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V. Rotation Practices and Erosion Control

V. Rotation Practices and Erosion Control

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soybeans were combined and the straw burned, there was a loss of

about 3 pounds of nitrogen per acre. Also, under comparable conditions,

Sears estimated that corn, oats, and wheat would remove 40, 26, and

36 pounds of nitrogen, respectively, but his estimates were made for a

rather low (40 bushel) yield of corn. Typical rotations for the midwest

farming areas usually suggest soybeans following corn in the cropping

sequence, as corn can utilize to advantage the nitrogen furnished by

turning under a deep rooted, small-seeded legume sod, whereas, wellnodulated soybeans do not benefit from the high level of nitrogen.

At Stoneville, Mississippi, with good weed control, soybean yields in

a continuous cropping system have been similar to yields produced in

a 2- or 3-year rotation with cotton.

The advantage of soybeans in the rotation cannot be explained altogether on the basis of the returns from the crop, according to Pond

(1950). Some of the other advantages given were ( 1 ) the labor requirements are low; ( 2 ) soybeans do not compete too seriously for labor at

peak periods; (3) soybeans can be planted later than other crops with

reasonable assurance that they will m a t u r e t h i s is especially important

in a wet spring; ( 4 ) as a cultivated crop they aid in weed control; ( 5 )

they stand drought better than some other crops; ( 6 ) soybeans do better

than many other crops on spring plowing; and ( 7 ) soybeans improve

the physical condition of the soil.

A crop rotation in which soybeans do not appear oftener than once

in three or four years aids in controlling certain diseases such as brown

stem rot. Where the soybean cyst nematode is a serious problem, nearnormal soybean yields may be obtained in a 2-year rotation if a nonsusceptible crop is grown during the year out of soybeans (J. M. Epps,

personal communication).





Soybeans improve soil tilth by shading and protecting the soil from

rain. The roots and the bacterial action they foster tend to loosen the

soil mass and make it more easily penetrated by nioisture and by roots

of the succeeding crop. Soybeans leave heavy compact soils in much

better physical condition than do corn and small grains (Calland, 1949).

Soybeans as a crop seem to be less depressing on soil productivity than

corn when judged by crop yield (Strickling, 1950).

Results from studies conducted in Minnesota show that corn yields

following soybeans are greater than those following oats. This yield

increase was attributed largely to greater residual nitrogen in the soil

after a crop of soybeans (Schmid et al., 1959).




Continuous cropping tends to build up the population of weeds of

certain species. Rotating crops with different growth patterns and management practices aids in controlling weeds. Thus, the most effective

approach to weed control is to consider it in relation to the entire rotation and utilize the advantage of good cultural practices on each crop.

With the development of selective herbicides, chemical weed control is

becoming more effective on many crops. Maximum pressure on the weed

population can be maintained by use of various herbicides on tolerant

crops in the rotation (Shaw, 1961).


In controlling weeds in one crop, due regard must be given to the

problem of residues or the long-term carry-over in the soil. In recent

years, many new herbicides have been developed for weed control in

cotton and corn. Some of these have long-persisting soil residues so

that they will give weed control throughout the growing season. Unfortunately, in some years, at some rates, and under some environmental

conditions, this residue will carry through the winter and injure the

soybean crop the following year.

In most cases, the rates of application used are such that the rate

cannot be lowered and effective weed control achieved. The residue in

the soil, however, will be greater in some years than in others, primarily

owing to differences in the rate of breakdown in the soil. Materials used

at the present time which may give residual injury to soybeans are diuron

on cotton, and simazine or atrazine and Randox-T on corn.



Soybeans have a mellowing effect on the soil, leaving the surface

loose and porous and in a favorable condition for seeding other crops.

According to Browning et al. (1943), three factors are responsible for

the looseness of the soil following a crop of soybeans: One factor is the

protection of the soil surface by the plants themselves, commonly

referred to as the “canopy effect”; if rainfall is limited up to the time

when foliage is large enough to protect the surface, the soil under the

soybeans will remain loose throughout the season. A second factor is

the desiccating action of the plant roots on the soil during July and

August when rainfall is often deficient and transpiration high. The root

system of the soybean is not as extensive as that of corn, and the unusually

heavy drain on the moisture supply of a limited soil mass reduces the

moisture content to a low level. Such desiccation has a loosening effect



on the soil. Since this zone is near the surface, the soil may be rewetted

by showers and then dried several times in a season. The phenomenon

has an effect on the soil similar to freezing and thawing. The third factor

is aggregation resulting from decomposition of the roots, tops, and

nodules. The incorporation of soybean roots and nodules increases ,the

number of larger-sized aggregates.

