UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE PHOSPHATE OF SOUTHERN CALIFORNIA SOILS IN RELATION TO CITRUS FERTILIZATION H. D. CHAPMAN BULLETIN 571 APRIL, 1934 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/phosphateofsouth571chap The Phosphate of Southern California Soils in Relation to Citrus Fertilization 1 2 H. D. CHAPMAN 1 Field experimentation and general experience have proved conclu- sively that it is necessary to apply nitrogenous fertilizers to the majority of California soils in order to maintain citrns-frnit yields and tree vigor. On the other hand, none of the long-continued fertilizer experiments with citrus trees have given any evidence of need for phosphate fertil- izers. These experiments furnish substantial support for the belief that, at least in so far as citrus is concerned, southern California soils are well supplied with available phosphate. In spite of this evidence many citrus growers annually apply large quantities of phosphate or phosphorus-containing materials to their groves. The continuation of this practice is accounted for by the preva- lence of various ideas among orchardists. Some believe that the use of nitrogen alone produces fruit of poor quality and that it is necessary to balance the nitrogen with phosphate or potash or both. Still others are influenced by the idea that the amounts of those elements which are removed by the crop should be replaced by fertilizer application. Again others, in lieu of specific information regarding the phosphate of their particular soils, prefer to- apply phosphate, feeling that excessive amounts will do no harm whereas a deficiency might prove detrimental; in other words they believe that it is safer to apply phosphate than to omit it. A scientific system of fertilization is one designed to add to the soil only such elements as are actually deficient. As long as the supply of a given constituent in a soil suffices to meet the needs of the crop or crops being grown, it would seem unnecessary to add more to it, particularly in view of the fact that a part of the applied material may be lost by leaching or else be fixed by the soil in an unavailable form. The perennial question confronting every citrus grower then, is whether or not the supply of essential elements in his particular orchard is sufficient to meet the needs of the trees. In the absence of specific data on his soil, the grower should be guided by such information and experience as can be brought to bear on his particular case. i Eeceived for publication October 16, 1933. 2 Paper No. 292, University of California Graduate School of Tropical Agricul- ture and Citrus Experiment Station, Riverside, California. s Assistant Chemist in the Citrus Experiment Station. [3] 4 University of California Experiment Station Results recently secured in the course of chemical and pot-culture investigations have provided further evidence that, in so far as citrus is concerned, the majority of the soils of southern California are well supplied with available phosphate. For the benefit of those who are interested in citrus culture these results are here presented. The field experimental evidence which bears on this question is also reviewed briefly. CHEMICAL AND POT-CULTURE STUDIES A large number of soil samples representing most of the important soil types of southern California have been collected and analyzed for phosphate. Many of these were drawn from places where it was known definitely that no fertilizer had ever been applied. The data obtained from these samples afford a fairly reliable basis for judgment as to the phosphate conditions in the unfertilized soils of these various types. At the same time samples were taken from many citrus orchards, and a comparison of the data obtained from these with those of nearby un- fertilized soils has made it possible to draw reasonably reliable conclu- sions concerning the effects of previous orchard management on the phosphate content of the soil. Coincident with these chemical studies, pot cultures have been made in order to secure further information concerning the adequacy and availability of the phosphate of the various soil types. METHODS OF SAMPLING AND ANALYSIS Soil samples each composed of from 6 to 8 individual cores were taken from representative parts of typical citrus orchards in the various citrus districts and also from nearby unfertilized soil. In most cases the samples represent the first 6 inches, the second 6 inches, and the second foot of