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VII. Seed Quality and Seed Treatment
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.
THE MANAGEMENT OF SOYBEANS
1. Effectiveness of Nodulution as a Source of Nitrogen
A mutation type was reported by Williams and Lynch (1954) which
does not develop nodules when inoculated with the soybean nodulating
bacterium Rhizobium japonicum. This mutation, due to a single recessive
gene, has been incorporated into several nodulating and nonnodulating
near-isogenic lines of different maturities and is providing an excellent
tool for the study of nitrogen fertility problems.
Perhaps the best measures of effectiveness of nodules in supplying
the soybean plant with nitrogen are the results of studies using one of
these pairs of near-isogenic lines which differ in ability to nodulate. At
Ames, Iowa, when 20 tons of ground corncobs per acre were added to
the soil prior to planting to reduce the available nitrogen, nodulated soybeans produced 41 bushels per acre without nitrogen and 43 bushels
when 600 pounds per acre of nitrogen was added. The nonnodulated
strain produced 16 bushels per acre without nitrogen and 41 bushels with
600 pounds per acre of nitrogen. The 25-bushel yield increase for
nodulated over nonnodulated soybeans where no nitrogen was applied was
attributed to nodule activity. The 25 bushels of seed contained 96
pounds of nitrogen (C. R. Weber, personal communication). In a similar
comparison of nodulated vs. nonnodulated soybeans on Sharkey clay
at Stoneville, Mississippi, nodulated soybeans yielded at the rate of 42
bushels per acre with 41 per cent protein whereas nonnodulated soybeans
yielded at the rate of 14 bushels per acre with only 28 per cent protein.
The seed from nodulated soybeans contained 112 pounds more nitrogen
per acre than seed from nonnodulated soybeans. In the same general
area, other strains of soybeans produced seed yields as high as 60 bushels
per acre which contained 192 pounds of nitrogen per acre, or an increase
of about 160 pounds per acre over that for the nonnodulated strain.
It appears that under conditions of low available soil nitrogen, nitrogen fixation from nodulation can furnish nitrogen for soybean yields of
at least 60 bushels per acre with fixed nitrogen amounting to as much
as approximately 160 pounds per acre. On soils high in available nitrogen,
the amount of nitrogen supplied by the fixation process appears to be low.
As far back as 1900, it was noticed that nodules of legumes inoculated
with certain pure cultures were more active than those produced by
bacteria already in the soil (Erdman, 1949). Some strains of bacteria
are more efficient than others in fixing nitrogen, some are more aggressive
in nodulating roots, and some are more competitive in the soil. Means
et al. ( 1961), using a chlorosis-inducing strain of Rhizobium japonicum
and mixing it in varying proportions with other strains, found that as
little as 1.1 per cent of this strain in the mixture with another strain
JACKSON L. CAR-
A N D EDGAR E. HARTWIG
caused 85 per cent of the nodules on certain soybean varieties. Almost
without exception, a given nodule contains only one bacterial genotype.
A chlorosis-inducing strain has been found in the Mississippi delta area
that is aggressive on the variety, LEE, and causes typical chlorosis symptoms on plants around 6 weeks old (Clark, 1957). The symptoms are
transient-that is, they are present for a short period and then new
leaves growing out appear normal. Field observations indicate that the
bacterial strain is relatively efficient in nitrogen fixation, though for a
week or two symptoms may be alarming to a soybean grower.
2. Methods of Znoculation
The generally accepted method for applying inoculum to the seed
is to apply the bacteria adsorbed in a humus-peat carrier to slightly
moistened seed or to make up a slurry of the inoculum in water and
apply this. The quantity of seed inoculated should be limited to that
which can be planted before the seed coats have completely dried.
Results from several studies showed that 83 per cent of the inoculant
was retained upon moistened seed whereas only 8 per cent of the inoculant was retained upon dry seed (Clark, 1956). Recent studies with
precoating of soybean seed have shown no advantage over traditional
methods of inoculation.
3. Survival of Bacteria in the Soil
Studies in Illinois showed good survival of bacteria in soils that
had not grown a crop of soybeans in thirteen years (Lynch and Sears,
1952). Similar results were obtained in North Carolina after twelve
years of continuous cotton. Noninoculated plots were well nodulated
and produced yields similar to yields of inoculated plots (E. E. Hartwig,
4. Effect of Seed Treatment on Inoculation
Under some conditions, seed treatment with the organic fungicides
Arasan, Captan, or Spergon will result in improved stands of soybeans.
