 wheat 
 
 Aver- 
 
 Plot 
 
 1914 
 
 1915 
 
 1916 
 
 1817 
 
 1918 
 
 1919 
 
 1920 
 
 1921 
 
 1922 
 
 1923 
 
 age 
 legume 
 
 38— Wheat-peas 
 
 39— Check 
 
 30.00 
 35.00 
 
 3.34 
 24.00 
 22.00 
 
 3.31 
 
 4.39 
 15.00 
 18.60 
 
 6.10 
 12.16 
 15.92 
 
 44.67 
 21.67 
 
 8.47 
 54.33 
 29.35 
 
 6.05 
 
 7.25 
 30.60 
 39.33 
 10.25 
 27.33 
 36.00 
 
 52.30 
 25.50 
 11.90 
 
 51.60 
 25.50 
 10.00 
 
 3.75 
 16.90 
 21.20 
 
 1.75 
 14.50 
 21.20 
 
 30.50 
 3.40 
 1.50 
 
 30.90 
 3.00 
 1.25 
 
 45 
 14 20 
 20.50 
 
 0.25 
 12.80 
 22.80 
 
 54.30 
 28.00 
 
 5.12 
 47.70 
 25.70 
 
 5.25 
 
 0.93 
 15.30 
 63.00 
 
 4.43 
 17.00 
 68.80 
 
 42.35 
 20.56 
 32.52 
 41.70 
 18.93 
 32.94 
 
 3.35 
 
 40 — Peas-wheat 
 
 41— Wheat- vetch 
 
 42— Check 
 
 6.06 
 4 55 
 
 43— Vetch-wheat 
 
 5.17 
 
 In this table the yields of wheat are expressed in bushels per acre 
 and the yields of legumes in tons of green matter per acre. It will be 
 noted that the average yield of wheat after green manure is on all 
 four plots greater than the yields of the adjacent check plots. Yet 
 the yields of wheat on plots 40 and 43 are lower than on plots 38 and 
 41, while the tonnage of green matter on these respective pairs of 
 plots is the reverse. It may be that more than five tons of legume 
 turned under per acre is an excess of green matter under these con- 
 ditions and when large amounts of green matter are turned under 
 yields of wheat are reduced. 
 
 If the ten crops of wheat after peas and the ten crops of wheat after 
 vetch are averaged and these compared with adjacent checks, the 
 influence of the green manure is more strikingly brought out. 
 
 TABLE 17 
 
 Influence of Green Manure on Yields of Wheat, "Wheat-Bushels, Peas and 
 Vetch — Tons Green Matter Per Acre 
 
 Average 10 crops, wheat after peas 37.44 
 Average 10 crops, peas after wheat 4.71 
 Average 20 crops, wheat continu- 
 ously (checks) 19.74 
 Average 10 crops, wheat after vetch 37. 34 
 Average 10 crops, vetch after wheat 4.86 
 
 lst-5 crops 36.98 2nd-5 crops 37.9 
 
 lst-5 crops 7.07 2nd-5 crops 2.35 
 
 lst-5 crops 23.50 2nd-5 crops 15.08 
 
 lst-5 crops 36.37 2nd-5 crops 38.28 
 
 lst-5 crops 7.14 2nd-5 crops 2.58 
 
28 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 From these figures it will be noted that the yields of wheat after 
 legumes turned under is considerably augmented, when compared with 
 wheat grown continuously. Also that the yields of wheat after legumes 
 was slightly greater the second half of the period than the first half, 
 although the rainfall during that period was below normal. The 
 amount of green matter turned under per acre was, however, consider- 
 ably less during the second half of the period than the first. 
 
 The influence of peas turned under is also brought out in another 
 rotation consisting of four plots (32-37) with checks and arranged so 
 that a crop of barley, peas, wheat, followed by fallow was harvested 
 each year. 
 
