UNIVERSITY OF CALIFORNIA PUBLICATIONS 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 A COMPARISON OF ANNUAL CROPPING, 
 
 BIENNIAL CROPPING, AND GREEN 
 
 MANURES ON THE YIELD 
 
 OF WHEAT 
 
 BY 
 B. A. MADSON 
 
 BULLETIN No. 270 
 
 August, 1916 
 
 UNIVERSITY OF CALIFORNIA PRESS 
 
 BERKELEY 
 
 1916 
 
Benjamin Ide Wheeler, President of the University. 
 EXPEEIMENT STATION STAFF 
 
 HEADS OF DIVISIONS 
 
 Thomas Forsyth Hunt, Director. 
 Edward J. Wickson, Horticulture (Emeritus). 
 
 Herbert J. Webber, Director Citrus Experiment Station; Plant Breeding. 
 Hubert E. Van Norman, Vice-Director; Dairy Management. 
 William A. Setchell, Botany. 
 Myer E. Jaffa, Nutrition. 
 
 Eobert H. Loughridge, Soil Chemistry and Physics (Emeritus). 
 Charles W. Woodworth, Entomology. 
 Ealph E. Smith. Plant Pathology. 
 J. Eliot Coit. Citriculture. 
 John W. Gilmore, Agronomy. 
 Charles F. Shaw, Soil Technology. 
 
 John W. Gregg, Landscape Gardening and Floriculture. 
 Frederic T. Bioletti, Viticulture and Enology. 
 Warren T. Clarke, Agricultural Extension. 
 John S. Burd, Agricultural Chemistry. 
 Charles B. Lipman, Soil Chemistry and Bacteriology. 
 Clarence M. Haring, Veterinary Science and Bacteriology. 
 Ernest B. Babcock, Genetics. 
 Gordon H. True, Animal Husbandry. 
 James T. Barrett, Plant Pathology. 
 Fritz W. Woll, Animal Nutrition. 
 A. V. Stubenrauch, Pomology. 
 Walter Mulford, Forestry. 
 W. P. Kelley, Agricultural Chemistry. 
 Elwood Mead, Eural Institutions. 
 H. J. Quayle, Entomology. 
 J. B. Davidson, Agricultural Engineering. 
 H. S. Eeed, Plant Physiology. 
 D. T. Mason, Forestry. 
 
 William G. Hummel, Agricultural Education. 
 John E. Dougherty, Poultry Husbandry. 
 S. S. Eogers, Olericulture. 
 *Frank Adams, Irrigation Investigations. 
 David N. Morgan, Assistant to the Director. 
 Mrs. D. L. Bunnell, Librarian. 
 
 division of agronomy 
 
 J. W. Gilmore W. E. Packard 
 
 P. B. Kennedy G. W. Hendry 
 
 E. L. Adams J. A. Denny 
 
 B. A. Madson L. K. Marshall 
 
 * In co-operation Office of Public Eoads and Eural Engineering, U. S. D. A. 
 
A COMPARISON OF ANNUAL CROPPING, BIENNIAL 
 
 CROPPING, AND GREEN MANURES ON THE 
 
 YIELD OF WHEAT* 
 
 By B. A. MADSON 
 
 It has been observed that the productive power of the arid and 
 semi-arid soil of the west is more often limited because of an insuffi- 
 cient supply of moisture or because of improper physical and biological 
 conditions than because of a lack of certain fertilizing elements. This 
 is especially true where dry farming is practiced, in which case the 
 cereals constitute the principal crop grown. In the more humid sec- 
 tions, where water is usually abundant, corresponding defects in 
 physical and biological conditions may generally be corrected by the 
 use of systems of crop rotation. Under arid conditions the develop- 
 ment of a comprehensive system of crop rotation is rendered somewhat 
 difficult because of the limited number of crops adapted to these 
 conditions. Some modification of this character, however, is necessary 
 in the system of grain farming in California, as evidenced by the fact 
 that wheat production in the state has decreased more than 80 per 
 cent during the past ten years, a condition which can be attributed 
 largely to the deleterious effects of continuous one-sided cropping. 
 
 To overcome this deficiency of crops, a year of fallow and addition 
 of organic matter to the soil by the use of cover crops have been ad- 
 vocated. The year of fallow is recommended primarily as a moisture- 
 conserving measure, to make possible an adequate supply of moisture 
 in the soil for a maximum crop at all times. The use of the cover 
 crop, on the other hand, is recommended as a means of maintaining 
 the supply of humus which is lost so rapidly by oxidation under con- 
 tinuous cultivation and cropping. Humus appears to be necessary to 
 keep the soil in good tilth, to stimulate bacterial activity and to main- 
 tain its maximum moisture-holding capacity. One or both of these 
 practices, together with proper cultivation, have been found under 
 many conditions not only to aid materially in maintaining the pro- 
 ductivity of dry-farm soils but in many cases to be absolutely essential 
 
 * The experiment discussed in this publication was outlined by and executed 
 under the direction of Dr. G. W. Shaw, in charge of the Agronomy work until 
 1913. The field work was conducted under the supervision of Mr. A. J. Gaumnitz, 
 in charge of the experimental work at Davis during the same period of time. 
 
