SOYBEAN PRODUCTION IN KENTUCKY PART II:
SEED SELECTION, VARIETY SELECTION AND FERTILIZATION
James H. Herbek and Morris J. Bitzer
Department of Agronomy
Planting high quality seed is essential
to a profitable production system and the first step to getting an optimum
stand. If you fail to plant good seed, you will have poor stands, slow
growth, weedy fields and lower yields. High quality seed has high varietal
purity, high germination (80% or above), uniform size, low foreign material,
no weed or other crop seed and little mechanical damage. Certified seed
is the main source meeting these requirements.
Select your seed carefully since seed
lots vary in quality. Selecting seed by sight and judgment is not good
enough since you cannot accurately tell whether good-looking seeds have
all the attributes of high quality seed. Analysis tags showing seed quality
factors are helpful and important.
Don't take chances on seed quality.
Purchase certified seed each year. The small per acre cost difference between
certified quality seed and low quality or unknown quality seed is relatively
inexpensive insurance to give your crop a good start. Yield comparison
studies from several states have shown that high quality seed is beneficial.
In these studies, the yield advantage for certified seed over non-certified
seed ranged from 0-3 bu/acre.
Many farmers still save their own seed
for planting, which is satisfactory if high quality is maintained and if
the seed quality is known before planting. Take extreme care that no damage
occurs to the seed during harvest, handling and storage. Clean the seed
to remove foreign matter, weed and other crop seeds and excessive splits.
Run germination tests to determine seed viability as close to planting
time as possible since soybean germination tends to decline over time.
Soybean seeds are inherently short
lived. They deteriorate more rapidly with age because of their high oil
content and are also more easily damaged during harvest, handling and storage
than any other major farm crop. For these reasons, mechanically damaged
seed and seed improperly stored for more than one year are poor risks for
Seed vigor may provide a more accurate
appraisal of seed quality relating to field emergence than the standard
germination test. While standard germination measures viability (i.e. the
seed's ability to germinate and develop into a normal seedling under ideal
field conditions), seed vigor measures field performance potential (i.e.
the seed's ability to establish a normal seedling under a wide range of
field conditions, including adverse conditions such as cool, wet soils).
As seed deterioration progresses, seed
vigor will decline before germination declines. Thus, 2 seed lots with
good germination could have great differences in plant emergence under
adverse field conditions if they differed in seed vigor. Vigor differences
among seed lots, if known, would be important to both the farmer and seedsman.
While many seed vigor tests have been
developed and described (these include seedling growth rate, speed of germination,
accelerated aging, cold, tetrazolium chloride and conductivity), they have
not become readily available as a routine seed testing procedure because
of inconsistent reliability or repeatability of the tests. Most of the
difficulties have arisen because of nonstandarized testing procedures.
Even though seed vigor tests are not presently standardized among seed-testing
laboratories, some of the vigor tests are being routinely used by seedsmen
to test soybean seed for vigor as an in-house management tool for monitoring
the quality of seed lots. Progress is being made toward reducing test variability.
As seed vigor testing procedures become standardized and accepted and if
the results are properly interpreted, soybean seed may eventually be labeled
for vigor, which would be valuable for the farmer as well as the seedsman.
Treating seed with a fungicide before
planting can help improve germination, seedling vigor and stands by reducing
seed decay, damping off and seedling blights. Seed treatment would be helpful
under the following conditions:
1. seed that is not of high
2. seed with less than 80% germination,
3. when weather conditions at
planting do not favor rapid germination.
Although seed treatment will improve
stands under these conditions, yield increases do not always occur. Seed
treatments have shown little advantage when used on high quality seed and
are not generally recommended with a germination of 80% or more. Likewise,
seed treatment will not improve germination of seed that has been mechanically
Materials for seed treatment are sold
under several trade names and formulations. Consult the product labels
for instructions about rates, methods of application and use restrictions
(particularly with regard to inoculation). Once seeds are treated with
a fungicide, they cannot be marketed or used for food, feed or oil.
