NDSU Crop and Pest Report
Soils


ISSUE 4  May 22, 2003

FERTILIZER FOR SOYBEANS

Just in way of review, here the NDSU guidelines for fertilizer application to soybeans-

Nitrogen:

Fields where soybeans are grown should be inoculated with a good quality soybean inoculation product. Peat-based, liquids and granular products have all been shown to be effective. In fields with a long history of soybean and good nodule development it is not as important to inoculate.

For first year soybeans and in fields which exhibited early season stress from severe iron chlorosis last time soybeans were grown, application of N may be beneficial if soil tests are below 50 lb/acre. Fertilize so that the sum of soil N plus fertilizer N equals about 50 pounds.

If soil N levels are higher than 50, it is not a concern. When the soybean runs out of N, it should be able to supplement its supply with help from the nitrogen-fixing inoculum that you supply.

Inoculation should not be applied before the day of seeding unless the label says you can. Even then, there are limits to the time that the inoculum will be viable. The shorter the time the better.

Phosphorus:

Soybeans seldom respond to P fertilizer application if the soil test is medium or higher. Generally, broadcast applications of P are better than banded P. If banding is the only option, band only if rows are 12 inches or narrower. Banding in wider row spacings concentrates the fertilizer too much, and unless you have access to 0-46-0, the N in the phosphate fertilizer has the potential to reduce stands too much to be useful. In narrow rows, up to 10 lb N (no urea!) with 18-46-0 or 10-50-0/11-52-0 is acceptable, but I would advise farmers to be conservative on rates.

Potassium:

If soil tests show high, no additional K is needed. Other nutrients- small areas of sulfur deficiency is possible on sandy, low organic matter ridges, however, the frequency of these events is quite low. Soybeans are not sensitive to low soil zinc levels, so ignore the soil test readings. Iron supplements have been inconsistent in response and their benefits are questionable. For a solution to iron chlorosis, the best method is during seed selection.

 

SOYBEANS AND SOIL SALT LEVELS

The North Dakota Soybean Council and Dr. Goos have an excellent list of iron chlorosis ratings, but no ratings of varieties are specifically listed for soil salt levels. Since the degree of chlorosis is often related to soluble salt levels, and because the soil in the greenhouse screening process also contains a high level of soluble salts, the list would also be a good estimate of relative salt tolerance. However, being salt tolerant does not mean that the yields in salty areas (EC greater than 1.5 mmho/cm) would be equivalent to a normal variety with low salt. In my studies a few years ago, the best an excellent variety could yield in a salty soil of between 1.5-2 mmho/cm was about 30 bushels, or about 60% of a normal yield. Of course varieties with low tolerance would have yielded zero, so 30 bushels seems like a lot. But compared to sunflowers, which would have tolerated those salt levels quite well, the yield was relatively low. Before deciding to seed soybeans into high salt areas, consider the likely prospect of half a normal yield at best when running your economic figures.

 

PHOSPHATE "INOCULUM"

Products are registered in North Dakota which can release phosphate into more available forms in a biological manner. These phosphate "inoculums" have a scientific basis. They are a formulation of soil fungus Penicillum bilaii which has the interesting characteristic of acidifying its immediate environment. If phosphates are "occluded" (coated with carbonate materials in high pH soils), these coatings can be dissolved in the zone of acidity formed by the organism and released into the soil solution for uptake by crops. I have visited with the researcher in Manitoba who developed this organism. The following are his observations:

  • The fungus would effectively release phosphates when pH was over 7, and carbonates are present.
  • The amount of phosphate released would be about 5-10 lb P2O5.
  • The fungus is not an "inoculant" in the sense that a nitrogen fixing bacteria is in a direct relationship with the plant. This fungus is placed with the seed, but it has no direct interaction with the crop.
  • The fungus is most effective on medium or higher soil test P soils, not in low and very low testing soils.

    In North Dakota, about 75% of fields average over pH 7. Of this 75%, most have areas within the fields that would test below 7. This would mean that the fungus would effectively provide supplemental P for only a low percentage of whole fields. In most fields, only parts of the field would benefit from its use. Dr. Goos at NDSU evaluated this fungus some years ago, and found that in low pH soils, P nutrition was not enhanced. The fungus cannot make P out of nothing. If soil tests are low, the product will have little benefit. Finally, at $240/ton 18-46-0, the phosphate cost of 5-10 lb P2O5 would be $1.10-2.20 . The fungus would not be expected to build or maintain soil test P levels, but the fertilizer P would. When evaluating the use of these products, consider the possible benefits and the chance of response from your fields.

     

    NITRATE LEACHING FOLLOWING RAINS

    In fields which have received more than two inches of rain the past couple weeks, it may be important to have your crop consultant keep an eye out for possible N deficiencies due to nitrate movement, or in the east for deficiencies due to nitrate leaching and/or denitrification. Supplemental N application may be needed to achieve good yields and quality in some crops. Plant analysis would be a better indicator generally than soil tests, although the soil sampling may reveal if the nitrate is still in the root zone.

    Dave Franzen
    NDSU Extension Soil Specialist
    (701) 231-8884
    dfranzen@ndsuext.nodak.edu


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