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V. Rotation Practices and Erosion Control
JACKSON L. C A R T E R AND EDGAR E. HARTWIG
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,
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).
THE MANAGEMENT OF SOYBEANS
C. EFFECTON WE^, POPULATION
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).
D. SOIL RESIDUESFROM HERBICIDES
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
JACKSON L. CARTIER AND EDGAR E. HARTWIG
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)
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.
THE MANAGEMENT OF SOYBEANS
OF PLANTINGTIMEON PLANTGROWTH
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
C . CHEMICAL
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-
JACKSON L. C A R T E R Ahm EDGAR E. HARTWIG
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
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
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-
THE MANAGEMENT OF SOYBEANS
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.
JACKSON L. CARITER AND EDGAR E. HARTWIG
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.