Since rhizobia are already present in most soils in older soybean-growing
areas, it is difficult to assess the effect of seed treatment upon survival
of rhizobia. In 1950, planting were made on newly cleared land at the
West Florida Experiment Station using ROANOKE seed which had been
untreated or treated with Arasan or with Spergon. Both Arasan and
Spergon reduced the effectiveness of inoculation. Seed yields from nontreated, inoculated seed were 22 bushels per acre as compared with 7
bushels for noninoculated seed and 13 bushels each for seed treated with
Arasan or Spergon and inoculated (E. E. Hartwig, unpublished).
THE MANAGEMENT OF SOYBEANS
5. Effect of Nitrogen Applications
Applications of nitrogen tend to retard nodulation of seedinoculated soybeans planted in rhizobia-free soil. Applications of
100 pounds of nitrogen at planting time in the Imperial Valley of California, where rhizobia were not present in the soil, resulted in poorly
nodulated soybeans which produced yields of 6 to 16 bushels per acre
with 27 to 34 per cent protein on a dry matter basis. Omitting nitrogen
application at planting time permitted good nodulation and resulted
in yields of 35 to 40 bushels per acre with normal protein content of
the seed (G. H. Abel, Jr., personal communication).
An extensive field trial in Arkansas in which nitrogen was applied
in a factorial experiment with phosphorus and potassium showed no
significant response from nitrogen applied at different stages of plant
development (Hardy, 1959). These results are typical of the many trials
showing no appreciable benefit from nitrogen fertilizer on well-nodulated
B. LIMINGAND PH LEVELS
1. p H and Plant Development
There seems to be general agreement that a pH of 6.0 to 6.5 is desirable for soybean production (P. R. Smith, 1956). In North Carolina,
under conditions where the pH of the soil was 4.2, nitrogen-fixing bacteria
could not function actively and a yield response was obtained from added
nitrogen. With additions of lime, soybean yields were related rather
closely to the degree of calcium saturation regardless of the sourcecalcitic or dolomitic. On two soils, CaS04 had a detrimental effect due
to the toxic effect of the sulfate ion and the salt injury (Welch and
Nelson, 1950). In another series of soil treatments in North Carolina, a
2.8-bushel increase resulted from lime alone, and a 7.2-bushel increase
from lime plus phosphate and potash (Collins et al., 1947). The pH of
these soils before liming ranged from 5.1 to 6.0.
A Richton silt loam in Arkansas with an initial pH of 4.9 produced
significantly higher seed yields after applications of 4 and 8 tons of
dolomitic limestone (Parks et al., 1959).
Soils in some areas have shown a tendency to be short of magnesium,
a minor element that is essential to green chlorophyll formation in leaves.
Magnesium and calcium must be in proper balance for best growth.
Surface applications of lime have been shown to move slowly
downward in the soil under humid conditions; thus, there is relatively
little advantage to deep placement by mechanical means. For a given soil
JACKSON L. CAR'ITER AND EDGAR E. HARTWIG
and climate, the rate and h a 1 depth of effect are functions of the amount
applied and the time elapsed (Brown et aZ., 1956).
2. Calcium and Magnesium Requirements
A soil in Illinois, with a pH of 4.1 and an exchangeable magnesium
level of 60 to 75 pounds per acre, gave increased yields of both corn
and soybeans from applications of 75 to 150 pounds of magnesium per
acre or 2 tons of dolomitic limestone (Key and Kurtz, 1960). The authors
considered a level of 150 pounds per acre of exchangeable soil magnesium to be adequate for field crops on soils of moderate to low exchange capacity.
A North Carolina soil low in magnesium showed a 4-bushel yield
response from 60 pounds of magnesium (Nelson et al., 1945).
A survey of farmer's fields in south Alabama showed that most fields
which appeared to be low in vigor had a p H of 4.9 or lower and/or
the available calcium below 200 pounds per acre. Soils with pH as low
as 4.4 and calcium as low as 120 pounds per acre were observed. Many
of these soils were also low in magnesium. A soil with a pH of 4.8
limed to give a pH of 6.2 gave a yield response of from 14 to 31 bushels
per acre when adequate potassium was supplied (Rouse, 1961).
