Spring is closing in on us quickly, and Jim Camberato, Chuck Mansfield and Shaun Casteel, all from the Agronomy Department at Purdue University, have gathered important information for you to consider before applying fertilizer your wheat fields this spring:
Reduced corn yield resulted in considerably more nitrogen (N) left in the soil after harvest in Indiana. The amount of N leftover depended on grain removal, fertilizer N applied and release of soil N. Drought-stricken corn grain removes about 0.7 pounds of N per acre. Indiana soils are quite variable in N release, ranging from as little as 50 pounds of N per acre to as much as 150 pounds of N per acre. Low organic matter and/or very poorly drained soils are on the low side of this range, while high organic matter and well-drained soils are on the high side. Leftover N can be estimated by subtracting crop removal from fertilizer N applied and a “guesstimate” of soil N released. However, there is a lot of uncertainty in this estimate. Measuring leftover N by soil sampling and laboratory analysis is a much better approach.
Typically in Indiana we do not consider leftover N because winter and spring rainfall removes the N from the crop root zone. Most of the N remaining in the soil at the end of the season is in the highly leachable form of nitrate (NO3-N). Nitrate has a negative charge and is repelled by soil particles (negative charge as well), so it moves downward with water. Indiana typically receives 18 to 24 inches of rainfall between October and April, which is sufficient to remove most of the NO3-N from the root zone of Indiana soils.
The amount of carryover N available for the following wheat crop or next year’s corn crop depends on the winter and spring rain as well as the amount of N left in the soil by the previous crop. Rainfall has been plentiful in the southern half of Indiana since last August (15 to 35 inches), but rainfall has been limited in the northeast and northwest (10 to 20 inches and less than 10 inches in some areas). Carryover NO3-N is much more likely in these areas than in southern Indiana.
A wheat crop has more potential than a corn crop to scavenge carryover N. Of course wheat planted in the fall has an advantage in that it will accumulate some N prior to dormancy. Wheat’s primary advantage is the established root system that can take up N early in the spring before corn is even planted. Drier soil from wheat crop water use also slows the loss of NO3-N from the soil. The amount of N in the soil at the stem erect stage just prior to jointing affects how much fertilizer is needed to optimize wheat yield. The University of Kentucky recommends no fertilizer be applied to wheat if the NO3-N content to a depth of 3 feet exceeds 120 pounds per acre, but no other guidelines exist for wheat based on soil NO3-N.
Since deep soil sampling is laborious, the more typical approach to adjusting fertilizer N rates has been to use the young wheat plant as an indicator of soil N supply rather than measuring soil NO3-N directly. Sampling wheat at the proper growth stage is important because, the tissue concentration changes rapidly with growth during this time period. At a given percent of N level in the tissue, there is only a difference of 20 pounds N per acre in the recommendation given by Virginia Tech and Kentucky.
Bottom line is wheat grown in Indiana may utilize and benefit from more carryover N than usual, especially in the northern area where rainfall to date has been limited. Spring fertilization rates necessary to optimize yield may be lower than what is needed following normal corn crops. The only guidelines for adjusting N fertilization are at the stem erect stage just prior to jointing, which is an ideal time to fertilize from crop production and N efficiency standpoints. Unfortunately, this timing is risky in wet soils, and it is later than many fertilize wheat.