Nitrogen carryover unlikely for most of Indiana
Last year’s drought and reduced corn yield in Indiana resulted in considerably more nitrogen (N) being left in the soil at the end of the growing season than normally occurs. Most of the leftover N was in the nitrate form which is subject to loss with excess soil moisture, both by drainage to the water table and via tile drains to the ditches and to the air through a process called denitrification. A dry winter and spring would have allowed some of the nitrate to carryover to the upcoming corn crop. Unfortunately in most of Indiana the winter and early spring have been anything but dry. Precipitation totals from late October through April (Fig. 1, upper panel) show that almost all of Indiana received more than 15 inches of precipitation during this 180 day span. Most of Indiana (except the southeast) received more precipitation than normal (Fig. 1, lower panel). Large areas of Indiana have received precipitation as much as 4 to 8 inches of above normal.
click image to zoomFig. 1. Upper panel is the observed rainfall for Oct. 21, 2012 through April 21, 2013. Lower panel is the departure from normal rainfall for the same time period. Data from: National Weather Service (http://water.weather.gov/precip/). Soil Analysis for Nitrate
Typically in Indiana we do not have significant carryover N because winter and spring precipitation remove nitrate (NO3) from the crop root zone. This year is no exception for most of the state. However, if you want to assess soil NO3 levels directly, soil sampling can be used. Sample representative field areas at depth intervals of 0 to 1 foot and 1 to 2 foot (15‐20 1‐inch diameter cores for each depth, composited and subsampled). Keep samples cold or spread thin to air dry shortly after sampling to minimize changes in the NO3 level of the sample. Send to a soil testing laboratory and request a NO3 analysis.
Results of the soil analysis are usually reported in units of parts per million (ppm) as NO3 or NO3‐N. If reported in NO3 divide by 4.5 to convert to NO3‐N. Contact the laboratory performing the test if there is any confusion as to the unit reported. 1 ppm NO3‐N in a 1 foot deep soil sample is equivalent to approximately 4 pounds of N per acre. Typical background NO3‐N levels at corn sidedress time are in the range of 5 to 10 ppm or 20 to 40 pounds per acre.
Fate of Fall and Spring Anhydrous Ammonia
Anhydrous ammonia (AA) applied this spring, particularly in April, is unlikely to have been lost because it remained in the ammonium form (NH4+) which is retained by the soil cation exchange capacity and is not subject to denitrification.2 Anhydrous ammonia bands do not immediately convert to NO3‐ because AA reduces the number of microbes that convert NH4+ to NO3‐, particularly when cold temperatures also reduce recovery of the microbes. Loss of NO3‐N from late fall AA applications are likely also minimal because soil temperatures have been consistently cold throughout the winter. If in doubt soil sampling can be used to assess soil N levels in fields where AA (or manure) was applied with two modifications to the procedure outlined above. click image to zoom
Since both NH4 and NO3 are plant available and much of the NH4 may not have been converted to NO3, request the laboratory measure both N forms. Most laboratories will report ppm of NH4‐N, but if ppm NH4 is reported divide by 1.3 to convert NH4 to NH4‐N. Background levels of NH4‐N are typically less than 10 ppm at corn sidedress time.
Nitrogen fertilizer is one of the most expensive and impactful inputs in corn management. Excess applied N reduces profit and negatively impacts the environment. Insufficient N reduces yield and profit.
If you believe carryover N from last year’s drought stricken crop is likely in your fields then soil sampling for NO3 and/or NH4 as a basis for a reduction in this year’s fertilizer application is wise. Otherwise utilizing the general N recommendations (which are based on an average level of background N over the previous 7 years) are warranted. Read the most current Nitrogen Management Guidelines for Indiana.