Approximately 16 pounds of phosphorus are present in 40 bushels
of soybean seed. North Carolina results indicated that a yield response
was obtained from applied phosphorus when their soil test showed less
than 40 pounds available phosphorus in the soil (C. D. Welch, personal
communication). Nelson ( 1946), on a coastal plain soil in North Carolina,
obtained 6.4 bushels p e r acre of soybeans on a soil low in available
phosphorus, but 33.8 bushels with added phosphorus. Response curves
have been drawn by Bray (1961) which show that with 10 pounds of
available phosphorus per acre soybean yields will be 75 per cent of the
maximum expected, whereas with 30 pounds of available phoshoms
soybean yields will be at 98 per cent of maximum. This response to
phosphorus assumes other elements to be in adequate supply. The
response curve for soybeans is almost identical with that for corn
soybeans were grown on a Wooster silt loam in Ohio at soil
phosphorus levels of 53, 30, and 11 pounds per acre. The percentage of
total phosphorus derived from fertilizer was inversely related to the
level of soil phosphorus. When radioactive superphosphate was applied,
the plants near maturity had obtained about 25 per cent of their phos-
THE MANAGEMENT OF SOYBEANS
phorus from the fertilizer on the high phosphorus soil and nearly 60 per
cent on the low phosphorus soil. The high and medium levels of soil
phosphorus gave increases in total dry matter of 38 per cent and 9 per
cent over the low level, but all seed yields were at a 32-bushel level
(Bureau et al., 1953). These results suggest that some other factors were
limiting seed production, since North Carolina experiments on a low
phosphorus soil showed grain : straw ratios to be relatively constant when
additional phosphorus was added.
FIG.8. Relation of available phosphorus to percentage of maximum yield of soybeans and other crops. (From Bray, 1961.)
The availability of all the essential elements obtained by plants from
the soil is affected in some way or another by the reaction of the soil.
Phosphorus in particular becomes less available as the pH value drops
below 6.5. This result may be due to the interaction of phosphate with
hydrated iron oxides to form a basic iron phosphate (Truog, 1938;
Klemme, 1949; Pearson, 1958).
In a sand nutrient culture, the removal of magnesium did not retard
phosphorus absorption, but did have a significant effect on the movement and final location of phosphorus in the plant, resulting in a higher
percentage of phosphorus in the vegetative parts and a lower percentage
JACKSON L. C A R T E R AND EDGAR E. HARTWIG
in the seeds. Thus, according to Webb et al. (1954), magnesium may
function as a carrier of phosphorus in the plant.
The ratio of phosphorus to potassium may be as important in some
cases as the phosphorus level. Miller et al. (1961a, b), on a Dickinson
fine sandy loam in Iowa with a pH of 6.6 and testing low in available
phosphorus and very low in available potassium, showed that over 80
per cent of the variation in soybean yield which they obtained was accounted for by the variation in phosphorus and potassium content of
some plant parts. The greatest yield increase was obtained from heavy
applications of both phosphorus and potassium, but the greatest yield
depression resulted from heavy additions of phosphorus and no addition
of potassium. The variety they were using in the study (HAROSOY) is
sensitive to high levels of phosphorus in relation to other elements in
the nutrient solution.
Howell (1954) found an increase in relative growth of soybeans as
phosphorus level was increased from 2 parts per million (ppm) to 10
in the nutrient solutions, but a marked difference in varietal response to
a wider range in phosphorus level. At levels as high as 112ppm, the
variety CHIEF responded favorably in growth but LINCOLN showed definite
symptoms of phosphorus toxicity. Howell and Bernard (1961) have
classified commercial varieties with respect to phosphorus response.
Weiss (1943) modified iron chlorosis by changing phosphorus level
in the nutrient solution, but the iron inefficient character does not appear
to be closely related to the phosphorus toxicity reported by Howell.
Forty bushels of soybean seed will contain approximately 50 pounds
of potassium. North Carolina data suggest that response to applications
of potassium is likely when the available soil potassium is less than 75
pounds per acre ( C. D. Welch, personal communication). Response
curves drawn by Bray (1961) suggest that when the soil test shows 50
pounds of potassium per acre the soybean yield will be approximately
50 per cent of the maximum whereas with a soil test of 200 pounds, the
soybean yield will be at 97 per cent of maximum (Fig. 9). As with
phosphorus, the expected response curve for soybeans closely approximates that for corn.