Studies in Iowa, Missouri, and Illinois show that land in soybeans

is no more subject to erosion than land in corn if the beans occupy

the same place in the rotation. Soybeans cause less erosion than corn

when they follow meadow in the rotation. The soil losses from second

year corn were larger because the soil tilth from meadow in the

rotation largely disappeared after the first year. On a Marshall silt

loam, the soil losses for soybeans were less than for corn under comTABLE 111

Soil Erosion from Corn and Soybeans with Different Tillage Practices (Marshall Silt

Loam, Soil Conservation Experimental Farm, Clarinda, Iowa, 1944-1947 )

Soil loss (tons/acre)

Tillage method

Up-and-down hill










Corn listed in 40-inch rows

Soybeans listed in 40-inch rows

Soybeans surface-planted in 40-inch rows

Sovbeans drilled in 7-inch rows

parable conditions (Table 111). Browning (1949) concluded that the

frequent criticism of soybeans as causing more erosion than corn is not


VI. Weed Control

Weeds constitute a major hazard in successful soybean production. Yield reductions of 15 bushels per acre have been measured from

competition of a moderate infestation of Johnsongrass in Arkansas

( Caviness and Taylor, 1960). Observations in Mississippi (E. E. Hartwig,

unpublished) show yield reductions of 50 per cent from competition from

pigweeds and 40 per cent from morning glory competition. Three-year

average yields in Illinois show a 10 per cent reduction in soybean yields

from competition of six giant foxtail plants per foot of row and a 28 per

cent yield reduction from competition of 50 giant foxtail plants per foot of

row. At least 50 per cent of the tillage requirements for producing a crop

of soybeans has been attributed to controlling weeds (Shaw, 1961). In

addition to yield reduction from competition, seed of plants such as

crotolaria are toxic to livestock and poultry. Consequently, presence of

any crotolaria seed requires the cleaning of soybean seed before it can

be used as food or feed.






Studies conducted at Stoneville, Mississippi, show that soybeans

planted May 1 to June 20 after the soil has warmed will emerge in 5

to 7 days. Soybeans planted April 10 required 12 to 14 days for emergence

(Hartwig, 1954). Similar results are reported for planting at a corresponding latitude in Japan (Nagata, 1960). In addition to more rapid

emergence for the May and June plantings, the Stoneville studies show

that these plantings grow more rapidly after emergence (Fig. 3). The

combination of rapid emergence and rapid early growth results in earlier

shading of the ground. Weeds will have greater difficulty becoming

established under these conditions. The later planting permits several

shallow cultivations prior to planting and destruction of many weed seeds

in the upper soil layer. Under Mississippi conditions, soil conditions will

be favorable for germination of weed seeds for a 6- to 8-week period

if soybean planting is delayed until mid-May.

Although preplanting shallow tillage has aided in controlling weeds

in Mississippi, work conducted under the shorter growing season at

St. Paul, Minnesota, showed that tillage of plowed ground prior to

seedbed preparation for soybeans was of no benefit in the control of

several annual weed species (Robinson and Dunham, 195s).

Illinois studies show that foxtail seed begins to germinate in midApril and continues to germinate for about one month. Delaying planting

until after May 15 permits many foxtail seedlings to be destroyed prior

to planting and aids appreciably in reducing the foxtail problem (Slife,




Iowa studies show that rotary hoeing when weeds were germinating

but not emerged and repeated once or twice at 5-day intervals reduced

weed infestations 70 to 80 per cent and soybean stands approximately

10 per cent. Delaying use of the rotary hoe until weeds had emerged

reduced the effectiveness of rotary hoeing (Lovely et al., 1958). Probst

and Luetkemeier (1959) considered two rotary hoeings (16 and 22 days

after planting) plus two cultivations (27 and 39 days after planting) to

be satisfactory for controlling weeds in soybeans under most Indiana




I. Pre-emergence Herbicides

Numerous pre-emergence chemicals have been evaluated on soybeans

but few have given consistently good control of weeds without injury to

soybeans, CIPC [isopropyl N-( 3-chlorophenyl ) carbamate], DNBP ( 0-



sec-butyl-4,6-dinitrophenol), PCP ( pentachlorophenol ) , CDAA ( 2chloro-N,N-diallylacetamide), and NPA (N-l-naphthylphthalamic acid)

have been used to some extent in different areas.

CIPC has given promising results in some tests, but results have not

been uniformly good because of inability of this material to control

broadleaf weeds at rates safe for use in soybeans. DNBP has appeared

promising on some soil types but generally does not give acceptable

weed control on the heavier soils. Volatility may cause injury to soybean seedlings. PCP and NPA give inconsistent weed control in soybeans.

CDAA has not proved to be superior to PCP or NPA (C. G. McWhorter,

personal communication ) . Amiben ( 3-amino-2,5-dichlorobenzoicacid )

appears to be one of the more promising pre-emergence chemicals for


Ohio results indicate that applications of 5 to 10 pounds per acre

of DNBP gave satisfactory results over a four-year period. Damage to

beans was observed, but this damage did not result in reduced yields

of soybeans (Willard, 1952).

Illinois data show Randox to be satisfactory for controlling annual

grasses. Randox is relatively soluble and for this reason usually gives

better results in seasons with limited rainfall (Knake et al., 1961). These

workers considered amiben to be one of the most promising chemicals.

A summary of weed control demonstrations conducted in Illinois in

1961 shows that 54 per cent of the growers using amiben reported good

weed control whereas only 13 per cent of those using NPA reported good

weed control.

