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VI. Effect of Cultural Practices

VI. Effect of Cultural Practices

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102



MARTIN G. WEISS



approximately one-third of the farms surveyed. I n Illinois corn and oats

followed soybeans on 67 and 25 per cent of farms, respectively.

On soils of low productivity, soybeans have by necessity replaced

another tilled crop without lengthening the rotation. Such a rotation

recommended for sandy soils in Wisconsin by Albert et al. (1947) is soybeans-oat8-legume, hay and seed-legume, seed, or hay and seed. Instead

of oats, ensiled corn and a winter grain may be substituted. When

grown for hay Trotter (1936) states that soybeans are a t times grown

in one-year rotations with winter barley in Missouri. Few data are

available at present which would indicate the relative merits of various

rotations including soybeans. The effect of soybeans on crop yields in

rotations is discussed in Section IX-2.

9. Fertilizers and



Soil Management



a. Response. Soybeans frequently have been classified as a (‘poor

land” crop. This concept probably originated from 2 sources: Frequently

soybeans yield relatively more than grain crops on soils of low productivity, and the response of soybeans to direct. application of commercial fertilizers is usually disappointing. However, marked yield variations are stimulated by differences in natural productivity of soil or

general fertility levels as conditioned by different soil management

(Cartter and Hopper, 1942; Lang and Miller, 1942; Norman, 1946; Vittum and Mulvey, 1944 and others). Some evidence is available, as

reported by Pierre (1944) and Norman (1946), that increases in soybean

yields as stimulated by high fertility levels are similar on a percentage

basis to yield increases exhibited by corn.

Although the response to direct application of fertilizers relative to

other crops has been low, under certain conditions material increases in

yield have been obtained. Correction of soil acidity with lime has, in

general, resulted in higher yields (Cartter, 1941; Collins et al., 1947;

Colwell, 1944; Nelson and Hart.wig, 1948; Pierre, 1944; Prince et al.,

1941 ; Vittum and Mulvey, 1944 and others). When soybeans were grown

on soils varying in pH from 4.6 to 7.7, Thatcher et al. (1937) found maximum yields resulted at p H 6.8. It is the contention of some workers that

the stimulation due to liming is attributable to fertilization with the calcium ion rather t.han to neutralization of the soil. Designation of soybeans as an acid-tolerant crop, according to Albrecht (1944) ,is equivalent

tjo intimating that the crop is tolerant to starvation. I n addition to increasing yield, application of lime was reported by Cartter (1941) to increase protein and decrease oil content of the beans.

On potash-deficient soils direct application of potash has resulted in

increased yields. Striking yield responses to pot,ash application were re-



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ported by Collins e f nl. (1947) and Nelson and Hartwig (1948) on Norfolk loamy, fine sand and Diinbar fine, sandy loam and by Colwell (1944)

on Portsmouth and Dunhar silt! loams in North Carolina. Lesser responses were reported by Cartter (1941) on Clermont silt loam in Indiana and by Prince et aE. (1941) on Ondawa loamy fine sand in New

Hampshire. In the latter work a slight, decrease in protein and increase

in oil percentage of seed were attributed to potassium application.

Similar changes in soybean composition as consequence of potassium

fertilization of soils in South Africa had previously been reported by

Viljoen (1937). Three varieties of soybeans grown on Coxville very fine

sandy loam in North Carolina were reported by Nelson et al. (1946) to

respond greatly in seed yield to applications of potassium. The yield increase was found to consist of a larger numbw of pods per plant, higher

degree of pod filling, greater seed weight, and improved seed quality. I n

contrast to other studies discussed later, maturity wag retarded by potassium application. Increased oil content of beans was reported by Colwell

(1944) with potash application and hy Adams et nl. (1937) with application of a combination of potassium and nitrogen.

Yield responses to phosphate h a w been reported (Cartter, 1941 ;

Collins s t al., 1947; Colwell, 1944; Tlang and Miller, 1942) but, in general,

responses to phosphate alone Iiavr not been as pronounced as to potassium. Applications of phosphate were even reported by Pierre (1944)

to reduce yields under certain conditions. Combinations of phosphate

and potassium have been reported by Lang and Miller (1942) to result

in higher yields on certain of the light-colored and sandy soils of Illinois.

In Indiana, combinations of these elements were found by Vittum and

Mulvey (1944) to stimulate yield and result in marked earlier maturity.