4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 to successful crop production. On the sandy soils near Ceres, in the 
 San Joaquin Valley, Shaw 1 found that by the use of the summer fallow 
 alone the yield of wheat could be more than doubled, while by the use 
 of green manure crops, combined with the summer fallow, the yield 
 was increased from 230 to 340 per cent, depending upon the amount 
 of organic matter added. Necessarily, however, the effect of these 
 treatments will vary not only with the total precipitation and its 
 seasonal distribution but more particularly with the type and char- 
 acter of the soil. 
 
 In order to secure some information as to the value of these treat- 
 ments under Sacramento Valley conditions, a biennial cropping ex- 
 periment was begun in 1907 on the University Farm at Davis, a portion 
 of which was previously discussed by Shaw in the bulletin above cited. 
 This project was designed to show the effects of fallow and of various 
 green manure crops in combination with the fallow, as compared with 
 continuous cropping on the yield of wheat. The results obtained in 
 this investigation since the publication of Professor Shaw's bulletin 
 are presented in this bulletin, with such discussions as these may 
 suggest. 
 
 Climate. — The climate at Davis is usually classed as semi-arid. 
 The normal precipitation is approximately 17.32 inches, the greater 
 portion of which falls during the winter months, while the months of 
 June to September are generally absolutely rainless. There is, how- 
 ever, a very wide variation in the seasonal precipitation, as indicated 
 by the subjoined table. This fact and the frequent occurrence of hot 
 north winds during the spring and summer months render moisture- 
 conserving measures imperative. 
 
 TABLE I 
 
 Eainfall at Davis 
 
 Seasons, 1907-08 1908-09 1909-10 1910-11 1911-12 1912-13 1913-14 
 Inches 13.16 22.48 11.35 23.18 9.41 8.84 28.70 
 
 Soil. — The soil type upon which this experiment was undertaken 
 is technically known as Davis silt loam. The surface soil is a rather 
 heavy silt loam varying in depth from four to five feet. It is, how- 
 ever, in fair physical condition, is relatively easy to work, and has a 
 maximum water-holding capacity of approximately 30 per cent, ac- 
 cording to field determinations. This surface soil is underlaid by a 
 layer of sand six to eight inches in thickness and this in turn under- 
 laid by a heavier clay loam. 
 
 i Calif. Bulletin No. 211, p. 267. 
 
EFFECT OF CROPPING SYSTEMS ON WHEAT O 
 
 The project consisted of fourteen one-tenth acre plats quite uni- 
 form in character and included five trials with various green manure 
 crops as compared with bare fallow and with continuous cropping. 
 Each test was made in duplicate, so that of the biennial cropped plats 
 one plat out of each series was cropped to wheat every year. During 
 the summer of 1907 all plats were fallowed in order to disintegrate 
 the plow sole and put the soil in as good physical condition as possible. 
 This was deemed advisable inasmuch as the field had been cropped to 
 grain for a number of years prior to its being used for experimental 
 purposes. 
 
 Treatment of Plats. — During the subsequent year all plats received 
 the same cultural treatment as far as conditions would permit. Plow- 
 ing was done in the late fall or early winter, as soon as the rain had 
 sufficiently moistened the ground. The surface was then worked down 
 to a good seed bed and the wheat and the green manure crop seeded 
 with a drill. The fallow plats were allowed to lie in the rough con- 
 dition until spring, when the weed growth was destroyed and the 
 surface worked down to an even mulch; this mulch was maintained 
 throughout the summer by an occasional cultivation with either a 
 harrow or a disc. On the green manure plats the cover crop was 
 turned under in the spring, after which the surface was worked down 
 to a mulch; this mulch was maintained throughout the summer in 
 exactly the same manner as on the fallow plats, with one exception. 
 Plats Nos. 38 and 39, upon which horse beans were grown as a cover 
 crop, were plowed in the spring, worked down and planted to Kaffir 
 corn during a part of the experiment. 
 
 When the wheat crop was removed from the plats in the summer 
 the stubble was immediately disced, the mulch thus formed aiding 
 materially in checking the loss of moisture from the soil by direct 
 evaporation. The treatment of the various plats and the yields ob- 
 tained are given in Table II. 
 