For current seed treatment recommendations,
get PPA-6, Fungicide Seed Treatment for Control of Grain Diseases,
from your county Extension office.
Selecting a variety is only one of
the many decisions a soybean grower must make. Yet, it is among the most
important. Soybean varieties can differ greatly, not only in yield potential,
but in many other characteristics. Using adapted varieties with the best
combination of desirable characteristics for your particular situation
will maximize returns. In selecting a soybean variety you should consider
1) maturity, 2) yielding ability, 3) standability,
4) shattering and 5) disease resistance.
Unlike many other crops, such as corn,
whose time of maturity largely depends on temperature, soybeans' maturity
is largely controlled by day length. A soybean plant is photoperiod sensitive,
which means it makes the transition from the vegetative to the reproductive
phase of growth in response to changing day length. The key to the initiation
of flowering, however, is actually the length of darkness.
Each variety depends on a critical
period of darkness to change from the vegetative to the reproductive stage.
When this critical night length is reached, the soybean variety will begin
flowering no matter how much vegetative growth it already has. An early
variety is sensitive to a shorter night and, therefore, requires fewer
hours of darkness to initiate flowering than does a later variety. A late
variety requires longer hours of darkness to initiate flowering, which
allows a longer period of vegetative growth, and thus it matures later
in the season. For this reason full-season varieties should be favored
over early varieties for late plantings. They have more time for vegetative
growth and therefore grow taller which results in a potential for better
For comparisons of relative maturity,
soybean varieties are divided into 10 maturity groups, which form narrow
bands from north to south across the US (Figure
1). These groups are ranked in succeeding order of maturity from the
earliest (Group 00) to the latest maturity (Group VIII). The earlier maturing
varieties which flower and mature under a relatively long day (shorter
nightlength) are best adapted to the short-season conditions of the northern
states. Each succeeding group requires a slightly longer growing season,
and is adapted further south with Group VIII requiring the longest growing
season. As maturity groups move south, varieties in each group require
slightly shorter days and longer nights to flower and mature.
Each variety is placed in the maturity
group where it will use most of the growing season and yet mature safely
before frost (i.e. adapted as a full-season variety). However, the maturity
zones for each group are relative: a variety can be grown outside its zone
and provide suitable performance, but the limit on adaptability is only
about 100 miles north or south. When a variety is grown out of its area
of adaptation, poor performance results. For example, when a variety adapted
to a southern latitude (shorter days and longer nights) is grown further
north, the longer day lengths and cooler night temperatures cause it to
remain growing vegetatively longer, resulting in an excessively tall plant.
Since the start of flowering may be delayed, it may not mature before frost.
Likewise, a variety adapted to a northern latitude, when grown further
south under shorter day lengths and warmer nights, will cease vegetative
growth earlier than normal, resulting in smaller plants and reduced yields.
Thus, for maximum yields grow soybean varieties adapted to your particular
Varieties best adapted in Kentucky
are in maturity Groups III, IV and V. In general, the earlier maturing
varieties (Group III) are best adapted in the northern portions of Kentucky
(approximately the areas north of the Western Kentucky Parkway) and the
later maturing varieties of Group V to the southern portions of Kentucky.
Varieties in Group IV are considered mid-season varieties for the state
and can be grown successfully in most areas of Kentucky. However, when
grown in southern parts of Kentucky, they are considered somewhat early
and when grown in northern parts, somewhat full-season. Remember that the
terms early, mid or full-season variety are relative terms with specific
context only for the area in which a variety is to be grown. That is, an
adapted full-season variety for one area would not be considered a full-season
variety if grown to the north or south. A Group V maturity variety, considered
full-season in southern Kentucky, would be considered very late if grown
in northern Kentucky and an early maturity variety if grown in southern
Varieties in the various maturity group
classifications are also related to plant growth type. Most varieties grown
in the North are indeterminate growth types. With indeterminates, flowering
occurs earlier in the season and occurs simultaneously with vegetative
growth for several weeks so that the vegetative and reproductive growth
periods overlap each other. Generally, varieties in maturity Groups 00
through Group IV are indeterminate. On the other hand, most southern varieties
are determinate. With determinates, flowering does not occur until most
of their vegetative growth has been attained. In general, varieties in
maturity Groups V through VIII are determinate. There is also a growth
type intermediate between the two main types called semi-determinates which
have some characteristics of both determinates and indeterminates in both
appearance and growth habit.
