Fertilizing Soybean
SF-719 (Revised), October 1992
W. C. Dahnke, Soil Testing and Soil Science Dept.
C. Fanning, Extension Soils Specialist
A. Cattanach , Extension Soils/ Sugarbeet Specialist
Soybean is a long season legume crop that do best on well-drained
fertile soil. While soybeans do not always respond to fertilizer the year that it is
applied, it is well known that soybeans do much better on fields in which the phosphorus
and potassium soil test levels have been built up by previous fertilization. Fields that
are high in available nitrogen should be planted to non-leguminous plants because legumes
can get their nitrogen from the air.
Inoculation:
Soybean, like other legumes, can fix nitrogen from the air but will take nitrogen from
the soil first if it is available. When available soil nitrogen is depleted, legumes will
get their nitrogen from the air if they have been inoculated with the right strain of
bacteria (rhizobia). The inoculum should be mixed with the seed at planting time. If the
seed has been treated with a fungicide, check to see if it is compatible with the inoculum
before using a seed treatment.
Yield Goal:
Yield is influenced by: 1) local climate; 2) soil type; and 3) management (timeliness
of field operations, plant population, variety, soil fertility, weed control, etc.). Yield
goals should be realistic. The are usually based on long-time averages and on management
ability of the grower but adjusted to conditions expected for the upcoming year (see
Circular SF-822).
Excessive fertilizer use, especially nitrogen and phosphorus, has potential to degrade
ground and surface water quality. Establishing realistic yield goals, carefully soil
sampling fields and fertilizing crops according to soil tests will help preserve water
quality.
Nutrient Recommendation:
Legumes without nodules or with ineffective nodules will respond to nitrogen
applications like any other crop. Since legumes have the ability to fix nitrogen, it is
important to inoculate soybean seed just before planting, especially on fields that have
not recently been planted to soybeans. Crop response to phosphorus and potassium are not
always noticeable in the year of application.
Approximately 60 percent of the phosphorus and 50 percent of the potassium taken by
soybean plants is removed from the field when the seed is harvested. One bushel of soybean
contains about 0.75 pound of P2O5 and over a pound of K2O
per bushel. The recommended broadcast rates of phosphate and potash for
soybean is given in Table 1.
Nutrient recommendations for soybean.
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Soil Test Phosphorus, ppm
------------------------------------
VL L M H VH
Yield Bray-I 0-5 6-10 11-15 16-20 21+
goal Olsen 0-3 4-7 8-11 12-15 16+
-------------------------------------------------
bu/a - - - - - lb P2O5/acre - - - - - - -
30 35 20 10 0 0
40 50 30 10 0 0
50 60 35 10 0 0
60 70 40 10 0 0
-------------------------------------------------
Soil Test Potassium, ppm
------------------------------------------
VL L M H VH
Yield Bray-I 0-40 41-80 81-120 121-160 161+
goal Olsen
-------------------------------------------------
bu/a - - - - - - lb K2O/acre - - - -
30 55 35 10 0 0
40 75 45 15 0 0
50 90 55 20 0 0
60 110 65 20 0 0
-------------------------------------------------
Bray-I P recommendation = (1.55-0.10 STP)YG
Olsen P recommendation = (1.55-0.14 STP)YG
Potassium recommendation = (2.2000-0.0183 STK)YG
The abbreviations used in the equations are as
follows:
YG = yield goal
STP = soil test phosphorus
STK = soil test potassium
Fertilizer Placement:
Phosphate and potash fertilizer can be applied broadcast and incorporated into the soil
before planting or applied as a starter at planting time. If applied as a starter, the
recommended placement of the fertilizer is in a band 2 inches to the side and 2 inches
below the seed. "Popup" (a small amount of fertilizer placed in contact with the
seed) should not be used on soybean. Soybean is very susceptible to fertilizer salt
injury.
Since phosphorus and potassium move very little in the soil, it is possible to
"build up" or increase the available level of these nutrients in the soil. The
application of approximately 20 pounds of P2O5 per acre will
increase the phosphorus soil test level by 1. In other words, if your phosphorus soil test
level is 5 and you prefer to operate at test level of 12, the application of 140 pounds of
P2O5 (305 pounds of 18-46-0) per acre thoroughly mixed in the top 6
inches of soil will raise the soil test level by 7. Likewise, the application of 10 pounds
of K2O per acre will increase the potassium soil test by 1.
Zinc Chlorosis:
Zinc (Zn) deficiencies have been found in isolated areas in the state. Problem areas
are generally limited to sandy soil. However, zinc deficiencies are not uncommon on soils
with high calcium carbonate levels at the soil surface or where topsoil has been removed
in leveling for irrigation.
The first symptom of Zn deficiency in soybean is usually a light green color developing
between the veins on the older leaves. New young leaves will be abnormally small. Bronzing
of the older leaves may occur. When the deficiency is severe, leaves may develop necrotic
spots. Shortened internodes will give plants a stunted, rosetted appearance.
A soil test for Zn is available from the NDSU Soil Testing Laboratory and is helpful in
identifying soils with suspected Zn problems. For soils testing very low or low in Zn,
(0-.5 ppm) rates of 3 to 5 pounds per acre of actual Zn as zinc sulfate (36% Zn) or 0.5 to
1 pound per acre actual Zn in an organic (chelate) form is recommended. For
marginal-testing soils (.51-1 ppm), use the same rates of Zn on a trial basis.
Broadcasting and plowing down Zn has been a successful method of application. A Zn
deficiency on young bean plants can also be corrected by a foliar application of zinc
during the first six weeks of growth. Use a foliar application of 1 pound of Zn in 25 to
30 gallons of water. Zinc sulfate and other soluble inorganic or organic sources are
suitable for foliar application.
Iron Chlorosis:
Iron (Fe) deficiency (chlorosis) may be observed in soybean, especially on high calcium
carbonate level soils during cool, wet periods. The youngest leaves of Fe deficient plants
will be distinctly yellow. The interveinal areas of the leaves will be bright yellow while
the veins remain green. Soil treatments for correcting Fe deficiences are not usually
effective. A suggested foliar treatment would be to dissolve 20 pounds of ferrous sulfate
in 100 gallons of water and apply at the rate of 10 to 20 gallons per acre. This will
usually quickly eliminate deficiency symptoms but may not result in a profitable yield
increase. Variety selection for high-lime soils can help counteract Fe chlorosis. Some
varieties are quite susceptible to iron chlorosis. Iron chlorosis is more common than zinc
chlorosis in North Dakota.
Other Nutrients:
To date, responses to other micronutrients have not been demonstrated in the state.
Remember that profitable yield of soybean is more likely on high testing soils at high
levels of management. Plant recommended varieties at optimum stands in narrow rows. Weed
control is very important.
SF-719 (Revised), October 1992
NDSU Extension Service, North Dakota State University of Agriculture and Applied
Science, and U.S. Department of Agriculture cooperating. Sharon D. Anderson, Director,
Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June
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