the soil, respectively. The samples were brought to the laboratory, air dried, and screened; water-soluble phosphate was determined in an extract of the soil made by shaking 100 grams of soil with 500 cc of C0 2 -free water. Acid-soluble phosphate was determined by digesting 2 grams of soil with 400 cc of 0.002 N sulfuric acid; this solvent buffered with potassium sulfate to pH 3.0. This is a minor modification of the Truog 4 method. Total phosphate was determined by the sodium carbonate fusion method. The blue colori- metric method was used for measuring the phosphate content of the va- rious extracts of the soil. 4 Truog, E. The determination of the readily available phosphorus of soils Jour Amer. Soc. Agron. 22:874-882. 1930. Bul. 571] Phosphate of Southern California Soils 5 phosphate in citrus orchards and adjacent unfertilized AREAS The analytical results are brought together in table 1. They show that without exception the orchard soils are higher, either in water-soluble phosphate or acid-soluble phosphate, or both, than the adjacent unfer- tilized soils; in many cases the difference is very striking. This is un- doubtedly due to the fact that citrus groves have been fertilized with phosphatic materials in amounts sufficient, not only to compensate for that removed by the crops of fruit and growth of the citrus trees, but also to augment the original phosphate content of the soil substantially. Since increases in phosphate over that of the adjacent unfertilized soil were found in every orchard irrespective of the age of the trees or the locality, it is safe to conclude that most citrus orchards of California now contain more phosphate than they did originally. Animal manures and various kinds of bulky organic materials, such as alfalfa hay and bean straw, are commonly applied to citrus orchards as fertilizers. The extensive use of these materials has arisen largely because of the recognized need for nitrogen and organic matter in citrus soils. However, these materials contain notable amounts of phosphorus and potassium, 5 and it is probable that no small part of the accumulated phosphate is to be accounted for by the extensive use of these materials. For example, animal manures or other bulky organic materials are the only fertilizers that have ever been applied to the areas from which samples 1, 9, 10, 40, and 48 were drawn. The results show that in every one of these soils there has been a substantial increase in phosphate. Many of the samples were drawn from orchards that had been fertilized with moderate to heavy applications of commercial phosphates and these also show marked gains. Since the data shown in table 1 indicate that the citrus orchards of various localities have made important gains in phosphate in conse- quence of previous fertilization, the question arises, is the amount now present in the soils adequate to meet the needs of citrus trees % The ade- quacy of the phosphate in a soil for a given crop is determined chiefly by the following factors : ( 1 ) The distribution of the phosphate in the soil, especially in relation to the rooting habits of the crop, (2) the sup- ply and availability of the phosphate present, and (3) the phosphate requirements of the plant in question. 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With but few exceptions, either the water-soluble data or acid-soluble data, or both, show evidence of significant penetration into the second six inches of soil; moreover, in many cases the movement of phosphate has extended into the second foot as well. In most citrus orchards absorbing roots are especially abundant in the zone immediately below the depth of cultivation. During winter months when cultural operations are largely discontinued, the writer has often observed active citrus roots within one or two inches of the surface. The shallow rooting habit of the citrus tree, on the one hand, together with the evidence as to phosphate penetration, on the other, justifies the be- lief that a portion at least of the phosphate which has accumulated in citrus soils is capable of being utilized. SUPPLY AND AVAILABILITY OF THE PHOSPHATE PRESENT The results of extended experiments with pot cultures of oats and alfalfa on representative unfertilized soils have shown that those which contain roughly from 100 to 150 parts per million of P0 4 , as measured by Truog's acid extraction method, are amply supplied. Except in soils which contain calcium carbonate this phosphate is usually readily