Soil sampling strategies for estimating residual nitrogen

Accurate fertilizer recommendations require representative soil samples. However it is often prohibitively expensive to collect the number of samples required to insure accurate recommendations. The objectives of this project were to determine inorganic N variability and sampling requirements in six fields with different cultural practices. Inorganic N concentrations and variability were evaluated in six corn (Zea mays L.) fields (20 to 80 acres) with management histories. Samples from the 0 to 2 ft soil depth were collected from a 200 ft grid. At each grid point, five samples were collected along transects between corn rows. Soil samples were analyzed for ammonium and nitrate N, Sampling location relative to the fertilizer band influenced inorganic N concentrations. By considering water and N inputs into the row and interrow areas separately, the relative importance of residual N remaining in row and interrow areas of the soil could be estimated. This estimate can be used as a guide for management decisions. Results shows that the best strategy for sampling banded fields may include collecting 15 to 30 composite cores from a zone located halfway between the row and the band.