 In comparing the data presented in Table II it will be observed 
 that the average yield of the two continuously cropped plats, Nos. 30 
 and 37, namely, 21.75 bushels per acre, is slightly higher than the 
 average of any of the biennial cropped plats. Of the biennial cropped 
 plats, Nos. 33 and 34, on which horse beans and fenugreek were grown 
 as a cover crop, gave the highest returns. In all fairness, however, 
 it must be said that the horse beans failed to produce much growth, 
 so that their effect as a green manure crop was practically nothing. 
 The fenugreek, however, made a fair growth, but had occasion to 
 influence but one of the six wheat crops. In reality, therefore, the 
 treatment of these plats prior to the use of the fenugreek was practi- 
 
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EFFECT OF CROPPING SYSTEMS ON WHEAT 7 
 
 cally the same as on the bare fallow. On plats Nos. 38 and 39 the 
 horse beans were likewise a failure. 
 
 Perhaps the most striking fact revealed by the data in Table II 
 is the rapid decrease in yield of the continuously cropped plats during 
 the progress of the experiment. In the case of the biennial cropped 
 plats there appears to be no consistent change in productivity^ with 
 the exception of plats Nos. 40 and 41, upon which rye and vetch were 
 employed as a cover crop. In fact, it is evident that had the experi- 
 ment continued a few years longer all the biennial cropped plats, with 
 the possible exception noted, would have surpassed in yield the con- 
 tinuously cropped plats. 
 
 It must also be remembered that all the plats were fallowed during 
 the summer of 1907, prior to the beginning of the experiment, so that 
 the condition on plats Nos. 30 and 37, for the first season at least, was 
 identical with that of the biennially cropped plats. The results clearly 
 indicate that the year of fallow very markedly affected the growth 
 on the continuously cropped plats for at least two seasons. This is 
 especially significant considering the short duration of the experiment. 
 Furthermore, one plat out of each series of the biennial cropped plats 
 was not cropped for the two seasons, 1906-07 and 1907-08, a condition 
 which might tend to vitiate the results somewhat, though the effect, 
 
 
 
 
 TABLE III 
 
 
 
 
 Eesults of the 
 
 Last Four Years of the Biennial Cropping Experiment 
 
 Plat 
 
 No. 
 
 1909-10 
 
 1910-11 
 
 1911-12 
 
 1912-13 
 
 Total 
 
 4 yr. 
 Yield per 
 acre, bu. 
 
 Av. annual Grand 
 yield per average, 
 acre, bu. bu. 
 
 30 Ck. 
 
 21.58 
 
 10.33 
 
 10.83 
 
 3.17 
 
 45.91 
 
 11.48 
 
 11.48 
 
 31 
 
 28.00 
 
 Fallow 
 
 44.83 
 
 Fallow 
 
 72.83 
 
 18.21 
 
 ► 17.67 
 
 32 
 
 Fallow 
 
 24.75 
 
 Fallow 
 
 43.83 
 
 68.58 
 
 17.14 
 
 33 
 
 34 
 
 Horse beans 
 28.91 
 
 34.83 
 Horse beans 
 
 Fenugreek 
 41.33 
 
 40.67 
 Fenugreek 
 
 75.50 
 70.56 
 
 18.87 
 17.56 
 
 1 18.21 
 
 35 
 
 Field peas 
 
 34.33 
 
 Field peas 
 
 31.42 
 
 65.75 
 
 16.44 
 
 I 16.71 
 
 36 
 
 29.25 
 
 Field peas 
 
 37.66 
 
 Field peas 
 
 67.98 
 
 16.98 
 
 37 Ck 
 
 22.41 
 
 11.00 
 
 6.66 
 
 5.08 
 
 43.18 
 
 11.29 
 
 11.29 
 
 38 
 
 Horse beans 
 
 j 
 
 
 
 
 ' 
 
 
 
 Kafir 
 
 28.58 
 
 Horse beans 
 
 28.58 
 
 57.16 
 
 14.29 
 
 > 16.83 
 
 39 
 
 31.83 
 
 Horse beans 
 
 
 
 
 
 
 
 Kafir 
 
 45.66 
 
 Horse beans 
 
 77.49 
 
 19.37 
 
 
 40 
 
 Rye & vetch 
 
 29.00 
 
 Rye & vetch 
 
 9.07 
 
 38.17 
 
 9.54 
 
 > 10.34 
 
 41 
 
 32.08 
 
 Rye & vetch 
 
 12.50 
 
 Rye & vetch 
 
 44.58 
 
 11.14 
 
 42 
 
 Burr clover 
 
 26.16 
 
 Burr clover 
 
 23.17 
 
 49.83 
 
 12.46 
 
 1 
 
 43 
 
 41.41 
 
 Burr clover 
 
 39.50 
 
 Burr clover 
 
 79.91 
 
 19.98 
 
 { 16.22 
 
8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 if any, is less apparent. Again, on the green manure plats the treat- 
 ment in some cases did not become effective until the third year, so 
 that only the last four years show a true comparison of the various 
 treatments. On the whole, therefore, it would probably be better to 
 regard the first two seasons ' results as preliminary and to include only 
 the last four years 7 results in comparing the merits of the various 
 treatments. The yields considered on this basis are given in Table III. 
 
