Managing nutrients following drought
As the 2012 fall crops are being harvested, many farmers are now inquiring if they should consider lowering their fertilizer field rates for 2013. Generally, the answer is "yes," but there are several factors to consider. Soil testing fields to determine mobile nutrients (nitrogen, sulfur and chloride) availability in the 0- to 24-inch soil profile depth is highly recommended as well.
Dave Mengel, Ph.D., K-State Extension Soils specialist, says that for producers planting wheat this fall in failed crop fields, a mobile nutrients profile soil test is a must. Phosphorous and potassium applications should also be checked through surface soil testing. For example, in the spring of 2012, most K-State lab soil test results (taken from wheat fields planted following a failed 2011 crop) advised little additional fertilizer nitrogen need. Providing excess nitrogen to wheat may have caused lodging. Since the weather was drier, the extra corn acres nitrogen stored well for the upcoming crop.
For those planting corn or grain sorghum next spring, Mengel advises to wait until late winter or early spring to take the profile sample. This will provide a better indication of the amount of residual nitrogen (remaining in the soil and residue) which will be available for the next crop.
Consider, also, to what extent the drought damaged the previous crop. Severely stunted corn (3 to 4 feet tall final height) that did not form any grain likely has high nutrient concentrations but very low total nutrient uptake because of the low level of dry matter produced. In those fields a portion of the applied nutrients will likely be present in the soil and potentially available for the 2013 crop.
In fields which grew to more normal height but had very poor yields due to heat and drought stress, there was likely near-normal nutrient uptake. Most nutrients are first taken into the vegetation and used/stored, but have limited translocation into developing grain. In this situation, residual nutrients in the soil likely will result in higher than normal content; but not to the degree in the more extreme drought situations.
The majority of the nitrogen, phosphorus, and sulfur in plant residue is present as protein and other organic compounds. For these nutrients to become available to plants, these compounds must be broken down and the nutrients mineralized. This process will normally take three or more years for high carbon to nitrogen ratios to become available for plant use. However, severely damaged crops have lower C:N ratios. This means that half the residue nutrients may be available for the next crops within months rather than years.
More information on crop nutrient fertility management is available through the K-State website at http://www.ksre.ksu.edu.