Soybean Soil Fertility
SF-1164, February 1999
D.W. Franzen, NDSU Extension Soil Specialist
Soybeans need the 13 mineral nutrients, nitrogen (N),
phosphorus (P), potassium (K), sulfur (S), calcium (Ca),
magnesium (Mg), zinc (Zn), manganese (Mn), copper (Cu), iron
(Fe), boron (B), chloride (Cl) and molybdenum (Mo). Of these,
North Dakota soils provide adequate amounts except for nitrogen,
phosphorus, potassium, sulfur and iron. Rare instances of
manganese and zinc deficiency have also been seen, but their
occurrence is of only minor importance.
Nitrogen
Although the atmosphere is 78% nitrogen gas, plants cannot use
it directly. Plants can use only ammonium-N, or nitrate-N.
Soybean is a legume and should normally provide itself N through
a symbiotic relationship with N-fixing bacteria of the species Bradyrhizobium
japonicum. In this symbiotic relationship, carbohydrates and
minerals are supplied to the bacteria by the plant , and the
bacteria transform nitrogen gas from the atmosphere into
ammonium-N for use by the plant.
The process of soybean infection by N-fixing bacteria and
symbiotic N fixation is a complex process between the bacteria
and the plant. The right species of N-fixing bacteria must be
present in the soil, either through residual populations from
inoculation of previous soybean crops or through inoculation of
the seed or seed zone at planting.
N-fixing bacteria are attracted to the roots by chemical
signals from the soybean root. Once in contact with the root
hairs, a root compound binds the bacteria to the root hair cell
wall. The bacteria releases a chemical that causes curling and
cracking of the root hair, allowing the bacteria to invade the
interior of the cells, and begins to change the plant cell
structure to form nodules. The bacteria live in compartments, up
to 10,000 in a nodule, called bacteroids. Each of the bacteroids
are bathed in nutrients from the host plant, and the bacteroid
takes N2 gas from the soil air and converts it to
ammonium-N using the enzyme nitrogenase, which consists of one
Fe-Mo (iron-molybdenum) based protein and two Fe (iron)-based
proteins. In this region, iron deficiencies can therefore
interfere with nitrogen fixation. Molybdenum deficiencies can
also stop N fixation but are not known to occur in North Dakota.
Figure 1. Nodules on soybean
roots. (20KB color photo)
N-fixation by nitrogenase must take place in an environment
without oxygen. However, bacteria and roots have to respire,
which requires oxygen. To get around this problem, nodules use
the same strategy humans do in oxygen transfer in the blood. The
transfer compound is leghemoglobin (closely related to our
hemoglobin). It results in a pink-red color of active nodule
interiors. Soybean plants with an abundance of nodules with
pink-red interiors are actively fixing N.
N-fixation is very energy intensive and does not come without
cost to the soybean. Ten pounds of carbohydrate are needed for
each pound of N produced.
Some researchers refer to carbohydrate movement in soybeans as
the "source-sink" relationship. Early in the growing
season, the source of carbohydrates is leaves and the main sink
is the nodule, in addition to the growing points of the plant.
After flowering, the activity of nodules decreases rapidly and
eventually stops due to lack of nutrient supply. The plant
changes the sink from the nodules to the seed production. Nodules
disintegrate and bacteria are released once again into the soil.
There are environmental conditions that limit N-fixation.
- Cold and heat. A temperature of 60-80 degrees is
ideal, while levels above or below this reduce bacterial
activity and slow the establishment of the N-fixing
relationship.
- High soil N levels. When soil N levels are too
high, nodule number and activity decreases. Roots do not
attract bacteria or allow infection, so N-fixation is
limited or non-existent.
- Drought. Poor plant growth does not allow the
plants to sustain nodules and plant growth. Nodule
activity is sacrificed.
- Excessive wetness. If soil pores are filled with
water, not air, there is no N to fix.
- Compaction. Compaction has been shown to affect
nodulating soybeans more than N fertilized legumes. If
there is no air, there is no N to fix.