 The results are tabulated as follows : 
 
 TABLE 18 
 
 Influence of Peas cn Wheat in a Four-Year Botation. Wheat Yield in 
 Bushels. Peas in Tons Green Matter Per Acre 
 
 Average yield of 10 crops of wheat 
 
 after peas 42.34 lst-5 crops 41.49 2nd-5 crops 43.20 
 
 Average yield of 10 crops of peas 5.06 lst-5 crops 6.09 2nd-5 crops 4.03 
 Average yield of 20 crops of wheat 
 
 adjacent check plots 33.36 19.90 lst-5 crops 24.88 2nd-5 crops 14.93 
 
 These figures show a slightly increasing yield of wheat following 
 peas turned under during the second half of the ten-year period, 
 though the average yield of peas turned under is very materially 
 decreased. The yield of wheat in this rotation is somewhat higher 
 than in the rotation of alternate wheat and peas. This would indicate 
 that a legume turned under once in four years with also a fallow 
 period is better than a legume turned under every alternate year. It 
 is recognized that this may not be an economic practice and such 
 results might not be obtained if the legume were harvested for forage 
 or seed. 
 
 The yields of barley in this rotation are also maintained at a 
 higher level than those on the continuously cropped plots, but it is 
 believed that in this case, barley following fallow, the crop is more 
 influenced by the previous fallow than it is of the crop of peas turned 
 under two years previously. 
 
 The data of this rotation as well as general observation would 
 indicate that a four-year sequence of crops with one year legumes and 
 one-year fallow will maintain the crop producing power of the soil 
 at a high level under the conditions existing at Davis. How profitable 
 this practice would be, would depend on many conditions, related to 
 stand, value and use of the legume crop. 
 
BULL. 393] . CROP SEQUENCES AT DAVIS 29 
 
 Wheat and Legume Combined. — One plot in the series (53) has 
 grown wheat continuously with a light seeding of burr clover or 
 Hubam in the spring. The stand of the legume has varied widely 
 during the ten-year period, and continuous record of the amount of 
 dry matter in legume restored to the soil has not been kept. However, 
 the results do not show an advantage in planting these legumes with 
 wheat. The average yield of the wheat crop for the ten-year period 
 was 20.56 bushels per acre with an average of 23.49 bushels during 
 the first half of the period and 17.6 bushels during the second half. 
 This plot showed a very low yield during the dry year of 1920, as did 
 the check plots. The records for the adjacent check were: average for 
 the ten-year period 20.34 ; first half of period, 25.98 and for the second 
 half, 14.70 bushels per acre. About the only advantage to be noted in 
 this combination, is that the yield during the second half of the period 
 has not declined so rapidly as on the adjacent check, or 18 checks 
 combined. 
 
 Comparison of Biennial Cropping With and Without Green 
 Manure. — By tabulating the yields from plots in various parts of the 
 experiment it is possible to compare the influence of green manures 
 and fallow in increasing yields. For this comparison, plot 38, wheat- 
 peas, and plot 41, wheat-vetch, have been chosen as representative of 
 the green manure plots, and plots 4 and 5 as representative of the 
 fallow plots, because all of these plots were in wheat the same years 
 throughout the period. In the following table the average yields of 
 wheat of these plots are given for the ten-year period only, comprising 
 five crops in each case, and the same years are chosen from the checks 
 as were in wheat on the other plots. 
 
 TABLE 19 
 
 Comparison of Green Manure and Fallow in Their Influence on the Yield 
 of Wheat. Average Yield in Bushels Per Acre 
 
 Wheat on plot 38 — wheat-peas 42.35 
 
 Wheat on plot 39 — wheat continuously 22.71 
 
 Wheat on plot 41 — wheat-vetch 41.70 
 
 Wheat on plot 42 — wheat continuously 19. 31 
 
 Wheat on plot 3 — wheat continuously 24.97 
 
 Wheat on plot 4 — wheat-fallow 40.07 
 
 Wheat on plot 5 — wheat-fallow 40. 17 
 
 Wheat on plot 6 — wheat continuously 24.62 
 
 Average yields on green manure plots (38, 41) 42.02 
 
 Average yields on their adjacent checks (39, 42) 21.01 
 
 Average yields on fallow plots (4, 5) 40. 12 
 
 Average yields on their adjacent checks (3, 6) 24.80 
 
 Average difference in favor of green manure over adjacent checks 21.01 
 
 Average difference in favor of fallow over adjacent checks 15.32 
 
30 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 It will be noted from these data that for a ten-year period there 
 is a difference in yield of wheat with fallow and with green manure 
 in a biennial cropping system, and the indications are that as time 
 goes on the difference will increase in favor of the green manure plots, 
 because even now the soil on the green manure plots appears more 
 friable and in better physical condition, than that of the fallow plots 
 or the adjacent checks. The measure of economy in these trials is not 
 shown by the figures, and except under special conditions the practice 
 of green manuring as here indicated may not be economical. 
 
