Nutrient management after a failed corn crop
Where the corn crop failed in 2011, farmers are asking questions on the best ways to handle their nutrient management programs for 2012. In most cases, the vast majority of the fertilizer that was applied to unharvested, failed corn should still be there in 2012 – either in the soil or in the crop residue. Farmers will need to do some soil testing to know for sure the nutrient status of fields with failed corn. Farmers will also want to have some idea of the amount of nutrients present in the corn residue remaining, and how quickly those nutrients will become available to crops.
There are a number of potential sources of nutrients other than applied fertilizers that could contribute to a 2012 wheat, corn, sorghum, or soybean crop. These include:
- Nitrate, sulfate, and chloride in the soil profile
- Phosphorus, potassium, and zinc in the surface soil
- Nutrients in crop residues
The first category consists of mobile nutrients, and the second category consists of immobile nutrients. The difference is important. Mobile nutrients are able to dissolve in soil water and can move through the soil in water, while immobile nutrients generally stay where applied. Of the 14 essential mineral elements, the common mobile nutrients we apply as fertilizer are N, S, and Cl, and the common immobile nutrients we apply as fertilizer are P, K, and Zn.
Mobile nutrients in the soil after failed corn
A very large portion of those mobile nutrients that were not taken up by the 2011 corn and/or wheat crops are likely still present in the top foot or two of soil. With the low rainfall in most of the state south of I-70, very little of the N will have been lost. In the K-State Soil Testing Lab, we are already seeing higher-than-normal soil test levels for N, reflecting an accumulation of unused nitrate N in the soil. Any unused sulfur (S) or chloride (Cl) would also be present in that top foot or two of the soil profile.
So the first tool a farmer should think about when planning his 2012 fertilizer program is a deep profile soil test for N, S, and Cl.
Immobile nutrients in the soil after failed corn
What about P, K, or Zn? Where these nutrients were applied to the 2011 corn crop, will they still be available for crops in 2012? When immobile nutrients such as P, K, and Zn are applied to the soil, they interact with different portions of the soil and are retained. Note the word “retained,” not “fixed.”
Phosphorus. Phosphorus reacts with the clay surfaces and the iron and aluminum coatings found on the soil particles and is sorbed to those surfaces. Sorption reactions occur in stages, and the initial stages are highly reversible. Sorbed phosphorus can be desorbed and go into soil solution, replacing the P taken up by plants. This is a buffering system which maintains a constant small quantity of P in the soil solution and supplies the P needed for good crop growth. This is how we store P in the soil and build soil test values, with little worry about that P being lost. Sorbed P is the primary P fraction in soils measured by a soil test. But the soil test only reflects a fraction of the total P present in the soil. In most Kansas soils, we have an 18:1 buffer factor. If we add 18 pounds of P2O5 and it reacts with the soil, becoming sorbed to the clays and other minerals present, the soil test will increase 1 ppm. If we remove 18 pounds P2O5 through crop uptake, the soil test value will drop 1 ppm.