Timely Topics: Grid Soiling Sampling: How Small? How Often? How Useful?
Timely Topics: Grid Soiling Sampling: How Small? How Often? How Useful?
Contributed by Dr. Steve Philips
Director, IPNI North America ProgramPart 1 (of 2 part series)
"As variable-rate fertilization increases around the U.S., some of the most common questions being asked are related to grid soil sampling. In a grid sampling strategy, the field is divided into areas of a pre-defined size and soil samples are collected from each grid area using one of several sampling strategies, i.e. grid point, grid cell, offset grid point, etc. Like traditional soil sampling, the objective of grid sampling is to assess the nutrient status of the field. The differ¬ence is that instead of a single average value for the entire field, a contour map is created using GIS software that shows the range and pattern of soil nutrient status, which is used to establish management zones within the field.
What grid size should I use? This is an important question, as grid size will affect the cost of establishing the pro¬gram. Ideally, the grid size will be small enough to accurately capture the range of nutrient status in the field; more highly variable fields should use a smaller grid size, while grid size in less variable fields can be larger. Research in Nebraska showed that increasing sampling density from 4.2 to 42 samples/A resulted in 45% of the field receiving a different N fertilizer rate recommendation and the average N fertilizer rate for the entire field was reduced by 14 lb N/A. However at another site, sampling densities of 14 samples/A and 1 sample/3.7A resulted in only 18% of the field receiving a different N recommendation and no change in the average N rate for the field."