On a Kalmia sandy loam in Alabama high in phosphorus but low in
potassium, the five-year average yield increase from 50 pounds of
potassium was 50 per cent (Rouse, 1961). Studies in North Carolina on
a soil very low in potassium showed a fourfold yield increase from
potassium. The addition of potassium caused greater retention of pods,
increased the degree of pod filling, and improved seed quality. The
THE MANAGEMENT OF SOYBEANS
application of 120 pounds per acre of potassium on this poor soil also
increased oil content about 2 per cent and reduced protein content
5 per cent (Nelson et al., 1945).
Indiana soybeans grown in rotation with corn and wheat gave equal
response to row or broadcast applications of potassium. The response was
proportional to the potassium applied and was closely correlated with
rainfall during the growing season (Barber, 1959).
FIG.9. Relation of available potassium to percentage
beans and other crops. (From Bray, 1981.)
yield of soy-
The abilities of peanuts, soybeans, corn, and cotton to absorb
potassium in small volumes of Ruston fine sandy loam were compared
in the greenhouse by Reid and York (1955). Peanuts, soybeans, and
corn absorbed essentially equal amounts of potassium, but cotton
tended to absorb slightly more under low potassium conditions. Potassium
deficiency symptoms appeared first and were most severe on corn
followed by cotton, soybeans, and peanuts. All four crops responded in
dry matter production to the application of potassium. In a second
cropping, the dry matter production in unfertilized soil for peanuts,
soybeans, cotton, and corn was 69, 85, 45, and 20 per cent, respectively,
of the plants fertilized with potassium.
JACKSON L. CARTIER AND EDGAR E. HARTWIG
Soybean seed is very sensitive to soluble-salt injury during germination, so potash should not be drilled directly with the seed. Placement
studies show best results are obtained from band placement 2 inches to
the side and 2 inches below the seed.
When studying cation uptake, the soil temperature is important.
Wallace (1957) found that the potassium content of soybeans increased
with temperature to 90"F., but the potassium in barley increased from
54" F. to 72" F. and decreased from 72" to 90".
In addition to calcium, magnesium, potassium, phosphorus, and
nitrogen, several other elements are essential for satisfactory development of the soybean plant. The elements iron, manganese, cobalt, sulfur,
boron, zinc, copper, and molybdenum are usually present in adequate
quantities, but soils occur in which one or more of these materials might
limit crop production.
The visible sign of iron deficiency in the soybean is a yellowing or
chlorosis of the leaves. Iron is required for chlorophyll synthesis and
respiration. In most soils the iron supply is adequate and conditions are
generally favorable for its absorption by soybean plants. Iron deficiency
symptoms are generally observed only on soils with a high pH and a
high calcium carbonate content. Weiss (1943) observed a differential
response of soybean varieties growing on a calcareous soil in Iowa.
Iron deficiency can be corrected by spray applications of ferrous sulfate.
Manganese, like iron, aids in the formation of chlorophyll. Manganesedeficient soybeans have a light green to yellow mottling between leaf
veins, Manganese deficiency symptoms are most likely to occur when soybeans are grown on soils limed to near the neutral level. According to
Ohlrogge (1950),low levels of available manganese in soils are associated
with a soil pH of above 6.3 on soils that have developed under a high
water table. This condition encourages the reduction of the manganese
to the soluble form which is consequently leached from the soil.
Manganese sulfate has been found to be more effective than manganese
oxide for correcting manganese deficiency in soybeans (Mederski et al.,
1960). Low soil temperature combined with high soil moisture was
conducive to the development of severe foliar symptoms of manganese
The relatively short time required for leaves to accumulate high
concentrations of manganese probably accounts for the effectiveness of
manganese sprays applied under a variety of field conditions. Manganese
can be applied dry at a rate of 25 pounds or more of manganese sulfate
per acre at planting time or as 10 pounds of manganese sulfate in 15
THE MANAGEMENT OF SOYBEANS
gallons of water per acre as soon as deficiency symptoms appear.
Repeated yearly applications of manganese are needed on fields where
this trace nutrient is known to be deficient. Manganese applied to the
soil changes to an unavailable form during the growing season (Mederski
and Jones, 1961).
Plant species and varieties differ in their capacity to take micronutrients from the soil. Studies at North Carolina showed that OGDEN
soybeans took up only 114 ppm of manganese from a field where LEE
soybeans in an adjacent row took up 197ppm.
Molybdenum, unlike manganese, boron, iron, copper, and zinc,
becomes increasingly available as the soil pH is raised (Reid et al., 1960).
