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Time to Update Soil Testing Forms for Soybean Yield Level

It is common knowledge that changing Midsouth soybean cropping systems have resulted in an upward trend in soybean yields in recent years (click here and here for documentation). It follows, then, that these increased yields will remove/have removed more nutrients from the soil than did the lower yields in previous years. Thus, producers who submit their soil samples to the various state soil testing laboratories for nutrient testing should be able to designate if they need a fertility recommendation for a low or high yield environment.

Forms that producers fill out when they submit their soil samples to these labs do not presently have a category for lower- or higher-yielding soybeans. See below examples.

University of Arkansas. The online soil testing form allows the producer to search for the crop code to use when submitting a soil sample. Presently, the search for soybean produces three codes–soybean for hay, soybean-full season, and soybean-doublecrop (requires small grain crop as first crop). There is no option for the producer to designate his expected yield goal, either high or low.

Louisiana State University. The soil test kit request form has only one category for “soybeans”. Again, there is no option for the producer to designate his expected yield goal, either high or low.

Mississippi State UniversityForm 76, which is the soil sample submission form, lists two codes for soybeans–soybeans and soybeans/small grain rotation. There is a code for “corn for grain–high yield”, but no such category for soybeans.

University of Tennessee. The Field Soil Submission Sheet has codes for soybean and small grain-soybean rotation. However, as with the other states, there is no option for the producer to designate his expected soybean yield goal, either high or low.

The following points are derived from the above narrative.

•   If fertility recommendations provided by the above soil testing labs are based on previous yield expectations, then they are under-recommending applications of essential nutrients for soybeans grown in known high-yield environments.

•   If fertility recommendations provided by the above soil testing labs are based on more recent yield trends, then they are over-recommending applications of essential nutrients for soybeans grown in known low-yield environments. This will result in an unnecessary expense for producers who are growing soybeans in low-yield environments.

•   If fertility recommendations provided by the above soil testing labs are based on a state average yield, then they are over-recommending applications of essential nutrients for soybeans grown in low-yield environments and under-recommending applications of essential nutrients for soybeans grown in high-yield environments.

•   Soil fertility recommendations for soybeans should be based on the expected yield level from the site of production. Producers will have good knowledge of this based on their yield histories for individual fields, and should have the option of designating that knowledge on soil sample submission forms.

Best Management Practices (BMP’s) for soybean production should include proper fertilizer application to sites with a known yield history (either high or low). Producers should have the option of designating this knowledge on soil sample forms so that fertilizer recommendations they receive are based on the yield expectation indicated by them.

This really boils down to an economic issue; e.g., producers with low-yield environments should avoid the expense of over-fertilizing their crop, and producers with high-yield environments should be applying fertilizer nutrients in an amount that will maximize the yield and subsequent net returns expected from other production inputs that were applied to achieve high yields.

Composed by Larry G. Heatherly, Jan. 2018, larryheatherly@bellsouth.net