 The results of the last four years have been decidedly in favor of 
 biennial cropping, with the exception of plats Nos. 40 and 41, on which 
 rye and vetch were employed as a cover crop. The difference in yield, 
 however, between the continuously cropped plats and the biennially 
 cropped plats is too great to be attributed to any other factor than the 
 effect of the treatment given. Unfortunately no determinations were 
 made on the green manure plats to ascertain whether or not there 
 was any relation between the total weight of organic matter added to 
 the soil and the yield produced. Attention must be called to the fact, 
 however, that the green manure plats, Nos. 40 and 41, which gave the 
 lowest average yield, not only for the last four years but throughout 
 the experiment were the only ones upon which a cereal was employed 
 as a cover crop. It is quite probable that the total organic matter 
 added was greater when rye was used as a part of the cover crop than 
 when a legume alone was used. This would seem to indicate either 
 that the growth of a larger cover crop tended to reduce correspond- 
 ingly the water content of the soil, thereby affecting the yield of the 
 following crop, or that the cereal (rye) exerted some influence on the 
 soil which affected the growth of the succeeding crop unfavorably. 
 On the whole, in comparing the effect of the various cover crops with 
 bare fallow the results would indicate that the cover crop was harmful 
 rather than beneficial. However, plats Nos. 33 and 34, upon which 
 horse beans and fenugreek were grown as cover crops, the former being 
 almost a total failure, gave a slightly higher yield than the bare fallow 
 plats. Even plats Nos. 38 and 39, upon which the cover crop of horse 
 beans was followed by Kafir corn during the first four years, gave as 
 large a yield of wheat throughout the experiment as did the field peas 
 and bur clover plats. "While the Kafir did not mature in most cases, 
 the yield of forage varied from four to eight tons per acre, the growth 
 of which must have caused a considerably greater draft upon the 
 moisture supply of the soil than did the cover crops. Furthermore, 
 the Kafir and other sorghums are generally considered to be "hard" 
 on the soil, greatly reducing the yield of the following cereal crops. 
 
 The rapid and consistent decrease in yield of both the continuously 
 cropped plats is striking. At first it would appear that this decrease 
 
EFFECT OF CROPPING SYSTEMS ON WHEAT 9 
 
 might be accounted for by an insufficient supply of moisture. For 
 the last two crops at least this is doubtless true, as the rainfall was 
 very low, the combined precipitation for the two seasons being only 
 18.25 inches. On the whole, however, there seems to be no close cor- 
 relation between the average yield of the continuously cropped plats 
 
 YEAE.S 
 W-08 1908-09 1909-10 1910-11 
 
 FIG. 1_ CURVES SHOWING THE fcELATIOM 5ETWEE/1 
 THE RAfMFALL AMD THE YIELDS Oh THE 
 PLATS CROPPED TO WHEAT CO/NTIMUOU5LY 
 
 and the seasonal precipitation, as is shown by figure 1, the yield de- 
 creasing continually while the precipitation was decidedly variable. 
 It is further worthy of note that in 1913, the second of two dry years, 
 the yield on plat No. 32, which was fallowed the previous season, was 
 43.83 bushels of wheat per acre, while plat No. 33, to which a cover 
 
10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 crop of fenugreek had been added the previous season, yielded 40.67 
 bushels of wheat per acre. The results obtained the succeeding season 
 (1914) further strengthened the belief that water alone was not the 
 limiting factor. While the experiment was technically closed at the 
 end of the 1913 season, all the plats, with the exception of No. 32, were 
 cropped to some cereal the next year, the results of which are given in 
 Table IV. Plats Nos. 30 and 31 were both planted to the same variety 
 of wheat. It will be noted that the yield of plat No. 30, which has been 
 cropped to wheat continuously, was only 11.17 bushels per acre in spite 
 of the fact that the seasonal precipitation was 28.70 inches. 
 
 TABLE IV 
 
 Yield of Cereals on the Plats of the Biennial Cropping Experiment, 
 
 Season of 1914 
 
 Yield 
 
 Plat 
 No. 
 
 Treatment, 1913 
 
 Crop grown, 1914 
 
 per acre, 
 bu. 
 