 The Effect of Milo in Rotation With Small Grain. — A four course 
 rotation with barley, milo, wheat and fallow has been carried out on 
 plots 26-31. This sequence is interesting both from the standpoint of 
 the feasibility of producing a profitable crop of grain- sorghum without 
 irrigation; and also from the standpoint of the effect of a cultivated 
 crop compared with a summer fallow. The yield data from the ten- 
 year period are given in the following table : 
 
 TABLE 20 
 Yield of Milo in Botation and Continuously, Bushels Per Acre 
 
 Milo in 4-year rotation, average 10- 
 year period 56.11 lst-5 years 68.16 2nd-5 years 46.5 
 
 Milo continuously, average 10-year 
 
 period 50.06 lst-5 years 73.34 2nd-5 years 31.44 
 
 Thus, it is indicated that milo holds up better in a rotation which 
 includes a fallow than it does under continuous cropping. It should 
 be noted, however, that under both summaries, the crop of '18 is 
 not included because the early fall rains and excessive bird damage 
 left no crop to harvest that year. 
 
 Summer Fallow and Moisture. — The summer fallow seems to have 
 at least three direct benefits on the crop of grain which is to follow. 
 If properly made and timed it conserves moisture, it causes an elabor- 
 ation of nitrates, and the land is ready to plant when the proper 
 seasonal and moisture conditions arrive. 
 
 Moisture determinations were made on all plots of this experiment 
 during the years 1914—17 inclusive. Samples were taken to a depth of 
 six feet on all plots and deeper on some plots, and the moisture cal- 
 culated as percentage of dry soil. In the following table the moisture 
 content on all plots fallowed during the year indicated is shown, and 
 for comparison the moisture content on the check plots adjacent to 
 the fallow plots is also shown. 
 
Bull. 393 
 
 CROP SEQUENCES AT DAVIS 
 
 31 
 
 TABLE 21 
 
 Moisture Percentages to a Depth of Six Feet on Fallowed Plots and 
 Cropped Plots (Checks) at the Close of the Summer Season for Four Years 
 
 Sampled Oct. 30. 1914 
 
 Sampled Oct. 29, 1915 
 
 Sampled Nov. 14, 1916 
 
 Sampled Oct. 30, 1917 
 
 Plot 
 
 Fallow- 
 
 Plot 
 
 Ad- 
 jacent 
 checks 
 
 Plot 
 
 Fallow 
 
 Moist- 
 ure 
 
 Plot 
 
 Ad- 
 jacent 
 checks 
 
 Plot 
 
 Fallow- 
 
 Plot 
 
 Ad- 
 jacent 
 checks 
 
 Plot 
 
 Fallow 
 
 Plot 
 
 Ad- 
 jacent 
 checks 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 Moist- 
 ure 
 
 20 
 31 
 37 
 
 21.40 
 17.98 
 18.53 
 
 21 
 30 
 
 36 
 
 14.59 
 14.57 
 15.03 
 
 4 
 19 
 29 
 35 
 
 13.35 
 
 19.92 
 18.64 
 18.67 
 
 3 
 18 
 
 30 
 36 
 
 8.66 
 14.51 
 12.26 
 10.16 
 
 17 
 
 23 
 28 
 34 
 
 19.74 
 19.74 
 18.07 
 18.83 
 
 18 
 24 
 27 
 33 
 
 18.17 
 16.80 
 15.56 
 14.57 
 
 4 
 
 20 
 22 
 25 
 26 
 32 
 
 14.98 
 19.57 
 19.01 
 20.22 
 19.17 
 18.19 
 
 3 
 21 
 
 24 
 27 
 33 
 
 15.61 
 15.82 
 
 12.86 
 14.94 
 17 40 
 
 Av. 
 Dif. 
 
 19.30 
 4 57 
 
 
 14.73 
 
 
 17 61 
 5.24 
 
 
 11.40 
 
 
 19.09 
 2 82 
 
 
 16.27 
 
 
 18.52 
 3.20 
 
 
 15 32 
 
 The data at hand is inadequate to explain the low differences for 
 the year 1916 and 1917; but notwithstanding this fact the amount 
 of moisture conserved is significant in its bearing on the crop the 
 following year, for it will be noted that the average yield of wheat 
 in 1917 on plots 17 and 23, which had been fallowed the summer of 
 1916, was 55.41 bushels per acre, while the average yield of check 
 plots 18 and 24 for the same season was 43.74 bushels, or a gain of 
 more than eleven bushels per acre in favor of fallow. Similarly the 
 average yield of wheat in 1918 on plots 4, 20 and 22 after fallow, was 
 46.1 bushels per acre, whereas the average yield on the adjacent check 
 plots 3 and 21 was 31.4 bushels. 
 