In most years, mid to full season varieties
for a given area will yield the highest. However, early varieties sometimes
outyield full-season ones because of unusual rainfall and other weather
conditions. In some situations an earlier variety may be preferred, such
as the need to establish a winter crop early in the fall or situations
where soybean harvest is frequently delayed by wet weather if not harvested
Planting several varieties with different
maturities can be of benefit, especially when large acreage are involved.
This management tool can spread harvest so that the entire crop has an
optimum harvest period; reduce weather risks to the crop during the growing
season and at harvest; and also reduce risks against serious losses from
diseases. If you want to have your crop mature over an extended period,
select varieties from different maturity groups or select among varieties
within a maturity group since time of maturity varies slightly (8 to 16
days) among varieties within the same group.
If several varieties with different
maturities are to be planted, the early maturing varieties should be planted
first since they perform best when not planted late. Mid and full season
varieties are usually considered multi-purpose and may be planted early
Selecting a variety from among those
with the highest yielding potential is obviously important. If other special
varietal characteristics are not a concern, then selection based on yielding
ability will net the most profit.
The results from the Kentucky Soybean
Performance Tests provide an excellent starting place for selecting varieties.
These tests containing varieties from both private companies and public
institutions are conducted at several locations in Kentucky and the results
are presented in an annual progress report available from your county Extension
office. In addition to annual and multiple year yield performance, these
tests provide data on lodging, plant height, pod height and maturity. Since
the relative performance of different varieties can vary from year to year
depending on weather and management, look at 2 or 3 year average yields
when selecting a variety. This provides a good indication of a variety's
stability and production potential over time and varying conditions.
A variety must be able to remain erect
throughout the growing season to produce maximum yields. If lodging occurs
before maturity, yields decrease. This decrease can be quite substantial
(15-30%) if it occurs during or before the pod and seed-filling period.
Lodging further increases losses at harvest due to reduced harvest efficiency.
Since varieties differ in tendency
to lodge, using resistant varieties controls lodging. Usually shorter varieties
will stand better than the tall ones. Although it can be controlled through
breeding, lodging is also greatly modified by the environment. Irrigation,
high fertility and high plant populations all accentuate lodging.
Varieties differ considerably in shattering
resistance (the ability to hold seeds in the pod). Shattering losses are
particularly harmful in that they diminish yield potential that has already
been achieved. Avoid varieties which shatter unless they are exceptional
in other respects and unless harvest can be done in a timely fashion. Avoid
any variety that cannot withstand shattering for 2 to 3 weeks after maturity.
Although shattering resistance was
not a main selection criterion in release of many of the older varieties,
more breeding programs today do consider shatter resistance. In general,
as a group, earlier maturing varieties for a given area of adaptation tend
to have less tolerance to shattering than do later maturing varieties.
However, this characteristic may be due to their earlier maturity since
great differences can be found among varieties in any maturity group. Environmentally,
successive periods of wet and dry weather before harvest accentuate shattering.
A practical and economical way of controlling
soybean diseases is to use resistant varieties. Since no one variety contains
tolerance or resistance to all diseases, in situations where a particular
disease is causing economic losses, choose a variety with resistance or
tolerance to that disease.