The components of yield increased by fertilization were found to include

number of pods per plant, number of seeds per pod and size of seed.

Seed quality also was improved in that number of damaged and purple

blotched beans was decreased and germination was increased.

Responses to application of certain other elements have, in fewer instances, been reported. Magnesium was found by Nelson et al. (1946)

consistently to retard maturity and slightly to increase bean yields of

t.wo of three varieties studied. Soil application of manganese sulfate

nearly doubled the yield of soybeans on manganese-deficient Maumee

loam in Indiana as reported by Steckel (1947, 1948). Early spray applications were found to be equally effective in correcting the deficiency,

Severe iron deficiency symptoms in soybeans when grown on high-lime

soils of the Webster series in north central Iowa were reported by Weiss

(1943). On soils of this series with approximately p H 8 two spray applications of 10 lbs. of ferrous sulfate a t weekly intervals during early



104



NlABTIN G . WEISS



growth stages were found by Nelson (1948) to result in soybean yields

varying between 20 and 35 bushels per acre whereas untreated plots produced virtually no beans.

Reports of growth responses of soybeans to incorporation of stran

and cornstalks into the soil have not been consistent. I n certain instances

(Norman and Krampitz, 1946) ground straw has been incorporated into

soil when reduction of available nitrogen has been desired for experimental purposes. This system is based on the premise that the microorganisms which decompose carbonaceous materials compete with the

plant roots for available nitrogen. In greenhouse experiments conducted

during both winter and summer Pinck et al. (1946) found that the addition of chopped wheat straw to Sassafras sandy loam materially reduced

dry plant weight and total nitrogen of soybean plants when harvested in

the green bean stage. Addition of the equivalent of 25 Ibs. per acre of

urea overcame the nitrogen deficit caused by two tons of straw but 100

lbs. of urea were inadequate to compensate for the depression in available

nitrogen accompanying a 4-ton per acre straw application. The above

results were not substantiated by field experiments on a prairie soil reported by Englehorn et al. (1947). Plowing under 4 tons of straw per

acre or twice the quantity of cornstalks produced by a 60-bushel crop of

corn resulted in no decrease in yield or nitrogen content of mat.ure soybeans. Midseason application of nitrogen, however, increased yields and

nitrogen content on the plots treated with crop residues.

b. Placement. Direct application of fertilizer in contact with soybean seed frequently has been observed to result in poor germination.

Phosphate and potash fertilizers, regardless of rate, were found by Probst

(1944) to inhibit emergence of soybeans when applied in contact with the

seed. Potash decreased emergence more severely than phosphate. When

placed in bands 1 inch to either side and a t the same level as the seed,

applications of 500 to 750 lbs. per acre of potash and phosphate failed

to reduce emergence materially. Applying potash in bands 2 inches to

each side and 1 inch below the level of the seed resulted in appreciably

greater stands in l-year trials in North Carolina (National Joint Committee on Fertilizer Application, 1947) than when the fertilizer Wac

placed 2 inches directly under the seed.

The placement of fertilizers in regard to manner of incorporation

into the soil has also been found to influence soybean yields. With certain conditions fertilizer broadcast and plowed under has been found t o

give materially higher yields of hay and seed than when broadcast and

disked in just prior to seeding (Drake and Scarseth, 1941; Enfield, 1943)

or when placed in bands to one side of the seed (National Joint Com-



SOYBEANS



105



mittee on Fertilizer Application, 1947). I n other areas the results have

been less conclusive (Pierre, 1944; Smith, 1943).

3. Seed Inoculation



The soybean, like other legumes, has the faculty of entering into a

symbiotic relationship with a species of root nodule bacteria. The species

compatible with soybeans is Rhizobium japonicum. When properly nodulated soybean roots may derive a considerable portion of the nitrogen

needed by the plant from the nodules which, in turn, is derived by the

organisms from atmospheric nitrogen. The amount of nitrogen fixed

by the root. nodule bacteria varies with factors such as the quantity of

available nitrogen in the soil, and is discussed in greater detail in Section

IV-2-a.

The manner in which atmospheric nitrogen is transformed into combined nitrogen is still unknown. The transformation is thought t o occur

in the nodules on the plant roots, and a number of theories have been

advanced as to the chemical reaction which must occur. Searches for

intermediate products in the nodules have, in general, proven fruitless.