 Thus it seems quite evident both from these data as well as from 
 general observation that a properly made summer fallow conserves 
 moisture to the advantage of the following crop. Whether this advant- 
 age is due directly to the moisture conserved or rather to the more 
 favorable distribution of moisture in the soil column or to the influence 
 of this moisture on the chemical and bacterial activities in the soil, 
 it is not possible to say. It is well known that both bacterial and 
 chemical forces are more active in a fallow soil during the summer 
 months than under stubble where conditions are otherwise similar, and 
 that as a result of these, the fallow soil is improved in its crop 
 producing power. Sievers and Holtz 9 in studies of silt loam soils and 
 their yields in Eastern Washington came to the conclusion that mois- 
 ture conservation under summer fallow practices serves its primary 
 purpose in the elaboration of nitrates. Specifically they state (p. 19) : 
 
32 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 "Available soil nitrogen and not total soil moisture is the limiting 
 factor in crop production in this region and that the real basis for 
 summer fallowing is to make the maximum amount of nitrogen ready 
 for plant use. ' ' 
 
 The data at hand and our observations indicate that similar con- 
 clusions can be drawn for the conditions of the present study, and 
 that the uniform distribution of moisture under summer fallow within 
 optimum limits for bacterial and chemical action, is of greater signifi- 
 cance than the total amount conserved. Emphasis must be placed, 
 however, on the primary importance of moisture, for it is upon this 
 that nitrate increase depends. As shown in table 22 below, the 
 moisture under the fallow is not only higher than under the checks, 
 but also more uniformly distributed ; and likewise the yields of wheat 
 are higher on plots fallowed than on the checks. 
 
 Trend of Moisture Under Continuous Cropping and Fallow. — For 
 the four years, 1914-17 inclusive, moisture determinations on all plots 
 were made to a depth of six feet. In general the percentage of 
 moisture for each foot declines as the season advances until late in the 
 summer or early in the fall when there is a recovery, even before the 
 rains set in. During the summer months currents of capillary moisture 
 are set upward while the evaporation from the soil surfaces or trans- 
 piration from leaf surface is at the maximum. When these losses are 
 retarded toward the latter part of the summer, the currents of capillary 
 moisture from the lower depths continue, and thus the moisture con- 
 tent of the entire soil column is increased. This tendency is illustrated 
 by the figures in table 22 (below) in which the percentage of moisture 
 in each of the upper six feet of soil are averaged. It will also be 
 noted that the fifth foot generally contains less moisture than either 
 the fourth or the sixth. 
 
 The influence of the summer fallow on the conservation of moisture 
 may be further indicated by the data contained in this table. The data 
 is for the season 1915-16. During this season samples were taken to 
 a depth of 6 feet on five different dates, at two- and three-month 
 intervals. During this season the rainfall was a little above normal 
 (3.65 inches) but only .21 of an inch fell after the first samples were 
 taken April 1st. For this comparison the four fallow plots (17, 23, 
 28, 34) of that season were taken with their adjacent check plots (18, 
 24, 27, 33). 
 
 These figures indicate that owing to a relatively moist soil at depths 
 below 6 feet the entire six-foot column has been pretty well supplied 
 with moisture under both crop and fallow. The first foot on the check 
 plots shows a greater tendency to dry, but in every instance the fallow 
 
Bull. 393" 
 
 CROP SEQUENCES AT DAVIS 
 
 33 
 
 plots show a greater percentage of moisture than do the check plots 
 for each comparable foot, and in general this same relationship holds 
 true for the seventh foot and the eighth. 
 
 TABLE 22 
 
 Trend of Moisture Under Continuous Cropping and Fallow. Season 1915- 
 1916, for Each Foot up to Six Ft. Depth. Samples Taken on Dates 
 Indicated, Moisture in Percentage of Dry Soil. 
 