Using inoculants
N-fixing bacteria are fragile organisms. Inoculants need to be
handled with care. Proper storage is critical. Make certain the
inoculant is fresh and has been stored in a manner recommended by
the manufacturer. Inoculation ahead of seeding is possible, but
check with the manufacturer to see if the shelf life of the
product will allow it. Some seed treatments are toxic to
inoculum. Captan and PCNB are very toxic to inoculum. Vitavax is
relatively safe up to 24 hours before seeding. Thiram produces
effects in between these two groups. Planter box treatments using
dry materials or auger treatments with liquids, fresh or frozen
materials are both acceptable if they give good coverage of all
seed. Granular treatments applied in-furrow at seeding have been
shown to be more effective than seed treatments in some studies
but are more expensive than planter box or transfer-auger
treatments.
Competition is possible from native strains of Rhizobium
bacteria which are less efficient than commercial inoculum.
Nodules from initial inoculation tend to be located on the main
top root near the surface, while native strains tend to grow on
the branches away from the seed zone. Native strains also siphon
off nutrients from the host, lowering the N-fixing ability of
more efficient strains. Native strains are sometimes much better
at infecting roots and can limit inoculation effectiveness. It is
important to inoculate with the best strains whenever soybeans
are planted, especially the first time, or in long rotations.
However, because of competition from aggressive strains, the
effectiveness of the inoculated strains will be reduced.
N recommendations for soybeans
In the central US soybean belt, it is uncommon for soybeans to
respond to pre-plant fertilizer N applications. However, North
Dakota soybeans are grown in a very different soil environment.
Research at NDSU has shown that as soil salt and carbonate levels
increase, nodule infection is reduced. Minnesota research at
Crookston has demonstrated that a modest level of N supplied to
soybeans before or at planting increases yields (Table 1),
probably due to increased early season plant health which results
in improved nodulation. South Dakota research on lower N sites
supports the use of N fertilizer supplements on soybean (Table
2).
Table 1. Affect of N-rate and inoculation on
soybean yields in the Red River Valley on a
low N site, Crookston, MN, 1988.
----------------------------------------
N-rate ------ Yield, bu/acre ------
lb/acre Uninoculated Inoculated
----------------------------------------
0 28.5 32.1
30 31.0 32.0
60 33.8 34.2
90 35.1 34.6
120 38.5 37.1
----------------------------------------
Differences between inoculation were not
significant.
Differences between N rates were significant.
Table 2. Influence of N fertilizer on soybean
yield, SD, 1993. Initial soil NO3-N
level at
site 1 was 59 lb N/acre, at site 2, 35 lb N/acre.
-------------------------------------
Soybean yield,
bu/acre
-------------------
N rate, lb/acre Site 1 Site 2
-------------------------------------
0 40.5 31.4
40 41.4 32.3
80 43.8 35.9
120 43.5 45.8
LSD 5% NS 11.3
-------------------------------------
If soil test N levels to 2 feet deep are less than 50-75 lb/acre,
apply the difference up to the 75 lb/acre level to compensate for
low early N-fixing activity, assuming that fertilizer/crop
economics allow and yield potential is sufficiently high. Do not
apply additional N if soil levels are over 75 lb/acre, as this
will reduce the potential for any significant later season nodule
activity. Since a 40 bushel soybean crop requires nearly 200
lb/acre of N to reach maturity, a large contribution by the
nodules is necessary. Soybeans can be grown without nodules if N
is supplied, but the fertilizer expenses are similar to those
required by a corn crop. It is better to apply more modest levels
of N and inoculate. If a field has a history of soybeans with
good nodulation, supplemental N would probably not be required
even if soil N levels were low.
Figure 2. N deficiency on
soybeans. (57KB color photo)
There have been occasional references to soybean responses
from late season N applications. Responses are most consistent in
irrigated fields with spoon-fed applications up to 40-50 lb/acre
N total. Dryland fields have only been occasionally responsive,
and only when yields above 50 bu/acre are possible. Because of
inconsistency of response and the cost of the practice,
late-season N applications are not advised (Table 3).