Soybean breeding programs emphasize
resistance to the most serious diseases in a state or region. In Kentucky,
the most serious problem is soybean cyst nematode. Selection of the proper
variety is very important if the soil is infested with cyst nematode and
such selection should be used along with other cultural practices. Presently,
resistant varieties are available for all 3 soybean maturity groups grown
in Kentucky. As new resistant varieties are released, be aware of their
resistant characteristics since more than one race of cyst nematode is
found in the state and also because a variety termed resistant is not necessarily
resistant to all races.
Soybeans grow best on highly fertile
soils. A soybean fertilization and liming program should be based on the
soil's fertility status. Use soil tests to determine available plant nutrients
in the soil. Soil tests, when used with past fertilizer management and
cropping history, provide guidelines for the fertilizer needs of soils.
Plant analysis, a laboratory determination of nutrient elements from a
sample of plant tissue, may verify a suspected nutrient problem or evaluate
a crop's nutrient status. Plant analysis is not a substitute for soil testing
but is most effective when used with a regular soil testing program.
Many field studies have been conducted
by the University of Kentucky to determine the needs for primary nutrients
and micronutrient in the state. Yield and soil test data from these studies
serve as guidelines in establishing nutrient recommendations. For current
lime and fertilizer recommendations, get AGR-1, Lime and Fertilizer
Recommendations, from your county Extension office.
Lime neutralizes soil acidity by correcting
the soil pH. Proper soil pH is important and will affect almost all the
fertility factors important to growing soybeans. If soil pH is too high,
it can lead to reduced manganese and zinc availability; or it can be too
low and cause problems with molybdenum deficiency, decreased phosphate
availability, or toxic levels of aluminum or manganese. The desired pH
level for soybeans is between 6.2 and 6.8, which allows for some safety
and assures that maximum production can be achieved.
The most frequent problem with soybean
production on Kentucky soils is a low soil pH. Soybeans respond very economically
to liming of acid soils. Since the soybean plant is a legume, it requires
the proper soil pH for good nodulation, nitrogen fixation and plant growth.
As the soil pH drops below 6.0, soybean yields will likely decrease, largely
due to molybdenum deficiency and reduced nodulation and nitrogen fixation,
Aluminum or manganese toxicity can be expected to be a problem as the pH
drops below 5.5.
Apply lime according to soil tests.
The most common source of lime for agricultural use is ground limestone,
available in most of the state. Since the quality of aglime can vary among
quarries, know its effectiveness as a liming material to neutralize soil
acidity so that proper rates can be applied. The neutralizing value of
aglime is determined by both purity and fineness and is an estimate of
the percentage of aglime that will be available to neutralize soil acidity
over 4 years. For further information on aglime see AGR-106, Determining
the Quality of Aglime: Relative Neutralizing Value (RNV), from your
county Extension office.
Lime can be applied anytime. However,
since it takes 6 to 12 months for lime to materially increase soil pH,
apply it at least 6 months before planting soybeans. Fall applications
are practical since they allow enough time for the lime to react before
the next growing season. For maximum benefits, thoroughly mix lime with
the soil, especially when the soil pH is very low and large amounts of
lime are needed. Although lime cannot be mixed into the soil on no-till
fields, surface applications are effective, but take longer before the
lime affects the acidity below the surface.
Soybeans require large amounts of nitrogen,
about 4 lb/bu. Since soybeans are a legume, they can provide the nitrogen
they need through nitrogen-fixing bacteria, which form nodules on the roots
and can fix atmospheric nitrogen for the plant, or from uptake of soil
mineralized nitrogen. Research has shown that a yield response to applications
of nitrogen fertilizer is unlikely if soils are maintained at proper pH
(limed) and the soybeans are well-nodulated. Consequently, no nitrogen
is recommended for well-nodulated soybeans. In fact, nitrogen added as
fertilizer can reduce or deter the fixation of nitrogen in the nodules.
However, an application of fertilizer nitrogen may be of value where:
•poor nodulation has occurred because
of poor inoculation,
•acid soils exist where molybdenum
is not used,
•other harsh soil environments that
result in nitrogen deficiencies.