Orcutt (1937), Umbreit and Burris (1938) and others have found nitrogen fractions of soybean nodules similar to those of other parts of the

plant with the possible exception of basic non-amino nitrogen which seems

slightly higher in nodules.

The nitrogen which has been accumlated through the fixation process

seems to pass directly into the soybean roots. No evidence of leakage

or excretion of nitrogenous substances into the substrate from nodules

of Manchu soybeans was detected by Bond (1938) by chemical analysis

of the sand in which the plants were growing or by growth response of

barley grown in pots with nodulated soybeans. I n certain other legumes

excretion of nitrogenous substances from the nodules has, however, been

demonstrated.

The advantages of inoculation are not always readily apparent to the

grower. I n fact, when adequate nutrient nitrogen is present in the soil,

increases in yield attributable to nodulation at times are negligible. The

nitrogen fertilization experiments described in Section IV-2-a demonstrate

that equally high yields can be produced with adequate quantities of soil

nitrogen by unnodulated plants as by nodulated plants. On soils where

combined nitrogen is a limiting factor for plant. growth, inoculation effects

notable changes. On a loess soil with moderate quantities of combined

nitrogen Norman and Browning (1943) and Norman (1944a) found the

percentage increases of inoculated over uninoculated soybeans of the

Mukden variety to be 31 per cent in yield, 11 per cent in protein content

of beans, 47 per cent in protein production per acre in beans, 24 per cent



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MARTIN G . W E N S



in oil production per acre, 29 per cent in nitrogen content of straw, and

a decrease of 5 per cent in oil content of beans. It was estimated that

under these conditions the application of 540 lbs. of ammonium sulfate

per acre would be required to equal the effects of inoculation.

Nodulated soybean seedlings grown on colloidal clay were found by

Hampton and Albrecht (1944a) to be higher in percentage of protein,

potassium, calcium, magnesium, and phosphorus than non-nodulated

plants. The authors postulated that the roots of nodulated plants, because of higher nitrogen content, have a more efficient physicochemical

system for the movement of ions from the substrate into the interior of

the root. Nodulated plants were further shown to have a considerably

greater top to root ratio, indicating approximately 50 per cent higher

efficiency of unit mass and surface of nodulated roots to produce tops

than non-nodulated roots.

The advisability of inoculating beans to be planted in soil which has

grown nodulated soybeans in previous years has received considerable

attention. Experiment. stations, in general, have recommended inoculation on fields not recently cropped with soybeans, or on acid soils, in

which medium the longevity of rhizobia is thought to be reduced. A

survey by Norman (1943) in an Iowa county in which soybeans had been

commonly grown and where soils are nearly neutral in reaction showed

nodules were absent in 8 per cent and soybeans were poorly nodulated

in 13 per cent of the fields sampled.

The presence of nodules is, furthermore, not in itself assurance of

maximum nitrogen fixation in the plant. Variability in efficiency in

nitrogen fixation with R. japonicum has been conclusively demonstrated

(Agati and Garcia, 1940; Andrews and Briscoe, 1943; Briscoe and Andrews, 1938 and others). Certain strains have been shown by Andrews

and Briscoe (1943) to be relat,ively efficient on limed soils whereas their

performance was poor on unlimed soils. Among cultures of rhizobia,

abundance of nodules is not closely associated with efficiency in nitrogen

fixation.

A further source of variation in efficiency of nitrogen fixation is the

genetic diversity of the host varieties. Diff erent.ial efficiency of bacterial

cultures on soybean varieties was found in the above researches. The

reason for inspection of legume inoculants by a number of states immediately becomes apparent. I n tl report of inoculant inspection in Indiana by Quackenbush (1946) it is st,ated that both fixation of nitrogen,

as judged by appearance of plants, and nodule formation are used as

criteria for classification of commercial cultures, since poor cultures may

produce a satisfactory number of nodules but plants may not obtain

adequate nitrogen. Total yield and total nit,rogen in soybean plants



SOYBEANS



107



were examined by Andrews and Briscoe (1943) as possible criteria for

efficiency indices. The two characters were highly associated when the

soybeans inoculated with the bacterial cultures were grown on unlimed

noils. On limed soils, however, a low association existed.