 
 April 1 
 
 May 25 
 
 July 7 
 
 August 29 
 
 November 4 
 
 
 Fallow 
 
 Checks 
 
 Fallow 
 
 Checks 
 
 Fallow 
 
 Checks 
 
 Fallow 
 
 Checks 
 
 Fallow 
 
 Checks 
 
 1 foot 
 
 21.61 
 23.50 
 
 27.77 
 
 20.24 
 22.69 
 28.34 
 
 16.86 
 21.51 
 26.85 
 
 13.30 
 14.31 
 19.73 
 
 17.27 
 21.81 
 
 25.77 
 
 9.91 
 13.54 
 20.75 
 
 12.99 
 19.59 
 23.39 
 
 7.85 
 12.59 
 18.92 
 
 13.86 
 19.71 
 23.72 
 
 £.20 
 
 2 feet 
 
 16.36 
 
 3 feet 
 
 23.23 
 
 
 
 Average 3 feet.... 
 
 24.29 
 
 23.76 
 
 21.74 
 
 15.78 
 
 21.61 
 
 14.13 
 
 IB. 66 
 
 13.12 
 
 19. C9 
 
 16.26 
 
 4 feet 
 
 30.18 
 30.08 
 
 32.76 
 
 30.04 
 29.35 
 33.27 
 
 28.75 
 24.77 
 30.58 
 
 26.20 
 27.48 
 26.28 
 
 26.95 
 26.49 
 30.27 
 
 25.71 
 22.26 
 29.32 
 
 24.43 
 22.42 
 30.27 
 
 22.89 
 21.95 
 28.31 
 
 24.75 
 23.95 
 27.85 
 
 25.19 
 
 5 feet 
 
 22.82 
 
 6 feet 
 
 26.66 
 
 
 
 Average 3 feet... 
 
 31 01 
 
 30.88 
 
 28.03 
 
 26.65 
 
 27.90 
 
 25.76 
 
 25.11 
 
 24-38 
 
 25.52 
 
 %! r .b9 
 
 Average 6 feet .. 
 
 27.65 
 
 27.32 
 
 24.88 
 
 21.21 
 
 24.76 
 
 20.25 
 
 22.18 
 
 18.75 
 
 22.30 
 
 20.57 
 
 Some Lessons From a Dry Year. — The season of 1919-20 was 
 exceptionally dry, not only in respect to total rainfall, which was 
 only 8.94 inches (8.29 inches below normal), but also in respect to 
 spring rainfall (Feb. Mar. and April), which was only 5.10 inches. 
 
 The effect of this shortage of moisture became noticeable by the 
 end of March, at which time the grain on fallow, and that after green 
 manure, stood out thrifty and vigorous compared with that on the 
 continuously cropped plots (checks) and on the manure plot (13). 
 By the middle of April, failure could be seen for all checks and con- 
 tinuously cropped plots, while those on fallow and green manure were 
 still promising. 
 
 Some yields during this year are interesting and are set forth 
 in the following table. 
 
 Evidence of the advantage of fallow and green manures during this 
 dry year is striking not only in relative yields but also in weights per 
 bushel of the grain harvested. In general where yields are lowest 
 the grain is lighter. These data corroborate to a certain extent the 
 fact that the duty of water or moisture is greater in soils of high 
 fertility, or crop producing power. The relative drought resistance of 
 the several crops above listed is also indicated. 
 
34 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 TABLE 23 
 
 Some Yields During the Dry Year, 1920 
 
 Bushels Pounds per 
 
 per acre bushel 
 
 Average yield of wheat on all checks (18) 3.2 49.83 
 
 Barley continuously, plot 8 16.4 45. 
 
 Rye continuously, plot 10 6.6 54 
 
 Oats continuously, plot 11 4.8 31 
 
 Milo continuously, plot 14 2.3 
 
 Wheat with legume continuously, plot 53 2. 1 44 
 
 Wheat with manure continuously, plot 13 2.2 38 
 
 Wheat after fallow, average 5 plots 28.34 56.2 
 
 Barley after fallow, average 2 plots 42.3 40 
 
 Wheat after peas turned under, plot 38 30. 5 56. 5 
 
 Wheat after vetch, average 2 plots 30.5 58 
 
 SUMMARY 
 
 With an annual rainfall of twenty inches or less, change of crops 
 and the inclusion of a fallow period are important factors in the 
 maintainance of high yields. 
 
