Allocating fertilizer resources for crop production

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The price for potash will be considerably higher this year than last year and supplies will be tight. Potash may be allocated based on past use. This is the result of an increasing worldwide demand for potash. Faced with this situation, farmers need to evaluate how they can best use the funds they have allocated for fertilizer inputs. Soil test results are an excellent guide for deciding how much lime, nitrogen, phosphorus and potassium to apply on each crop in each field. Collect soil samples and test as soon as conditions permit or work from recent soil test information, within the last 2 to 3 years.

Look first at the soil pH. If it is below 6.0, the best first investment is for lime. In many situations, the return on invest is best with lime, both short and long term. When the soil pH is below 6.0, applied fertilizer nutrients or indigenous soil nutrients are not used as efficiently by crops.

Nearly 70 percent of soils in Michigan used for field crop production contain more than adequate available phosphorus (soil test over 25 ppm) to produce top crop yields. For early planted corn, applying 25 lb P2O5 per acre as a starter is more than adequate. When the soil test is less than 25 ppm, then more may be needed for some crops. In general, spending limited resources on phosphorus will not provide the most potential for a good return on investment.

In contrast to phosphorus, less than 25 percent of the soils in Michigan used for field crop production contain adequate potassium. The probability for a good return on investment is generally better for potassium than for phosphorus. If the higher potassium price or a limited supply prevents the purchase of enough potash to meet the need for all fields, what is the best strategy to allocate the potash that can be purchased? The crop yield response to incremental additions of potassium or other nutrients can be characterized as the “curve of diminishing returns.” This means that the increase in yield from the first increment of potassium added (say 50 lb K2O per acre) is greater than the second. And return from the second increment is greater than from the third, and so forth. Eventually the cost of an additional increment of potassium is greater than the value of the yield increase. This being the case, it is better to apply or allocate potash first to fields with lower soil potassium values, where there will be a good yield response, than to those fields that have close to adequate levels. A soil test report will indicate these differences. The potassium recommendation for higher testing soils will be less than for low testing soils. One approach to deal with a reduced allocation is to only broadcast potash on fields where the recommendation is for more than 50 lb K20 per acre. For fields requiring less than 50 lb K20 per acre, use 20 - 25 lb K20 per acre in the starter fertilizer.

This can be especially beneficial when planting corn no-till or into high residue fields. Another approach is to reduce the amount of K2O recommended and applied to each field by a set amount, for example 25 lb K2O per acre. In this way, the impact on field yields from having to apply less potash on a whole farm basis will be minimized and return on investment in potash will be optimized for the resources available.  

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