Efficient nitrogen-fixing strainr of rhizobia, when passed successively

through the soybean host, tend to deteriorate in ability t o fix nitrogen,

whereas inefficient strains improve in this respect (Umbreit, 1944). Since

bacteria occurring in the soil must be assumed to have been derived from

one or more such passages, the grower has no assurance that the organisms are highly efficient even though the preceding crop was inoculated

with efficient strains. Repeated inoculation, therefore, is considered advisable by most soil bacterio1ogist.s to ensure the presence of adequate

quantities of active, efficient rhiaobia and, furthermore, to locate them

on the seed where they are needed by the young soybean seedlings.



4. Seed Germinability

a. Viability. The longevity of soybeans, relative to ot.her crop seeds,

is low. Under storage conditions encountered in Ontario, rapid decrease

in viability was found to occur when soybeans were stored for more

than three years (Laughland and Laughland, 1939). Viability was maintained for 5 years under Colorado conditions after which time Robertson

et al. (1943) found soybeans decreased rapidly in germination. Under

the same conditions the viability of small grains diminished only slightly

in 10 years. When stored in bins in Illinois, soybeans near the surface

were found by Burlison et al. (1940) to decrease appreciably in germination within 1 year from harvest. Within 2 years germinability of

beans 4 feet below the surface also had decreased. Relatively rapid

loss of viability a t the surface was attributed to greater absorption of

moisture. Under the conditions of high temperatures and humidity encountered in Puerto Rico, viability of soybeans stored for more than a

few months is consistently low (Stoddard, 1945).

High moisture content and high temperatures both have been shown

to decrease the longevity of soybean seed (Ramstad and Geddes, 1942;

Toole and Toole, 1946). Effert of temperature is readily demonstrable

by examination of the findings of Toole and Toole (1946). Soybeans

with 13.5 per cent moisture maintained a t 30°C. failed to germinate after

5 months, whereas full viability was maintained after 10 years of storage

a t -10°C. Soybeana maintained a t 20°C. with 18 per cent moisture lost

all viability in from 5 to 9 months, whereas with 8 t o 9 per cent moisture,

90 per cent germination resulted after 5 years of storage. High moisture

content and high storage temperatures of soybeans were found by Tervet



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MARTIN G. WEIISS



(1945) to favor incidence of fungi, particularly Aspergillus spp. Seedling growth was retarded accordingly.

High moisture content a t time of harvest in itself is not detrimental

to germination. Soybeans harvested with 66.9 per cent moisture content

were found by Robbins and Porter (1946) to be germinable. Exposure

of immat.ure soybeans to low temperature decreased germination roughly

in proportion to the decrease in temperature and degree of immaturity.

However, soybeans were found much more tolerant to low temperatures

than sorghum. Germinability of soybean seed with 32 per cent moisture

content or less was not reduced when frozen for 10 hours a t -20°F. With

a moisture content of 50 to 60 per cent, in most varieties seed germination

was not decreased by exposure t o temperatures not less than 20°F.

. b. Disinfectants and Protectants. The effect of disinfectants and

protectants on germination of soybeans has been studied extensively and

diverse results have been obtained. Under certain conditions increases

in germination have resulted following treatment of the seed with organic

mercury or strictly organic compounds (Allington et al., 1945; Davy,

1942; Hildebrand and Koch, 1947a; Johnson and Koehler, 1943;

Koehler, 1944a; Melhus et al., 1944; Petty, 1943; Porter, 1944, 194633;

Sherwin et al., 1948; and Stoddard, 1945), although under other conditions evidence of control is entirely lacking (Tervet, 1943). Increases

in germination generally resulted from seed treatment when the seed was

low in viability, damaged in the threshing operation, or when seed of

high viability was germinated a t reIatively low temperatures. Beans of

moderate viability were found by Sherwin et al. (1948) to give greater

response to seed treatment a t germination temperatures of 25OC. than

a t higher or lower temperatures. On the other hand, failure of high

germination of normally viable seed when germinated under low temperature conditions was found by Porter (1946a, 1946b) to be attributable

largely to the occurrence of “baldheads,” which he described as seedlings

which failed t o develop normally due to necrosis of the plumule in earl?

stages. The occurrence of baldheads was substantially reduced by treatment with protectants.

Reported instances of increased yields resulting from seed treatment

are less frequent (Hildebrand and Koch, 1947a; Koehler, 1944a). The

infrequent occurrence of increased yields from seed treatment is probably

due t o the ability of the soybean plant. to compensate for thin stands, as

discussed under Section VI-6. Even though less plants developed from

the untreated beans, adequate stand for maximum yields still occurred.