Table 3. Affect of foliar treatment of soybeans
at flowering and early pod-set. MN.
-------------------------------------
Treatment Yield, bu/acre
-------------------------------------
Control 50
N-P-K, Iowa State* 48
10-34-0, 28%, +
potassium sulfate 43
-------------------------------------
*Iowa State mix was a total of 96N-22P2O5-35K2O-5S,
from four applications of a 20 gal/acre spray solution
containing a total of 20-5-7.5-1.5S total analysis. N from
urea, K and S from potassium sulfate, and potassium
polyphosphate. The 10-34-0, 28% and potassium sulfate
treatment also tested at U of MN above was applied in
four applications.
Phosphorus
Soybeans react to broadcast applications of P better than
banded applications with or near the seed. Soybeans appear to
prefer their entire rooting zone bathed in nutrients, rather than
having nutrients concentrated in a small area of the root zone.
Soybeans have a different, more tap-rooted habit than grassy
plants like wheat and corn, which often respond more efficiently
to banded fertilizer. Several recent studies confirm that
broadcast application of P is better than banded P (Table 4).
Table 4. Affect of P placement on soybeans,
NB, 1981. P test very low.
-------------------------------------
P rate --- Yield, bu/acre ---
P2O5/acre Starter Broadcast
-------------------------------------
0 34 34
23 35 42
46 36 42
69 37 43
-------------------------------------
If soil test levels are low to very low, then a separate
application of broadcast P is justified. However, if soil test
levels are medium or higher, the level of response of soybean to
P fertilizer is small, not justifying a separate P application.
Soybeans are excellent scavengers of P at medium or higher soil
test levels. It would be better at medium or higher soil test
levels to front-load the crop prior to soybeans or apply more to
the crop following soybeans than to apply P to that soybean crop.
The most common fertilizer practice in the central US soybean
belt is applying extra P to the previous corn crop and allowing
soybeans to utilize residual P from the soil. The practice has
been successful for over 30 years.
Even though a broadcast application of P may result in several
more bushels of soybeans than a banded application, some
producers will elect to apply P with the seed. NO
fertilizer of any kind is recommended with soybean seed in a 15
inch row or wider (Table 5), because soybean seed is more
sensitive to salt than corn. However, using a double-disc drill
with 6 inch spacings, up to 10 lb N/acre may be applied to
soybeans as a P fertilizer (do not use urea). With air-seeders,
risk to soybeans will be decreased with fertilizer spread across
the seed zone. Even though it is possible to apply up to 10 lb
N/acre with a 6-inch row spacing, dry weather at planting will
increase the risk of injury. Sandier textures and low moisture
soils may show more stand injury than other areas of the field.
Again, the best recommendation for P application is to broadcast
it (Table 6).
Table 5. Seed-placed fertilizer effects on soybean
stand and yield, SD, 1993. Row width, 30 inches.
Average of MAP and DAP treatments at two sites.
-------------------------------------
Final Yield,
Rate, P2O5 stand,% bu/acre
-------------------------------------
0 100 35
12.5 62 33.5
25 36 26.5
50 14 18
-------------------------------------
Table 6. Nutrient
recommendations for soybean.
-----------------------------------------------
Soil test P, ppm
------------------------------
VL L M H VH
Yield N Bray 0-5 6-10 11-15 16-20 20+
goal Olsen 0-3 4-7 8-11 12-15 16+
-----------------------------------------------
bu/a -------- lb P2O5/acre --------
30 50-75* 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 K, ppm
------------------------------------
VL L M H VH
Yield N Bray 0-40 41-80 81-120 121-160 160+
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-P1 recommendation = (1.55-0.1 STP)YG
Olsen P recommendation = (1.55- 0.14 STP)YG
Potassium recommendations = (2.2-0.0183 STK)YG
The abbreviations used in the equations are as follows:
YG = yield goal
STP = soil test phosphorus
STK = soil test potassium
* Total of soil test N to 2 ft. and supplemental N.
