Optimum sample size of nitrate nitrogen in soil sensing and mapping

Temporal and spatial variability of soil parameters is very important for precision agriculture. It is necessary to determine how many samples should be collected for representing the feature over one grid space in soil sensing. Test area located in a growing cornfield was 4.2 m×4.2 m with 0.6 m×0.6 m sub-grid spacing. The soil parameter analyzed was nitrate nitrogen. Seven tests were conducted. The first sampling was carried out a month later after seeding, and the final one was made ten days later after harvesting. The variation of nitrate nitrogen was analyzed in order to clarify the relationship between sample size and estimation error of the nitrate nitrogen level. Data obtained provide non-normal distribution. Analyses of the data from various stages of growth show that estimation errors increased with depth. When only one sample is collected from a grid to estimate the level of nitrate nitrogen, the estimation errors were almost 50% (given α≈0.10). When collecting five samples from one grid (sample size was 5), estimation error would decrease to 25%. Keeping estimation error below 30% with significant level 0.1 required about 3 of sample size in common growing stages, but at least 15 of sample size after top dressing.