Considering all data, seed treatment of soybeans would not seem warranted except under the following conditions: When seed is of low viability due to age or severe weather damage (Allington et al., 1945) ; when



SOYBEANS



109



it is desired to plant a t a low rate of planting (Koehler, 1944b) ; or when

abnormally low germinating t.emperatures are anticipated (Porter,

1946b).

The seed treatment compounds most commonly used in the above

researches could be classified as strictly organic protectants, such as

Spergon and Arasan, the organic mercury disinfectants, such as New

Improved Semesan Jr. and New Improved Ceresan, and others such as

Cuprocide (copper oxide) and Fermate (ferric dimethyl-dithiocarbamate). Cuprocide inhibits germination and is considered injurious t o

soybean seedlings (Heuberger and Manns, 1943; Johnson and Koehler,

1943). With the exception of Cuprocide, when a substantial response t o

seed treatment occurred, some degree of response to all of the above

protectants and disinfectants occurred provided adequate quantit,ies were

applied. No consistent difference between the mercury and non-mercury

compounds was in evidence.

The effect of the disinfectants and protectants on nodulat-ion of the

soybean has been a subject of considerable interest. I n a study by

Appleman (1942) the number of nodules on plants grown from inoculated

beans in sterile substrate was not appreciably reduced by prior treatment

with either of two organic mercury compounds or a copper oxide disinfectant. However, nodules developed only on lateral roots and not on

the tap root as in the untreated check. The aut,hor postulated that with

seed treatment a zone of bacteriostatic action was set up around the seed

in which area legume bacteria are inactivated t o the degree of preventing

their entrance into root hairs. Definite inhibition of nitrogen fixation

was reported by Johnson and Koehler (1943) in plants when seed had

been treated with Cuprocide. Inhibition of nodulation on the t a p root

accompanied treatment with Ceresan and Semesan, Jr., whereas Spergon

only partially inhibited t a p root nodulation. Field observations of inoculated seed planted in soil not inhabited with nodule bacteria, in general,

have shown that all seed treatments appear to be detrimental to nodulation but not to the extent, of causing nitrogen defficiency symptoms in the

plants (Allington e t al., 1945; Koehler, 1944a; Petty, 1943). The organic

mercury compounds appear to inhibit nodulation t o a greater degree than

the strictly organic protectants (Allington e t al., 1945). I n soils containing the nodule organisms, field observations have failed to detect

decreases in nodulation attributable to seed treatment (Allington e t al.,

1945; Koehler, 1944a; Petty, 1943; Stoddard, 1945).

c. Hormones. Treatment of soybeans with various hormones has

been the subject of several investigations (Bartholomew, 1944; Kiesselbach, 1943; Youden, 1940). The hormones studied by the various investigators included indolbutyric acid, napthalene acetic acid, Rootone,



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MARTIN G. WEISS



Staymone, Grain 0 and Du Bay-120 FF. Stimulation of plant growth

as measured by top growth or seed yield was not in evidence. Certain

of the substances tended to inhibit germination of the seed.

5. Time of Planting



The effect of time of planting on yield of soybeans varies with geographical location and has been studied wherever soybeans have gained

prominence. In general, when adapted varieties are planted at successive

intervals throughout the first month following the frost-free date, differences in yields attributttblc to dates of planting are not great. As

reported in the corn belt by Burlison et al. (1940) and in the cotton belt

by Henson and Carr (1946) successively later plantings result in progressively lower yields. As shown by Weiss et d. (1949) varieties respond differentially in this respect. Early varieties, which do not

normally require the entire growing season to mature, may be planted

considerably later than adapted varieties without incurring yield reductions. On the other hand, yields of relatively late varieties, which can

utilize the entire growing season, may be reduced by any delay in planting following the frost-free date.

Time of maturity, in general, is delayed by later planting. However,

the delay in time of maturity is not as great as the delay in planting. In

South Africa Viljoen (1937) found 4 to 5 days delay in planting retarded

maturity 1 day, whereas iq the corn belt Weiss et al. (1949) reported

that a 3-day delay in planting retarded maturity 1 day. Varieties differing in earliness do not respond alike in this respect. Maturity of late

varieties, which can utilize the entire growing season, is not deIayed by

late planting as greatly as in early varieties. I n Mississippi, Henson and

Carr (1946) reported that a delay in planting of 50 days resulted in

delaying maturity of an early variety 26 days and of a late variety 2

days. Similar but less extreme reports were reported by Weiss et al.