available. Reference to table 1 (acid-soluble data) shows that of the 56 orchard soils sampled, only 3 contain less than 100 parts per million of P0 4 ; many contain amounts greatly in excess of this quantity. More- over, most of the contiguous unfertilized soils likewise contain phos- phate in amounts exceeding this quantity. However, the unfertilized soils from certain areas, notably Fallbrook, Escondido, Chula Vista, Imperial Valley, and certain of the Ramona soil types found in Orange and Los Angeles counties are somewhat low in phosphate. Other data in harmony with those of table 1, but not in- cluded in this bulletin, substantiate this general conclusion. In addition, as previously mentioned, pot cultures with oats and alfalfa have shown that the phosphate in calcareous soils, though readily soluble in dilute acid, is less available than in noncalcareous types. This latter observa- tion is in harmony with results obtained by workers elsewhere. The question arises, is the supply or availability of phosphate in the above-mentioned types too low to meet citrus needs ? The conditions of low supply and availability noted are based on the behavior of quick- growing plants. As already stated one of the important factors involved Bul. 571 Phosphate of Southern California Soils 13 in the question of phosphate adequacy is the phosphate requirement of the particular plant in question. It is well known that plants differ markedly in this respect. For example, many investigators have observed that buckwheat will make satisfactory growth in soil which contains a supply of phosphate entirely inadequate for another crop, such as tomatoes. The reasons for such differences are not fully understood. Length of growing season, character of the root systems, differences in the growth habits and metabolism of different plants are all involved. Although little is known about the phosphate requirements of citrus in relation to that of quick-growing plants, certain deductions can be made based (1) on the comparative composition and length of growing season of the two, and (2) on the results of certain phosphate fertilizer experiments. TABLE 2 Phosphate Bemoval by Various Crops* Crop Yield Phosphate, (P0 4 ) in pounds Wheat. .. . 30 bushels 28.2* 27.7* Oats 25.9* 30 bushels 24 1* 33.3* and new growth 28.8* Citrus fruit 22,500 pounds fresh fruit, 4,653 pounds (dry weight) tree 32.69f * The figures represent phosphate removal by the entire harvested crop. The data for the field crops are taken from: Lyon, T. L., and H. O. Buckman. The nature and properties of soils. The MacMillan Co., New York. p. 245. 1929. t The figures for citrus fruit were computed from data given in: Hume, H. Harold. The cultivation of citrus fruits. The MacMillan Co., New York, p. 287. 1926; and in: Barnette, R. M., etal. The mineral analysis of a nineteen year old Marsh seedless grapefruit tree. The Citrus Industry 12(3) :5, 6, 34. 1931. As shown in table 2, the amount of phosphate removed by an average crop of oranges plus that required for the production of the annual new growth of the tree, is not greatly different from that removed by the cereals, corn, clover, and potatoes. However, annual crops must secure their phosphate in a growing period of from 2 to 4 months, whereas the citrus tree probably absorbs phosphate continuously throughout the year. Therefore, it is probable that the average rate of phosphate ab- sorption by citrus is not nearly so rapid as with the annuals. This sug- gests that citrus trees may be able to satisfy their phosphate requirement from soils containing lower total supplies of available phosphate than is necessary for quick-growing plants. The results of certain pot-culture and field fertilizer trials, briefly described in the following sections, support this theory. 14 University of California Experiment Station Pot-Culture Studies. — Following the growth and harvest of alfalfa in a series of differentially fertilized soils, very young sweet-orange seed- lings were transplanted into the same pots and allowed to grow until they had attained a growth roughly corresponding to that made by the alfalfa. The relative responses of the alfalfa and citrus to phosphate >¥ NK NP N . s*-^ NPK Fig. 1. — Relative response of alfalfa and citrus seedlings to phosphate addi- tions. Soil No. 18053, Hanford snnd from Santa Ana. additions in these various soils is shown by the illustrations in figures 1, 2, and 3. Soils 18053 and 18064 are markedly deficient in phosphate