Molybdenum is needed by the plant and in the symbiotic fixation of
nitrogen or in the reduction of nitrate (Anderson and Spencer, 1950;
Evans, 1956). Soils in need of molybdenum are quite rare in the United
States. Parker and Harris (1962), working in Georgia on a soil with a
pH of 5.6, obtained yield responses from applications of 0.2 pound
molybdenum per acre equivalent to applications of 2 tons of limestone.
The application of molybdenum further increased protein content of
the seed on a limed soil. Untreated plots yielded at the rate of 30
bushels per acre, but with the addition of molybdenum or limestone,
yields of 50 bushels were produced. Caution should be observed by
applying molybdenum only to soils that are deficient in this element, as
plants may accumulate levels that are toxic to animals (Stout and
Cobalt is accumulated from the soil by plants, which in turn become
the primary source of cobalt for animals. While cobalt is essential for
animals and for synthesis of vitamin B12, plant needs are met by as little
as 1part per billion in nutrient solutions. One part per million produced
toxic symptoms and resulted in growth reduction (Toth and Romney,
1954). In many field trials in the Midwest, as little as 2.5 g. of cobalt
applied on a bushel of soybean seed at planting time caused observable
toxic symptoms on the unifoliate leaves (A. J. Ohlrogge, personal communication). If cobalt is applied to soil to enrich the resulting crop for
animal feeding, extreme care should be exercised not to apply toxic
Sulfur, essential to plant life, is a part of methionine and other amino
acids in protein and also occurs in the vitamins thiamine and biotin.
According to Baxter (1952), aside from functioning as a building material,
sulfur is important in formation of chlorophyll and holds essential
elements such as iron and manganese in solution. The average sulfur
content of soybeans is about 0.16 per cent. Sulfur requirements of soybeans have been found to be lower than for cotton (Kamprath et al.,
JACKSON L. CAR-
AKD EDGAR E. HARTWIG
1957). It usually enters the plant from the soil as the sulfate ion, and
on most soils, especially those fertilized with superphosphate, the sulfur
supply is adequate for the needs of the plant.
Boron is essential to normal cell division and growth and the general
metabolism of plants (Russell, 1957). In a few instances, yield increases
have been obtained from boron application (S. R. Wilkinson, personal
communication), but usually a soil that grows good alfalfa has adequate
‘loron for soybeans. Boron deficiency occurs at higher boron levels in
the presence of high calcium levels (Reeve and Shive, 1944). Boron
deficiency in soybeans is easily corrected by light applications of borax,
but treatments should be made with caution as high levels of the element
are toxic to the plant (Mederski and Jones, 1961). Boron toxicity decreases with increasing concentration of calcium ( Berger, 1949).
Zinc is essential for soybeans, though needed in very small amounts
(Seatz and Jurinak, 1957). Zinc deficiency produces a light brownishyellow color to the leaves. The symptoms are more severe in cold, wet
weather, disappearing in warmer sunny weather. Where symptoms are
severe, yield may be severely reduced (Weldon and Chesnin, 1959).
Apparently, zinc is readily redistributed in the plant as the most severe
symptoms occur on the older leaves (Viets et al., 1954). Heavy liming of
a soil may lower the availability of zinc; also, heavy phosphorus fertilization may reduce zinc absorption (Bingham and Martin, 1956). Treatments of 50 pounds Es-Min-El or 5 pounds of zinc sulfate corrected
zinc deficiency in soil from the Black Belt of Mississippi. In this
case, a vegetative response of soybeans (LEE) to zinc fertilization was
obtained, although severe chlorosis of the unfertilized plants did not
occur (Nelson, 1956).
Copper deficiency may cause severe stunting of growth, but moderate
deficiency may merely reduce yields. The rate of photosynthesis is low
in copper-deficient plants, and there is evidence that the element is
involved in oxidation-reduction reactions and as an enzyme activator
(Reuther, 1957). Response of soybeans to copper has been observed on
peat and muck soils in the Everglades (Allison et al., 1927) and in
Indiana ( S. R. Wilkinson, personal communication).
Aluminum, commonly present in the soil in large amounts, may be
essential to plants, though this is difficult to demonstrate, and it is not
likely that it will ever be deficient (Bear, 1957). Aluminum is toxic to
soybeans only in strongly acid soils, and proper liming to pH 6.0 will
ordinarily correct any toxicity (Kamprath, 1958).
Chloride content of the tops in a soybean variety (LEE), on plots
receiving variable amounts of KCl, was found to be low and essentially
constant, whereas, the chloride content of corn tissue varied with the