(1949).

Delayed planting usually results in less plant height, slightly less

lodging, and smaller seed size (Weiss et al., 1949), particularly in relatively late varieties, and better seed quality (Henson and Carr, 1946).

Composition of soybeans has been shown to be influenced by the time

of planting. A decrease in oil content (Viljoen, 1937; Weiss et al., 1949),

a slight increase in protein content (Viljoen, 1937), a material increase

in iodine number of oil (Weiss et al., 1949), and ti slight decrease in ash

content (Viljoen, 1937) accompany delayed planting. Certain associations among characters were reported by Weiss et al. (1949). Among

the means of five dates of planting the following attributes were found

to be significantly correlated: Large seed size with low iodine number,



SOYBEANS



111



lateness of maturity with low oil content, lateness of maturity with high

iodine number of oil, low mean temperature during the seed developmental period with high iodine number of oil, high oil content with low

iodine number of oil, and high protein content with low oil content.

Little association was obtained between seed size and oil content, seed

size and protein content, or maturity date and protein content.

6. Method and Rate of Planting



Methods of planting soybeans alone include broadcast planting,

drilling in rows with 7- or 8-inch spacing, and planting in adequately wide

rows to permit intertillage. Because of uneven depth of planting and

subsequent lack of uniformity in germination, the broadcast planting

method has rapidly lost favor in the areas of heavy production. Drilling

of soybeans with the conventional grain drill was the first generally

accepted practice in the heavy production areas. During the past decade

the method of growing soybeans in rows to permit row cultivation has

gained favor particularly in the northwestern part of the corn belt. The

shift toward rowed planting is undoubtedly attributable to its greater

efficiency in weed control, which constitutes a greater advantage in the

northern areas where low mil temperatures prevent adequate destruction

of weeds prior to planting.

Soybean yields obtained when drilled a t various row widths in four

corn belt states were summarized by Weber and Weiss (1948) and together with results reported for southeastern Kansas by Zahnley (1942)

are illustrated in Fig. 4. The yields are presented as gross yields and

no compensation has been made for differential planting rates in the

various row widths. No weed control was generally practiced on the

drilled beans after their height no longer permitted harrowing, whereas

row widths from 21 to 42 inches were additionally intertilled.

Highest yields were consistently attained over a period of years with

the narrowest, intertilled rows, spaced 21 inches apart. Drilled beans

and 28-inch rows gave slightly lower yields, and yields further decreased

with wider row widt.hs. Substantiation of these experimental data were

obtained by Calland (1946) in a canvass of 4200 growers in 48 principal

soybean producing counties in Illinois, Indiana, and Ohio. On the average, narrow rows, spaced 18 to 28 inches, gave highest yields, medium

width rows, 30 to 36 inches, yielded slightly less, and wide rows, 38 to 42

inches, and solid drilled fields gave similar yields and ranked lowest

among the methods of planting. Lower yields in drilled beans relative

to narrow rows was attributed by many investigators t o greater competition by weeds. In seasons during which adequate control of weeds

was permitted in the drilled plots, maximum yields with this method of



MARTIN G . WEISS



112



planting frequently resulted. These observations are substantiated by

yield data collected in New York by Wiggans (1939). When row widths

of 8, 12, 16, 24, and 32 inches were compared for 4 years in plots kept

weed-free by hand cultivation, maximum net yields of 38, 35, 34, 33,

and 30 bushels per acre, respectively, were obtained. At all row widths

approximately six plants per square foot, gave maximum yields, although



{:

"



KANSAS SYEARS



7



14



21

28

INCHES BETWEEN ROWS



35



Fig. 4. Effect of row widths on yield of soybeans in five corn belt stat.es (Data

of Zahnley, 1942; and Weber and Weiss, 1948).



the author notes that this number would undoubtedly vary for different

varieties and growing conditions. Regardless of row width, the number

of plants per unit area required to give maximum yields was thought t o

be constant. Greatest possible uniformity of distribution was thought to

facilitate yield, thereby explaining the decreasing yields with increasing

row widths.

In most of the corn belt, corn planters and cultivating equipment are

used interchangeably on corn and soybeans. Width of soybean rows



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VI. Effect of Cultural Practices

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