Charles Wortmann, Extension Nutrient Management Specialist
Q: What are the implications of the 2012 drought for nutrient management in 2013? Do fertilizer rates need to be adjusted? How is the release of manure nutrients affected?
Let’s begin with nitrogen management. In samples we're seeing, residual soil nitrate levels are often unusually high. This is due to several factors related to 2012 weather conditions, especially in dryland and limited irrigation (water-limited) fields.
• Where yields were less than normal, uptake of nitrogen was less than normal in 2012.
• There was less leaching and less denitrification loss in 2012.
• Early crop maturity, less robust plants, and a warm fall resulted in more decomposition of crop residue, especially soybean leaves, contributing nitrogen to the soil.
• Soil organic N mineralization appears to have been more than usual in 2012, probably due to the relatively high temperatures. Reports indicate that residual soil nitrate levels are high following soybean as well as corn.
Given all this, there is an opportunity to greatly reduce production costs in 2013 by applying less nitrogen than normal before planting with the intention of applying a considerable amount in-season based on soil or crop condition. Soil sampling will be instrumental in determining how much fertilizer is needed and where it's needed.
Residual Soil Nitrate
Give credit to residual soil nitrate when determining nitrogen rates. Sample the soil to at least a 24-inch depth for all fields needing fertilizer nitrogen in 2013. Yields varied a lot in some water-limited fields in 2012; therefore, more residual nitrate is likely to be present where corn yields were reduced.
Consider targeted soil sampling for residual nitrate determination and then varying the rate of nitrogen application across the field; the 2012 yield maps will be a valuable tool in defining these target areas.
The UNL fertilizer guidelines give credit to only about 55% of the residual nitrate-nitrogen. Crop availability is expected to be more than 55% in 2013 because of low potential for loss to leaching and denitrification.
Consider withholding much of the total nitrogen for in-season application through a side-dress application or fertigation. In-season nitrogen rate can be determined by using the pre-sidedress nitrate test or by "reading" the crop canopy. This can be done by using a chlorophyll meter or comparing the crop canopy reflectance across the field with a reference strip that received a high N rate before planting.
Fertilizer nitrogen rates should be adjusted according to the previous crop; 45 lb/ac less nitrogen is applied after soybean than after corn. Give the 45 lb/ac credit in 2013 as in other years, even for fields where the soybean crop was poor in 2012.
N and S Availability from Manure
Availability of nitrogen from manure will be affected for 2013. In water-limited fields, breakdown of manure applied in 2011 and 2012 was likely less than normal. The potential for organic N and sulfur release from manure is greater than normal in 2013 for rainfed fields; however, this is not well predicted. To use manure and fertilizer nitrogen well, apply less fertilizer pre-plant than normal and plan to side-dress apply some nitrogen based on an in-season soil or crop canopy assessment.
P, K and Zn Availability
What about other nutrients? Soil test levels of phosphorus, potassium, and zinc are also expected to be higher than normal in 2013 for water-limited fields, primarily because the 2012 crop removed less than would occur in a more typical year.
If there was late summer and fall precipitation, decomposition of crop residue was more than normal due to less robust plants, early maturity, and a warm fall; this also may have contributed to increased soil test levels. Therefore, less fertilizer is likely to be needed for these nutrients in 2013, but test the soil. Use a 0-8 inch sample to assess availability of phosphorus, potassium, and zinc.
This information will be very valuable for 2013 management. However, the soil test results of 2012-2013 are likely to be atypical compared with more normal years and not a good indication of nutrient availability for future years.
The drought also may have affected release of phosphorus, potassium and zinc from manure. Generally sufficient quantities of these nutrients are applied with a manure application so that some delay in release is not a major concern.
The unusual soil nutrient status of water-limited fields indicates that the spring of 2013 is not a good time for grid soil sampling. Grid soil sampling is a significant investment intended to be useful for eight or more years. In a very atypical year grid sampling provides a poor basis for nutrient management over future years.