Nutrient management after a failed corn crop
So how does this relate to planning for 2012? Any P applied in 2010 or 2011 for this year’s crop that was not taken up by the corn was sorbed onto clays and other minerals. This creates a new equilibrium in the soil, and will increase the soil test values for P. The higher soil test values will result in a lower P fertilizer recommendation.
Potassium. Potassium is a charged cation, K+, which is attracted to and retained on the soil’s cation exchange capacity (CEC). Like sorbed P, exchangeable K maintains a constant supply of K in the soil solution to support plant growth. Also like P, this exchangeable K can be measured by a soil test, and it is a highly buffered system. With K, every 4-8 pounds K2O added will increase the soil test 1 ppm, and every 4-8 pounds removed will lower the soil test 1 ppm. The buffer factor is a function of CEC and soil minerals present. On low-CEC sandy soils this factor is closer to 4, while on high-CEC silty clay loams the value will be closer to 8. Any K applied and not taken up by the 2011 corn crop would have been retained on the CEC in the surface soil and remains available for 2012. And, the higher K soil test values will result in lower K fertilizer recommendations for 2012.
Zinc. With zinc, a third mechanism, chelation, occurs and retains applied zinc. Soil organic matter is a strong natural chelating agent, much like some of the synthetic compounds we buy as fertilizer sources. Zinc sulfate added to soil slowly dissolves. A portion reacts with the organic matter and is retained in soluble, natural organic matter chelates. The vast majority of the zinc that moves to plant roots for uptake is present as a natural soil organic matter chelate. Again, this can be measured by a soil test, and there is a common buffer factor of about 10:1 with our DTPA soil test. If we add 1 pound of Zn, the DTPA soil test value will increase by about 0.1ppm.
Testing for soil nutrients
The bottom line for soil nutrients is that any N, P, K, S, Zn, and Cl added as fertilizer and not taken up by crops is still likely there, and can be measured by soil tests. The mobile nutrients (N, S and Cl) will need to be measured using a deep profile test, while the immobile nutrients (P, K, and Zn) can be measured using a surface sample.
What about the nutrients taken up by the 2011 crop?
We sampled 15 fields across the eastern two-thirds of Kansas in mid-July to determine the actual condition of the 2011corn crop, yield potential, and nutrient content of the corn plants. These fields ranged from those that had essentially died prior to tasseling to some with the potential for outstanding yield. The full results from these measurements can be found in e-Update 312, from August 18, 2011.