for alfalfa, as is shown by the growth increases resulting from phosphate additions. These are calcareous types, and illustrate some of the most extreme cases of phosphate deficiency found in any of the soils studied. On the other hand, citrus seedlings growing in these same soils showed Bul. 571 Phosphate op Southern California Soils 15 no effect whatever from the addition of phosphate, indicating that for these plants the original soil supply is apparently ample. For purposes of comparison the results on another soil showing no evidence of phos- phate deficiency for alfalfa is also included (fig. 3). USI Fig. 2. — Relative response of alfalfa and citrus seedlings to phosphate addi- tions. Soil No. 18064, Yolo loam from Tustin. Field Trials. — Three of the field trials with citrus, the results of which are briefly reviewed later in this paper, were located on soils of low phosphate availability (table 5, Orange County Farm, Santa Ana; C. C. Chapman orchard, Fullerton; and Chula Vista Experiment). The complete absence of increases in yield from phosphate in these field 16 University of California Experiment Station trials again suggests that the citrus tree may be able to grow normally on soils which contain an inadequate supply of available phosphate for some quick-growing plants. NPK NPK NK NP ^*~- ^8^»* N Fig. 3. — Relative response of alfalfa and citrus seedlings to phosphate addi- tions. Soil No. 18058, Yolo clay loam from Santa Paula In general the aforementioned lines of evidence indicate that, as far as citrus is concerned, the unfertilized soils representative of citrus areas contain an adequate supply of available phosphate. It has been shown by the data of table 1, however, that most of the soils of commercial citrus orchards contain more phosphate now than the adjacent unfer- Bul. 571 Phosphate of Southern California Soils 17 tilized types. A portion at least of this accumulated phosphate is ac- cessible to citrus roots and additional evidence indicates that it is more soluble and inferentially more available than that originally present. For example, the data recorded in table 3 show that the acid solubility of the phosphate in orchard soils exceeds that of the contiguous unfertil- ized types. This difference in solubility is again shown by the increased water solubility of the phosphate of orchard soils as shown in table 1. TABLE 3 Comparative Solubility of Phosphate in Fertilized and Unfertilized Soils Laboratory No. Treatment Years fertilized Total P0 4 , parts per million in soil Acid-solu ble P0 4 Soil type Parts per million in soil Per cent of total phosphate Hanford sandy loam ( 15915 None 1,220 114.0 9.4 \ 15917 Fertilized 40 4,820 2,580 53 5 Hanford gravelly / 16107 None 1,500 172 11 5 sandy loam \ 16110 Fertilized 15 3,760 1,280 33 Hanford loam f 15601 None 2,250 338 15 \ 15633 Fertilized 22 3,590 966.0 26 9 Ramona loam / 15662 None 1,460 178 12.2 | 15658 Fertilized 30 2,350 880 37 5 Placentia loam f 15738 None 2,350 288 12.2 \ 15653 Fertilized 22 4,890 2,178 44 5 Placentia loam / 17400 None 1,920 196.0 10 2 \ 17403 Fertilized 35 2,400 475.0 19 8 Yolo clay adobe f 15714 None 1,630 560 33 3 \ 15718 Fertilized 18 2,090 654 31 3 Hanford loam f 18036 None 2,440 207 8 5 \ 15730 Fertilized 14 3,000.0 1,020 29.4 Hence, the results of chemical and pot-culture studies indicate that the present supply of phosphate in citrus orchards is ample to meet the needs of the citrus tree. The results of field trials briefly reviewed in the following section offer still further evidence in harmony with this con- clusion. PLOT EXPERIMENTS The average yields of citrus fruit that have been obtained from those plots of several different fertilizer experiments which bear on the phos- phate question are assembled in table 4. The important point brought out by these data is that added phosphate has uniformly failed to in- crease yields. On the other hand, marked effects have been produced by nitrogen. 