2. Post-emergence Herbicides

Several workers have reported good results from the use of 2,4-D to

remove broadleaf weeds from soybeans, whereas other workers consider

the material too hazardous to be used. In Ohio, studies indicated that

in the cornbelt there are no important weeds which are more sensitive

to 2,4-D than soybeans (Willard, 1952). In these studies, rates of 1/48

pound per acre to

pound per acre were applied at three stages of

growth from two true leaves to first flowers.

Slife (1953) indicates that 2,4-D can be applied to soybeans 4 to 8

inches tall to remove broadleaf weeds. A rate of 1/16 pound of acid

per acre is suggested to kill cockleburs, giant ragweed, and pigweed.

The treatment is suggested only for areas where weeds are an extremely

serious problem.

The herbicide 4( 2,4-DB) gives promise of being less toxic to soybeans

than 2,4-D but similar to 2,4-D for killing certain .broadleaf weeds.

Preliminary results from Mississippi ( McWhorter et al., 1961) in-



dicate that a mixture of

pound of diuron plus 1% pounds of a surfactant may be used to remove young weeds from soybeans. The surfactant greatly increased the activity of diuron on weeds so that a concentration of herbicide, too weak to damage soybeans, could be used.

VII. Seed Quality and Seed Treatment





Seed quality in the soybean is influenced by the variety and the

environment during seed development, as well as by the conditions

under which the seed is harvested and stored. Varieties must be able

to withstand a period of rain and unfavorable weather which may frequently occur at harvest time. One of the objectives in varietal improvement is selection for good quality seed and resistance to weathering

damage at maturity. Most of ,the recommended varieties have good seed

quality when grown within their area of adaptation.

Unfavorable weather during the ripening period, frost occurring

while the beans are still green, or exposure to damp periods after the

beans are fully mature may cause damage and poor seed quality (Morse

et al., 1950; Howell et al., 1959). Severe drought can affect seed development, resulting in green seed and oil having high refining loss (Cartter

and Hopper, 1942; Howell, 1956).

Rough handling in threshing or cleaning, especially when moisture

content of the seed is low, causes both externally visible and internal

damage, though the latter may not be discovered until the seeds have

been germinated (Moore, 1957; Humphrey, 1958; Colbry et al., 1961).

Hard seeds, or those that fail to absorb moisture for several hours

when soaked in water or placed in a germinator, are found occasionally

in seed samples. Normally they present no serious problem.

Soybean diseases, especially pod and stem blight, downy mildew,

frogeye, and purple stain may affect the seed quality or injure germination. Comparative growth of normal and abnormal seedlings in germination tests indicate that the normal seedlings produce the most vigorous

plants. Decay of cotyledons is a serious injury and only seedlings with

healthy or very slightly decayed cotyledons should be included in the

percentage of germination (Anderson, 1960).

In Iowa, soybean germination tests in the seed laboratory correlated

closely with those in the field. Greenhouse germinations in sand were

lower, probably owing to a large population of seed-rotting fungi in

the greenhouse sand (Sherf, 1953). Seeds of low vigor will be affected

more by adverse field conditions than will seeds of high vigor (D. F.



Grabe, personal communication ) . Soybeans germinate most rapidly at

86" F., and if in sand, the most favorable moisture level is 15 per cent

water based on dry weight of sand (Delouche, 1953). Methods of

measuring seed viability are important to the seed trade (Sprague,

1958).Price per bushel of pure live seed, calculated by dividing the price

per bushel by the purity times germination, is a simple way to determine

planting value of the seed (Everson, 1957).


Seed treatment with a fungicide is not recommended as a general

practice when seed with high germination is planted. Stands may be

increased by seed treatment when seed having a germination of 85 per

cent or less is planted. Although seed treatment seldom results in

increased seed yields (Howard W. Johnson et al., 1954; Chamberlain and

Koehler, 1959), the improved stands resulting from seed treatment aid

in giving soybeans a competitive advantage with weeds. Studies by

Howard W. Johnson et al. (1954) show that seed may be treated at any

time between harvest and planting with equal effectiveness. The most

satisfactory time for treating seed would be as it is cleaned. The materials

Arasan, Captan, and Spergon have proved to be most satisfactory for

treatment of soybean seed. Before any lot of seed is treated, it may be

a good practice to check the germination with and without the fungicide

to determine the beneficial effect of seed treatment on each seed lot.

VIII. Nutrient Requirements

Nodulated soybeans do not respond to nitrogen fertilizer as do

non-legume crops and because of this, gained a reputation of not responding to direct fertilization, a reputation that is not justified. In order

better to understand the nutrient response of soybeans in comparison

with corn, the total energy in the protein-oil seed of soybeans has been

compared with the largely carbohydrate seed of corn (Howell, 1961).

These studies show that in terms of total energy per acre, a 45-bushel

soybean yield is equivalent to a 100-bushel yield of corn.




When properly nodulated, soybean roots may derive a considerable

portion of the nitrogen needs of the plant from the nodules through the

fixation of atmospheric nitrogen. Weiss (1949) presented a detailed

review of the literature in this field, indicating the importance of nodulation to yield and composition of the crop.

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