 In 1913 a series of plots was laid out at Davis, California, These 
 plots were designed to ascertain the influences of certain crop 
 sequences, summer fallow, green manures, legumes and continuous 
 cropping on the crop producing power of Yolo loam and Yolo silt 
 loam, the two soil types over which the plots extend. 
 
 The seasonal and spring rainfall exert an important influence on 
 the yield of cereal crops; and of the two influences, spring rainfall is 
 the more important. 
 
 On the whole, the Yolo loam seems to be more productive of wheat 
 under the conditions than the Yolo silt loam. 
 
 In 1916, when the annual rainfall was 21.63 inches, the moisture 
 under the eighteen check plots (wheat continuously) to a depth of 
 six feet, was reduced from an average of 26.88 per cent on April 1 
 to an average of 17.55 per cent on September 1. On November 1 the 
 moisture content had risen to an average of 19.67 though no rainfall 
 had been received between the two last samplings. This increase 
 previous to the last sampling was noted for each succeeding foot except 
 the fourth. 
 
 Continuous cropping decreases the yield of cereals ; and the 
 decrease is augmented with the passage of time. This conclusion is 
 borne out by experiments under many other climatic and soil con- 
 ditions. The depressing effect of continuous cropping is especially 
 marked during dry years. 
 
Bull. 393] CROP SEQUENCES AT DAVIS 35 
 
 Cultivated summer fallow tends to maintain high yields. It 
 should not be assumed, however, that this maintainance will be con- 
 tinuous over a long period of time. The principal influence seems to 
 be due to a more equal distribution of moisture rather than to the 
 total amount conserved. 
 
 The economy of summer fallow depends on its frequency, the 
 presence of legumes or cultivated crops in the rotation and the amount 
 and distribution of rainfall. In general, however, yields decline as 
 the period between fallow years increases. 
 
 Manure, if too coarse and if applied too abundantly, may be detri- 
 mental, especially during dry years. Its effect is related among other 
 things, to its composition and to the moisture supply. 
 
 Green manures of peas and vetch maintain the yields of wheat at 
 a high level, but the results would indicate that during dry years too 
 much can be added for the best results. The optimum amount may 
 be around five tons per acre. 
 
 LITERATURE CITED 
 
 i Holmes, L. C, and Nelson, J. W. 
 
 1913. Reconnaissance soil survey of the Sacramento Valley area, California. 
 U. S. Dept. Agr. Advance Sheets Field Operations Bur. Soils. 
 91-93. 
 
 2 Brezeale, J. F. 
 
 1924. The injurious after effects of sorghum. Jour. Am. Soc. Agron. 16: 
 
 689-700. 
 
 3 Hawkins, R. S. 
 
 1925. The deleterious effect of sorghum on the soil and on the succeeding 
 
 crops. Jour. Am. Soc. Agron. 17: 91. 
 
 * Schreiner, Oswald, and Reed, H. S. 
 
 1909. The isolation of harmful organic substances from soils. U. S. Dept. 
 Agr. Bur. Soils Bull. 53: 51-53. 
 
 5 Madson, B. A. 
 
 1916. A comparison of annual cropping, biennial cropping and green 
 manures on the yield of wheat. California Agr. Expt. Sta. Bull. 
 270: 1-14. 
 e Klein, M. A. 
 
 1915. Studies in the drying of soils. Jour. Am. Soc. Agron. 7: 72-75. 
 7 Kelly, W. P., and Mc. George, W. 
 
 1913. The effect of heat on Hawaiian soils. Hawaii Agr. Expt. Sta. Bull. 
 30: 5-7. 
 
 s Sievers, F. J., and Holtz, H. F. 
 
 1922. The silt loam soils of Eastern Washington and their management. 
 Washington Agr. Expt. Sta. Bull. 166: 56:59. 
 s Sievers, F. J., and Holtz, H. F. 
 
 1922. The silt loam soils of eastern Washington and their management. 
 Washington Agr. Expt. Sta. Bull. No. 166. 
 
36 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 ACKNOWLEDGMENTS 
 
 The experiments upon which the data and discussions of this bul- 
 letin are based, and during the period under discussion, have been 
 under the immediate charge of Professors Madson and Hendry and 
 Mr. Conrad ; and it is to them that credit is due for carrying out the 
 details of the experiments. In working out the plan and method of 
 procedure valuable assistance was rendered by Professors Madson 
 and Hendry. The photographs were taken by Professor Hendry. 
 
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