18 University of California Experiment Station TABLE 4 Besults of Field Fertilizer Trials with Citrus Location Soil type Fruit Period represented by yield data and number of years averaged§ Plot Treatment Rubidoux Plots, Riverside Citrus Experiment Station, Field I, Riverside Chaffey Junior College, Ontario Sierra loam Ramona loam Hanford stony sandy loam Fontana Farms, Fontana Hanford gravelly sand Arlington Heights Placentia loam Navel and Valencia oranges, Eureka andLisbon lemons Navel oranges Navel oranges Navel oranges Navel oranges 1912-1932 (Navels, 21 year av. ; Valencias, 20-year av. Eurekas and Lisbons 15-year av 1927-1931 (5 years) 1915-1930 (15 years) 1918-1922 (5 years) 1916-1920 (5 years) 4C 3C 2C 4A 3A 2A 2A, 4A, 6 A 1C,3C, 5C, 7C 1A 2C IB 2D 4C 3A 4D 3B 14B 38B 41B 12B None None Dried blood Dried blood Dried blood, sodium ni- trate, potassium sulfate, superphosphate Covercrop Urea, covercrop Urea, treble superphos- phate, covercrop Urea, potassium sulfate, covercrop Urea, potassium sulfate, treble superphosphate, covercrop Ammonium sulfate Tankage (8-8) Complete (6-8-2) Ammonium sulfate, cover crop Tankage (8-8), covercrop... Complete (6-8-2) , covercrop None None Sodium nitrate Sodium nitrate, super- phosphate Sodium nitrate, gypsum ... Sodium nitrate, gypsum, superphosphate Sodium nitrate, potassium sulfate Sodium nitrate, potassium sulfate, superphosphate Sodium nitrate, potassium sulfate, gypsum Sodium nitrate, potassium sulfate, gypsum, super- phosphate Covercrop Covercrop Ammonium sulfate, cover- crop Dried blood, covercrop * Average of both oranges and lemons, t Treatment No. § The apparent discrepancy in some casse between inclusive dates and the years averaged is accounted for by failure to secure yield data in one or more of the years during the period in question. Bul. 571] Phosphate of Southern California Soils 1!) TABLE 4— Concluded Location Arlington Heights— {Concluded) Orange County Farm, Santa Ana C. C. Chap- man orchard, Fullerton Chula Vista Naranjo, Tulare County Soil type Placentia loam Hanford sandy loam Hanford loam Kimbal sandy loam San Joaquin loam Fruit Navel oranges Valencia oranges Valencia oranges Lemons Navel oranges Period represented by yield data and number of years averaged§ 1916-1920 (5 years) 1924-1929 (5 years) 1924-1931 (7 years) 1915-1920 (5 years) 1916-1923 (6 years) Plot 17B 39B 19B 2A, 2B 3A.3B 4A.4B 2A, 2B, 2C 3A, 3B, 3C 4A, 4B, 4C Treatment Dried blood, sodium ni- trate, potassium sulfate, covercrop Dried blood, sodium ni- trate, potassium sulfate, superphosphate, cover- crop Dried blood, sodium ni- trate, superphosphate, covercrop Dried blood, ammonium sulfate Dried blood, ammonium sulfate, superphosphate Dried blood, ammonium sulfate, potassium sul- fate, superphosphate ammomun Dried blood sulfate Dried blood, ammonium sulfate, superphosphate Dried blood, ammonium sulfate, potassium sul- fate, superphosphate Ammonium sulfate.. Dried blood Complete Sodium nitrate Tankage (8-8) Complete (4-10-2). Average annual yield per tree, pounds 162.8 125.0 145.8 279.31 265. 2 % 239. 5% 217.2$ 228.4J 222. 9% 207.01 138.01 138.01 93.0 101.0 77.0 % Converted from field boxes to pounds using 50 pounds per box. 1 Converted from field boxes to pounds using 46 pounds per box. § The apparent discrepancy in some cases between inclusive dates and the years averaged is accounted for by failure to secure yield.data in one orjnore of the years during the period in question. The suggestion has often been made that the lack of response to phos- phate applications may possibly have been due to the failure of the phos- phate to penetrate into the root zone of the soil. As bearing on this ques- tion, soil samples taken from a number of the phosphate plots of these experiments and from plots to which no phosphate has been applied, have been investigated. Determinations were made of water-soluble and dilute acid-soluble phosphate. The data reported in table 5 show that in all cases the applied phosphate has penetrated, to some extent at least, into the root zone. In the case of the experiments of longest duration, namely, the Rubidoux, Chaffey, and Fontana trials, the penetration of the phosphate has been very striking. 20 University of California Experiment Station CO CO CO OO CO CO CM M- t- 00 U, CO CO oo -* c CN ^ O OO lO t^ CO lO O0 CO •*i 1(3 1- O i- ■* « <* CO US CO CO CO CM CO CO CM <>? i-H CO CO »o >^ *".S 03 1 o o o 00 cv CO t— co CO CM O "* o 1 CO 00 CO OO •O CO c 5 O CM Tf CO - 1< iC -* OS T)< OO O OS -» CO O 3 ■* co «o 1- H i—l CO T-I CO CD CO 3 o on 1 , T3 DC "3 03 IN to * © OS CM OS J 00 if CM (M O CO CM 03 t-- 1- lO CO ■*! S lO CC 5 t^ CO co OS °3 T*^ CO CM CO CO > i-. 