 30 
 
 Wheat 
 
 Bobs Wheat 
 
 11.17 
 
 31 
 
 Fallow 
 
 Bobs Wheat 
 
 25.42 
 
 32 
 
 Wheat 
 
 Fallow 
 
 
 33 
 
 Wheat 
 
 Beldi Barley 190 
 
 58.20 
 
 34 
 
 Fenugreek 
 
 Beldi Barley 190 
 
 65.20 
 
 35 
 
 Wheat 
 
 Imp. Beldi Bar. 1601 
 
 53.80 
 
 36 
 
 Field Peas 
 
 Chevalier Bar. 5865 
 
 65.60 
 
 37 
 
 Wheat 
 
 Chevalier Bar. 5865 
 
 29.40 
 
 38 
 
 Wheat 
 
 Chevalier Bar. 5865 
 
 52.80 
 
 39 
 
 Horse Beans 
 
 Burt Oats 293 
 
 40.00 
 
 40 
 
 Wheat 
 
 Burt Oats 293 
 
 44.69 
 
 41 
 
 Eye and Vetch 
 
 Burt Oats 293 
 
 47.66 
 
 42 
 
 Wheat 
 
 white Aust. Wh. 12113 
 
 28.16 
 
 43 
 
 Burr Clover 
 
 White Aust. Wh. 12113 
 
 24.37 
 
 Plat No. 31, which had been fallowed the previous season, yielded 
 25.42 bushels of wheat per acre. The other plats in the experiments 
 were cropped to various cereals, so that an exact comparison cannot 
 be made. The results on the other continuously cropped plats and the 
 two adjoining plats, Nos. 36 and 38, which were seeded to Chevalier 
 barley, are also of interest. Plat No. 37, which had been continuously 
 cropped to wheat, yielded only 39.4 bushels of barley per acre. Plat 
 No. 38, cropped to wheat the previous season, which was preceded by 
 a cover crop of horse beans, yielded 52.8 bushels per acre, and plat 
 No. 36, upon which had been grown a cover crop of field peas in 1913, 
 yielded 65.6 bushels of barley per acre. It does not seem probable 
 that in either instance the difference in yield could be attributed to a 
 
EFFECT OF CROPPING SYSTEMS ON WHEAT 11 
 
 difference in moisture supply, as all plats were abundantly supplied 
 with moisture throughout the growing season. In fact, the heavy 
 spring rains kept the soil nearly saturated until the crop was fully 
 headed. 
 
 From the data obtained it would certainly appear that some factor 
 other than the lack of moisture was responsible for the decline in yield 
 under continuous cropping. Considering the results obtained on the 
 plats in which rye was used as a part of the cover crop, it would appear 
 that the cereal exerted some unfavorable influence upon the succeeding 
 grain crop. Whether this condition is due to some toxic substance 
 left in the soil, to the development of fungi, as maintained by Bolley, 
 or to some effect of the fibrous root system on the physical, chemical 
 or biological condition of the soil is still undetermined. The results, 
 however, would suggest that some effect of this character was the real 
 limiting factor. The fact that most of the plats upon which a legum- 
 inous cover crop had been grown gave a lower yield than the bare 
 fallow plats might be explained on the basis of moisture, the cover 
 crop using a portion of the moisture which might otherwise have 
 served to supply the wheat crop. This explanation seems plausible, 
 since the difference in yield during seasons of high rainfall was very 
 slight, the greatest difference occurring during the drier seasons. It 
 would seem, therefore, that while the addition of organic matter may 
 be of value, its beneficial effect is not sufficient to compensate for the 
 water used in its growth, nor does the soil appear to be seriously 
 lacking in any constituent which the cover crop might add. 
 
 From an economic point of view, the bare fallow unquestionably 
 produced the greatest net returns per acre. The cost of maintaining 
 a good summer fallow, including plowing and the necessary cultiva- 
 tion, is usually slightly less than the cost of producing a crop of wheat. 
 The annual increase in yield of 6.29 bushels per acre, which was ob- 
 tained from the fallow plats, is therefore largely clear gain. In other 
 words, to be economically a success, the fallow plats would have to 
 produce only twice as large yields every alternate year as was pro- 
 duced by the continuous cropped plats annually, whereas the actual 
 average production of the fallow plats every two years was 35.35 
 bushels per acre, as compared with 11.38 bushels per acre, which was 
 the annual production of the plats cropped continuously. 
 
 In the case of the green manure plats, while the yields were consid- 
 erably larger than those obtained under continuous cropping, they 
 were, on the whole, somewhat lower than the yield obtained on the 
 fallow plats. Furthermore, the cost of producing and handling the 
 green manure crop is at least 50 per cent greater than the cost of 
 
12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 maintaining the fallow, including as it does the cost of putting in the 
 crops, the cost of the seed and additional plowing in the spring, be- 
 sides the same summer cultivation that is given to the fallow. On the 
 whole, therefore, the actual economic returns attending the use of a 
 green manure crop is but slightly greater than those obtained under a 
 system of continuous cropping. 
 
