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. Research Question Fertilizer recommendations are only as good as the soil sample collected. Typically, a composited sample contains between five and 20 separate cores from a field. However, when N fertilizer is band applied, the fertilizer recommendation may be highly inaccurate if the composite sample contains this number of cores. Techniques are needed to accurately assess residual soil N and reduce sampling errors without increasing sampling requirements. Literature Summary Four different sources of error (lab, temporal, spatial, and management induced) have to be considered in developing soil sampling protocols. Reducing errors associated with any one of these parameters should reduce fertilizer recommendation errors. Study Description Soil samples from the surface 2 ft were collected from a minimum grid spacing of 200 by 200 ft in six fields. At each grid point, five individual samples were collected at different points relative to the fertilizer band. The total number of soil samples collected from each field ranged from 190 to 455. Samples were analyzed for ammonium N and nitrate N. Applied Questions How do we sample fields where N was band applied? If samples were randomly collected from a no‐tillage field where 120 lb of N/acre was band applied, then for the estimated residual N to be within 20% of the mean 80% of the time, approximately 57 individual cores should be composited. Many farmers are unwilling to collect this number of samples. Systematic soil sampling may reduce sampling requirements. However, samples must be collected from soil zones that contain both fertilizer and soil N. For example, if samples are collected from crop rows, then residual N in the field may be underestimated, while if collected from bands then residual N may be overestimated. An efficient sampling protocol for the Elkton no‐tillage field where 120 lb of N/acre was band applied could include compositing 15 to 30 cores from a zone located halfway between the row and the fertilizer band (Fig. ). This strategy reduced sampling requirement by at least 50% when compared with random sampling. The influence of sampling location and the number of subsamples included in a composite sample on the probability or frequency of obtaining a sample containing a nitrate N concentration that was within 20 % of the field mean in a no‐tillage field where 120 lb N/acre was band applied.