3 (C O 00 ^ O0 © CN Tf CO 1-^ t^. CO CO O CM O CM o o o 5-3 r- oo r-- I s - CM CO CO O CD 00 t^ 00 CO CM ■"* d Ph CO CM ■* e CM i-l r-H •«*i CM 00 i-c CM CM CO O CM 1 O T ~ l -. Tr- 3 S8 o 03 cw r^ cc CO 1ft OS CO iO CO o o lO os 03 c3 «.s »o CN oo T— c CM ■^ OS (-, (H O O £ s 03 a CO CM H E- o> * lO oc IN N OS lO CO t^ CO OS i-H i-l 03 03 OS CO 2 3 CO c3 c: «vi o CM CO CC CN CS CO OO u O r-i S c 03 CS CM CM H h 1 5 "" Ph co CM 03 CO CO 00 O CO CO O OO lO -^i O •^ co a P c ™ c O X a p 0) o3 a. 03 03 a t-i & | 03" 03 : X z 1 6 S c 1 *i 2 5 ^ g » o fl T3 ^ 73 03 3 03 "C CO 'C o 5 T3 .2 1 •§ 1 CO r- «*» fc £ Q Q fc t3 p O P P < c Q P Q Q o PC H cq a 2 C£ CO 3 CM CM CO i-l o CM 1-1 x»1 CN . ! >> g G r S o: co C > -o 3 03 a 2 03 JO 2^ q 03 JO -»? 03 'E > T3 C3 -a 13 fl (-. CO 1 g 'o CO 2 O s O T3 K 03 ■2 >, O "3 03 03 o3 co 03 03 03 02 « ffl rt W " w s CO o a 2 a) "53 0) 't .2 ft a CO 03 3 II a a "5 « o p - s 03 e 3 1 a 09 «§| 03 C o s P P c3 03 S 5 3 1=1 o O C3 O 03 § £ 03 u O O 03 r3 Bul. 571] Phosphate of Southern California Soils 21 Although certain criticisms may be made of some of these plot ex- periments there is no plausible explanation of the uniform failure of phosphate to give increases in yield, other than that the soils of these experimental areas already contained an adequate amount of available phosphate. These experiments were well distributed as to locality and soil type, and since the chemical studies show that the amount and availability of the phosphate in the various commercial orchards is now higher than in many of the soils of these orchards under experiment, there can be but little reasonable ground for doubt about the general ap- plication of these field results. As has been pointed out, the unfertilized soils representative of cer- tain types or areas are somewhat low in phosphate. It is conceivable that of these, some may be found in which the supply or availability of phosphate is inadequate for covercrops. In cases where covercrops fail to grow normally, tests 6 with phosphate should be made. SUMMARY Chemical and pot-culture studies have shown that most of the unfer- tilized soils in the principal citrus areas of southern California are well supplied with phosphate and that, with the exception of the calcareous types of soil, the phosphate is in a readily available state. Chemical studies have further shown that citrus-orchard soils of California have made substantial gains in phosphate as a result of past fertilization practices and that this phosphate is more soluble than the phosphate of the unfertilized soils. The results of fertilizer trials in pot cultures, and in certain fields, and a consideration of the amounts of phosphate annually absorbed by citrus trees as compared with that of quick-growing crops, leads to the con- clusion that citrus trees may be able to secure ample phosphate from less available forms and lower total supplies than certain annual crops. The use of phosphate or phosphorus containing materials in 9 differ- ent field trials with citrus, for periods ranging from 5 to 20 years, has in no case resulted in significant increases in the yields of fruit. Chemical studies indicate that the applied phosphate has penetrated into the root zone. 6 In citrus orchards where adequate provisions are made for the maintenance of nitrogen, this test can be carried out as follows: Select several representative areas about 15 feet square between the tree rows in various parts of the orchard. Prior to seeding the covercrop, broadcast and work into the surface soil about 10 pounds of 20 per cent superphosphate. Observe from time to time the growth of the covercrop on this as compared to adjacent untreated areas. The results should reveal whether or not phosphate is needed. 22 University of California Experiment Station Based on the above lines of evidence, it is concluded that, in so far as citrus trees are concerned, the majority of the soils of commercial citrus orchards in California contain at present an ample supply of available phosphate. It is possible that covercrops, when grown in citrus orchards that are located on calcareous types of soil or on those relatively low in phos- phate, will be benefited by applications of phosphate. In cases where satisfactory growth of covercrops has not been obtained, tests with phos- phate should be made. ACKNOWLEDGMENTS The author wishes to acknowledge his indebtedness to Dr. W. P. Kelley for advice and helpful criticism in connection with the chemical and pot-culture investigations reported herein, and for assistance in the preparation of the manuscript. Thanks are also extended to Dr. L. D. Batchelor; to H. E. Wahlberg, Farm Advisor of Orange County; and to Professor C. J. Booth, of Chaffey Junior College; for the use of unpub- lished data from which part of the average yields in table 4 were computed. 13m-5,'34