 EFFECTS ON THE SOIL 
 
 It is usually maintained that the addition of organic matter to the 
 soil by the use of green manure crops will aid in increasing its humus 
 content. In humid sections this is true, and experiments have shown 
 that the plowing in of a cover crop has increased the humus content 
 in some cases 1 per cent. Under arid conditions, however, the increase 
 in humus content by the use of a cover crop has been found in most 
 instances to be very slight. As has been pointed out by Scofield, 
 under arid conditions the low moisture content of the surface soil, 
 together with high temperature and high degree of aeration, tends to 
 stimulate direct oxidation, rather than humification, so that in a short 
 time the larger proportion of the organic matter may be lost without 
 adding materially to the humus content of the soil. Hilgard also 
 states that "in the absence of a sufficient degree of moisture to co- 
 operate with the agencies of humification the final results in the soil 
 is practically the same as in the 'fire fanging' of dung. The organic 
 matter is almost wholly destroyed by direct oxidation with or without 
 the aid of minute organisms, leaving essentially only the ash behind to 
 be reincorporated with the soil." 
 
 Unfortunately no systematic studies were made to determine the 
 effect of the various treatments upon the soil in the experiments at 
 Davis. A few tests were made in the spring of 1913 by Mr. S. E. 
 Ooodall, at that time a senior student in the department of agriculture, 
 with a view to determining the effect of the various treatments on the 
 humus content of the soil, as well as on the nitrogen content of the 
 humus. The results thus obtained are given in Table V. 
 
 The soil samples upon which these determinations were made were 
 taken in January, about ten months after the last cover crop had been 
 plowed in. The results indicate that no increase in humus content 
 occurred following the application of the green manure crop. While 
 a variation of .28 per cent of humus was obtained in the first foot, both 
 extremes occurred on plats to which the last cover crop had been added 
 nearly two years previous. The difference in the average humus con- 
 tent for the various treatments is too slight to be significant. The 
 fact that the humus content of the first foot is slightly less on the 
 
EFFECT OF CROPPING SYSTEMS ON WHEAT 13 
 
 1913 
 
 
 
 
 TABLE V 
 
 
 
 Humus and 
 
 Humu 
 
 s Nitrogen Content of 
 
 Soils, January, 
 
 Plat 
 
 No. 
 
 Per 
 
 cent of H 
 
 A 
 
 umus 
 
 Per cent Nitrogen in Humus 
 
 A 
 
 1st ft. 
 
 2nd ft. 
 
 3rd ft. 
 
 1st ft. 
 
 2nd ft. 
 
 3rd ft. 
 
 30 
 
 1.12 
 
 .86 
 
 .62 
 
 15.0 
 
 11.2 
 
 19.4 
 
 31 
 
 1.02 
 
 .90 
 
 .54 
 
 8.5 
 
 8.5 
 
 13.5 
 
 32 
 
 • .98 
 
 .90 
 
 .54 
 
 10.6 
 
 10.6 
 
 14.7 
 
 33 
 
 1.10 
 
 .90 
 
 .54 
 
 9.5 
 
 7.5 
 
 10.4 
 
 34 
 
 .94 
 
 .82 
 
 .52 
 
 8.5 
 
 9.8 
 
 10.4 
 
 35 
 
 1.06 
 
 .88 
 
 .72 
 
 9.4 
 
 11.2 
 
 12.2 
 
 36 
 
 .94 
 
 .80 
 
 .60 
 
 10.1 
 
 9.8 
 
 10.7 
 
 37 
 
 1.06 
 
 .80 
 
 .64 
 
 8.0 
 
 9.9 
 
 11.7 
 
 38 
 
 1.14 
 
 .94 
 
 .70 
 
 10.3 
 
 10.8 
 
 13.4 
 
 39 
 
 1.18 
 
 1.02 
 
 .62 
 
 9.9 
 
 10.5 
 
 16.4 
 
 40 
 
 1.18 
 
 .90 
 
 .62 
 
 10.2 
 
 8.9 
 
 9.6 
 
 41 
 
 1.10 
 
 .96 
 
 .66 
 
 9.5 
 
 7.3 
 
 10.1 
 
 42 
 
 1.14 
 
 .70 
 
 .54 
 
 8.8 
 
 10.0 
 
 12.2 
 
 43 
 
 1.22 
 
 .74 
 
 .56 
 
 8.4 
 
 8.9 
 
 8.6 
 
 Av. of continuously- 
 cropped plats, 
 Av. of fallow plats, 
 
 1.09 
 1.00 
 
 .83 
 .90 
 
 .63 
 .54 
 
 11.5 
 9.5 
 
 10.5 
 9.5 
 
 15.5 
 14.1 
 
 Av. of green manure 
 plats, 
 
 1.10 
 
 .86 
 
 .61 
 
 9.4 
 
 9.5 
 
 11.4 
 
 fallow plats than on the other plats corroborates the usual observations 
 as to the effect of summer cultivation on the oxidation of the humus. 
 