Nitrogen deficiency is characterized
by slow, small growth, small, pale green leaves, yellow, old leaves, and
early defoliation. Nitrogen deficiencies due to poor nodulation can normally
be overcome with nitrogen application before early pod set.
Consistent yield responses to phosphorus
(P) fertilization of soybeans have been generally limited to soils testing
very low and low in available P even though each bushel removes about 0.9
lb P2O5. With medium testing soils, responses to
direct phosphorus fertilization have been inconsistent and normally quite
small. Little or no yield response to direct fertilization can be expected
on soils testing medium-high to high in P. In general, the highest soybean
yields have been obtained on soils having a medium to high soil test.
Phosphorus is considered an immobile
element because it can react with the soil in ways that minimizes its movement
with soil water. For this reason, leaching of P from Kentucky soils is
of little importance. However, loss of P by topsoil erosion can be a concern.
Several sources of fertilizer P are available (liquids, fluid suspensions,
and dry sources). Performance of the material is considered equally effective
for agronomic purposes when used at recommended rates and properly applied.
Material preference is primarily related to ease of handling and cost.
Phosphorus deficiency symptoms may
be hard to recognize. They generally appear as thin and dwarfed stems,
lack-lustre leaves and early defoliation. Leaves tend to be more erect
and form an acute angle with the stem. A limited supply of P reduces the
number and efficiency of nodule bacteria. Correcting a P deficiency in
the same crop year is difficult.
Soybeans remove higher amounts of potassium
(K)in the grain (about 1.4 lb K2O/bu) than do most other crops.
Yield responses on the average for K are very similar to those obtained
for P under similar soil test levels. Soybeans are likely to respond to
K fertilization on soils testing very low or low in available K. Medium
testing soils give infrequent, small responses to K fertilization. Soybeans
are not likely to respond to direct application of K fertilizer on medium-high
to high testing soils.
Potassium is also considered an immobile
element because it can react with soil fractions and be retained by the
soil's cation exchange capacity. As a result, leaching of K is of little
importance except for very sandy soils. Several sources of K fertilizer
can be used, all equally effective.
Potassium deficiencies appear as stunted
growth with shortened internodes. Leaf edges are generally curled downward
(cupped) and the leaf margins show yellowing and browning, especially on
lower leaves. Correction through prevention (i.e. soil testing) is the
Application of P & K
For direct application of P and K to
soybeans, broadcasting is most convenient and practical, especially with
large amounts. Fertilizer does not have to be applied directly to soybeans.
Soybeans do well on residual fertility from previous crops, if enough fertilizer
is applied to the previous crop to also meet the soybeans' needs. This
practice is often used when double-cropping soybeans with small grains.
The required amounts of P and K for both crops can be applied to the small
grains in the fall.
Banded or row applications are suggested
where small amounts are being used and are more efficient than broadcasting
on low testing soils. Rates suggested for broadcasting on low testing soils
could be reduced by 1/3 if banded. Since soybeans are particularly susceptible
to fertilizer (salt) injury, avoid fertilizer contact with the seed. Banded
applications should be 2 inches to the side and 2 inches below the seed.
Do not place fertilizer with the seed or stand reduction may occur, especially
under dry conditions.
The increasing amount of conservation
tillage limits the methods of application to essentially surface broadcasting,
particularly with no-tillage. As a result, producers often worry whether
enough nutrients are available, particularly when they have not been incorporated.
Research indicates that fertilization is not a major obstacle in no-tillage
and that surface applications of P and K have not reduced yields. When
soil tests are medium or higher, yield results are comparable to tillage
systems where fertilizer has been distributed in the soil. It has been
suggested, however, that soils with low fertility levels be built to medium
or high soil test levels before using conservation tillage.
Foliar fertilization of soybeans is
not recommended as a means of supplying the major nutrients (N, P, and
K). The large requirements for these nutrients means applying them to the
soil is the only practical way. Numerous foliar fertilization studies in
Kentucky showed inconsistent results, with essentially no increase in yields,
with yield reductions in some cases due to foliar burning.