For fields growing soybeans for the first time and
where nodulation is reduced by soil conditions. In
fields with a history of good nodulation, no
fertilizer N should be required.
Potassium
Most soils in North Dakota have high K levels. However, some
sandier textured soils in the beach ridges west or east of the
Red River Valley are lower in K. Sandier hilltops in the glacial
till plain or in residual materials west of the Missouri River
may also be lower in K. Some limited soil testing based on
general landscape will show whether K is needed in these areas.
When soils are dry, even high K testing soils can show K
deficiencies. Ridge-till producers often include K in their 2 by
2 banded starter fertilizer applications to compensate for
limited soil K availability in these dryland cropping systems.
Potassium should be either broadcast or banded, with the seed and
fertilizer separated. Do not apply potassium fertilizers with the
seed (Table 6).
Figure 3. Potassium deficiency.
(20KB color photo)
Sulfur
Although sulfur deficiency is possible, few reports of sulfur
deficiencies exist. Deficiencies are most possible on sandy
textured hilltops, beach ridges, and eroded areas with low
organic matter.
Soil pH
Soybeans grow best around a pH of 6.5. Lowering pH is usually
not an option because of the cost of amendments and the formation
of salt if the application is successful. However, low pH levels
have been found in North Dakota. Sampling by landscape position
provides much better information regarding soil pH than composite
testing. Application of limestone would be justified if soil pH
is lower than 6.
Zinc
Soybeans are usually not sensitive to low soil zinc levels and
grow well at zinc test levels much lower than sensitive crops
like dry beans and corn. A recent North Dakota study with 10
locations and nine varieties with and without zinc revealed no
significant differences even at soil test levels as low as 0.2
ppm. Although rare zinc deficiencies have been observed in North
Dakota, it is not a problem most producers need consider. Zinc
deficiency is expressed as a light green color developing in
between the veins of younger leaves on sandy, low organic matter
soils with very low (less than 0.2 ppm) zinc levels.
Iron
Soybeans are susceptible to low soil iron availability. Iron
deficiency is expressed as yellowing in between veins on younger
leaves. This yellowing is called "chlorosis." Iron
chlorosis is not seen until the first trifoliar leaf emerges,
since prior to this stage iron from the seed is translocated to
new growth. At emergence of the first true leaves, iron becomes
an immobile nutrient and the plant must rely on soil availability
to supply iron needs.
Figure 4. Iron chlorosis (45KB color photo)
Iron chlorosis in this region is different than chlorosis
reported in the central US soybean growing belt. High soil
carbonates, increased solubility of bicarbonate caused by soil
wetness, and the presence of elevated levels of soluble salts
have been shown to influence the presence and severity of iron
chlorosis in soybeans in North Dakota and northwestern Minnesota.
Cold temperatures also aggravate the problem in some spring
seasons.
Application of iron-EDDHA chelate appears to be most helpful
in correcting chlorosis, but it is expensive at rates greater
than 0.5-1 lb/acre. Research is under way to determine if a
combination of chlorosis-tolerant varieties, low rates of
iron-EDDHA (0.5-1 lb/acre) applied with the seed, and foliar
application of a number of iron fertilizers may be effective in
combating chlorosis. Many soybeans are also treated with
post-emergence herbicides during expression of chlorosis.
Research is now being conducted to determine if some herbicides
are generally safer to use than others.
Figure 5. Herbicide
phytotoxicity differences when applied during chlorosis. (62KB color photo)
There are genetic differences among soybean varieties for
susceptibility to iron chlorosis. To combat chlorosis, plant the
most iron chlorosis tolerant varieties available in a maturity
range. If soybeans are seeded in rows (22 inches or wider),
cultivation tends to dry the soil, reducing soluble bicarbonate
levels and chlorosis.
SF-1164, February 1999
|