 In the nitrogen content of the humus, it is not clear just why the 
 humus in the soil of the continuously cropped plats should be higher 
 in nitrogen than that of the other plats. The most probable expla- 
 nation would be that less of the organic matter left in the soil as wheat 
 roots was lost by direct oxidation than in the case of that applied as 
 cover crops. In the latter case, in the process of oxidation it is quite 
 likely that a large portion of the nitrogen was lost as well. Another 
 possibility that suggests itself is that a more rapid rate of nitrification 
 occurred when the green material had been added than on land that 
 had been cropped, so that a larger portion of the nitrogen was changed 
 to the nitrate form. In the absence of nitrate determinations, how- 
 ever, this can only be assumed. 
 
 From the results obtained in this experiment it would appear that 
 soils such as those at Davis contain sufficient organic matter to meet 
 the demands of a normal crop. While the addition of organic matter 
 by the use of a cover crop may tend to stimulate bacterial activity and 
 the liberation of plant food, the benefit thus derived is not sufficient 
 to counterbalance the draught made upon the soil moisture by the 
 growth of the crop. Furthermore, because of the rapid oxidation 
 
14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 which takes place in the soil the cover crops did not increase the humus 
 content, but were entirely lost before the next wheat crop was grown. 
 
 On the other hand, the bare fallow greatly stimulated the crop- 
 producing power of the soil, though it is evident that if long continued 
 the humus content will be seriously depleted. It will doubtless, there- 
 fore, eventually be necessary to add organic matter in some form if 
 the crop-producing power of these soils is to be permanently main- 
 tained. It would also appear that there was considerable difference in 
 the comparative value of the various crops for green manure purposes, 
 though the exact reason for this is not clear. 
 
 Again, while the highest economic returns were obtained from the 
 bare fallow, it does not hold that biennial cropping as practiced in 
 this experiment is necessarily productive of the greatest returns. It 
 is quite likely that from an economic point of view the use of the fallow 
 every third or even every fourth year would give larger total returns 
 over a long period of time than biennial cropping. As an integral 
 part of a dry-farm rotation, however, its value can not be questioned. 
 
 CONCLUSIONS 
 
 1. The marked decline in wheat production in California has been 
 caused by the deleterious effects of continuous one-sided cropping, 
 together with inadequate cultivation. 
 
 2. Under a system of continuous cropping to wheat the yield de- 
 creases very rapidly to a point below profitable production. 
 
 3. The year of fallow is one of the most effective means of retaining 
 the productive power of semi-arid soils. 
 
 4. The use of green manure crops increases the productivity of the 
 soil, but not to the same extent as does the bare fallow. 
 
 5. There appears to be a decided difference in the effect of the 
 various cover crops. Apparently a cereal should not be used as a cover 
 crop in a cereal rotation on soils such as those on the University 
 Farm at Davis. 
 
 6. The organic matter added to the soil by the cover crop appears 
 to be practically lost by oxidation the following summer. 
 
 7. The frequent use of the fallow will probably hasten the depletion 
 of the humus content of the soil, even though it greatly increases its 
 productive power. 
 
 8. The use of cover crops, even if they be legumes, does not seem 
 to increase the content of humus or of humus nitrogen in the soil to 
 any appreciable extent; the favorable effects of cover crops that are 
 frequently observed must, therefore, be attributed to other causes than 
 an increase of the humus in the soil. 
 
STATION PUBLICATIONS AVAILABLE FOE FREE DISTRIBUTION 
 
 REPORTS 
 
 1897. Resistant Vines, their Selection, Adaptation, and Grafting. Appendix to Viticultural 
 Report for 1896. 
 
 1902. Report of the Agricultural Experiment Station for 1898-1901. 
 
 1903. Report of the Agricultural Experiment Station for 1901-03. 
 
 1904. Twenty-second Report of the Agricultural Experiment Station for 1903-04. 
 
 1914. Report of the College of Agriculture and the Agricultural Experiment Station, July, 
 
 1913-June, 1914. 
 
 1915. Report of the College of Agriculture and the Agricultural Experiment Station, July, 
 
 1914-June, 1915. 
 
 No. 
 168. 
 
 169. 
 
 178. 
 184. 
 
 185. 
 
 195. 
 207. 
 208. 
 212. 
 213. 
 216. 
 
 225. 
 227. 
 230. 
 241. 
 242. 
 246. 
 248. 
 
 249. 
 250. 
 
 Observations on Some Vine Diseases 
 in Sonoma County. 
 
 Tolerance of the Sugar Beet for Alkali. 
 
 Mosquito Control. 
 
 Report of the Plant Pathologist to 
 July 1, 1906. 
 
 Report of Progress in Cereal Investi- 
 gations. 
 
 The California Grape Root-worm. 
 
 The Control of the Argentine Ant. 
 
 The Late Blight of Celery. 
 
 California White Wheats. 
 
 The Principles of Wine-making. 
 
 A Progress Report Upon Soil and Cli- 
 matic Factors Influencing the Com- 
 position of Wheat. 
 
 Tolerance of Eucalyptus for Alkali. 
 