Soils in Kentucky usually supply adequate
secondary and minor elements for optimum soybean growth. Only molybdenum
and manganese deficiencies have been observed to any extent in Kentucky.
Both usually result from improper soil pH levels.
Molybdenum--Symptoms of molybdenum
(Mo) deficiency are stunted plants with pale green or yellow leaves typical
of nitrogen shortage. In fact, these symptoms are caused by a lack of nitrogen
rather than a lack of Mo in the leaves. Mo is required by the nitrogen
fixing bacteria in the soybean nodules. Without adequate Mo, soybeans fail
to nodulate properly and show nitrogen deficiency symptoms. The availability
of soil Mo to plants is closely related to soil pH. As soil acidity increases
(lower soil pH), the availability of soil Mo decreases and deficiencies
Mo deficiencies should not occur if
soils are properly limed to pH 6.2 or above. Therefore, the best recommendation
is a good liming program. However, when lime hasn't been or cannot be applied
far enough ahead of planting to get the pH above 6.2 at seeding time, Mo
application is recommended. An application of 1 to 2 oz of sodium molybdate
(0.4 to 0.8 oz of elemental molybdenum)/acre as a seed treatment is satisfactory
for applying Mo where no seed inoculant is needed. If seeds are to be inoculated
at the same time, they must be planted immediately or the bacteria's viability
will be reduced considerably. A safer method, if seed is to be inoculated,
is to broadcast the Mo on the soil by dissolving the sodium molybdate in
water and spraying it on the soil before final seedbed preparation.
Molybdenum should not be used as a
substitute for a good liming program since the proper pH has many other
beneficial effects in addition to Mo availability. Also, when the pH level
is too low (below 5.5), Mo may not be effective. ln Kentucky, the use of
Mo should be considered an emergency treatment to permit growing a soybean
crop until soil pH can be adjusted through a good liming program.
Manganese--Manganese (Mn) deficiency
in soybeans is characterized by interveinal chlorosis on newer leaves,
while the veins remain green. The severity of the deficiency can be determined
from the interveinal leaf color as it varies from pale yellow to almost
white. In Kentucky, Mn deficiency has been mainly confined to medium and
fine-textured soils having a pH of 6.5 or above. High pH and poor drainage
are contributing factors; however, treatments to reduce soil pH are not
Foliar applications of Mn on deficient
soybeans after symptoms appear have provided satisfactory results and have
been superior to soil applications made at planting. Foliar manganese spray
is recommended in two forms: chelated manganese or a solution of manganese
Suggested References and Related Publications
University of Kentucky Publications
AGR-1, Lime and Fertilizer Recommendations
AGR-5, When to Apply Lime and Fertilizer
AGR-16, Taking Soil Test Samples
AGR-19, Liming Acid Soils
AGR-57, Soil Testing: What It Is and What It Does
AGR-92, Sampling Plant Tissue for Nutrient Analysis
AGR-106, Determining the Quality of Aglime: Relative Neutralizing Value
AGR-111, Soybean Varieties
AGR-128, Soybean Production in Kentucky-Part I: Status, Uses and Planning
AGR-130, Soybean Production in Kentucky-Part III: Planting Practices
AGR-131, Soybean Production in Kentucky-Part IV: Weed, Disease and
AGR-132, Soybean Production in Kentucky-Part V: Harvesting, Drying,
Storage and Marketing
4BB-04PO, Kentucky 4-H Soybean Project
PPA-6, Fungicide Seed Treatment for Control of Grain Diseases
Kentucky Soybean Performance Tests (Annual Progress Report)
How a Soybean Plant Develops, Sp. Rept. 53 (Available from Iowa State
Univ., Ames, Iowa) (For sale only)
Modern Soybean Production (Available from American Soybean Association,
St. Louis, MO) (For sale only)