 Grape Vinegar. 
 
 Enological Investigations. 
 
 Vine Pruning: in California, Part I. 
 
 Humus in California Soils. 
 
 Vine Pruning in California, Part II. 
 
 The Economic Value of Pacific Coast 
 Kelps. 
 
 Stock-Poisoning Plants of California. 
 
 The Loquat. 
 
 BULLETINS 
 No. 
 251. 
 
 252. 
 253. 
 
 254. 
 255. 
 256. 
 257. 
 261. 
 
 263. 
 265. 
 266. 
 
 267. 
 268. 
 269. 
 270. 
 
 271. 
 
 Utilization of the Nitrogen and Organic 
 Matter in Septic and Imhoff Tank 
 Sludges. 
 
 Deterioration of Lumber. 
 
 Irrigation and Soil Conditions in the 
 Sierra Nevada Foothills, California. 
 
 The Avocado in California. 
 
 The Citricola Scale. 
 
 Value of Barlev for Cows Fed Alfalfa. 
 
 New Dosage Tables. 
 
 Melaxuma of the Walnut, "Juglans 
 regia." 
 
 Citrus Diseases of Florida and Cuba 
 Compared with Those of California. 
 
 Size Grade for Ripe Olives. 
 
 Cottony Rot of Lemons in California. 
 
 A Spotting of Citrus Fruits Due to the 
 Action of Oil Liberated from the Rind. 
 
 Experiments with Stocks for Citrus. 
 
 Growing and Grafting Olive Seedlings. 
 
 Phenolic Insecticides and Fungicides. 
 
 A Comparison of Annual Cropping, 
 Biennial Cropping, and Green Ma- 
 nures on the Yield of Wheat. 
 
 Feeding Dairy Calves in California. 
 
 CIRCULARS 
 
 No. No. 
 
 65. The California Insecticide Law. 134. 
 
 69. The Extermination of Morning-Glory. 135. 
 
 70. Observations on the Status of Corn 136. 
 
 Growing in California. 13 7. 
 
 76. Hot Room Callusing. 138. 
 
 80. Boys' and Girls' Clubs. 139. 
 
 82. The Common Ground Squirrels of 
 
 California. 
 
 83. Potato Growing Clubs. 140. 
 
 106. Directions for Using Anti-Hog Cholera 
 
 Serum. 
 
 107. Spraying Walnut Trees for Blight and 141. 
 
 Aphis Control. 
 
 108. Grape Juice. 142. 
 
 109. Community or Local Extension Work 
 
 by the High School Agricultural De- 143. 
 partment. 
 
 113. Correspondence Courses in Agriculture. 144. 
 
 114. Increasing the Duty of Water. 145. 
 
 115. Grafting Vinifera Vineyards. 
 
 117. The Selection and Cost of a Small 146. 
 
 Pumping Plant. 
 
 118. The Countv Farm Bureau. 147. 
 121. Some Things the Prospective Settler 148. 
 
 Should Know. 149. 
 
 124. Alfalfa Silage for Fattening Steers. 150. 
 
 126. Spraying for the Grape Leaf Hopper. 151. 
 
 127. House Fumigation. 152. 
 
 128. Insecticide Formulas. 
 
 129. The Control of Citrus Insects. 153. 
 
 130. Cabbage Growing in California. 
 
 131. Spraying for Control of Walnut Aphis. 154. 
 
 132. When to Vaccinate against Hog 
 
 Cholera. 155. 
 13 3. County Farm Adviser. 
 
 Control of Raisin Insects. 
 
 Official Tests of Dairy Cows. 
 
 Melilotus Indica. 
 
 Wood Decay in Orchard Trees. 
 
 The Silo in California Agriculture. 
 
 The Generation of Hydrocyanic Acid 
 Gas in Fumigation by Portable Ma- 
 chines. 
 
 The Practical Application of Improved 
 Methods -of Fermentation in Califor- 
 nia Wineries during 1913 and 1914. 
 
 Standard Insecticides and Fungicides 
 versus Secret Preparations. 
 
 Practical and Inexpensive Poultry Ap 
 pliances. 
 
 Control of Grasshoppers in Imperial 
 Valley. 
 
 Oidium or Powdery Mildew of the Vine. 
 
 Suggestions to Poultrymen concerning 
 Chicken Pox. 
 
 Jellies and Marmalades from Citrus 
 Fruits. 
 
 Tomato Growing in California 
 
 "Lungworms" 
 
 Lawn Making in California. 
 
 Round Worms in Poultry. 
 
 Feeding and Management of Hogs. 
 
 Some Observations on the Bulk Hand- 
 ling of Grain in California. 
 
 Announcement of the California State 
 Dairy Cow Competition, 1916-18. 
 
 Irrigation Practice in Growing Small 
 Fruits in California. 
 
 Bovine Tuberculosis.