The second advantage of grid sampling and identifying the phosphorus variability is the environment. Livestock farmers usually find areas of fields that have phosphorus levels higher than the traditional composite soil test; these areas usually do not warrant any additional phosphorus. Livestock farmers also frequently discover areas of fields that are lower in phosphorus than the composite soil test; these areas may need additional phosphorus. This additional information can really help in administering a nutrient management plan by being more precise in manure application.
Along with this article you will see an image of the phosphorus results of Grid Soil Sampling on a dairy farm field. The phosphorus analysis of the composite sample of this field is 36 ppm Bray 1-P; this is higher than the 21 ppm Bray 1-P recommended for corn and alfalfa production. Based solely on a composite soil test result, we might completely pass on applying any manure to this field. Notice that the phosphorus levels on the left side of the image are substantially higher than the right. In fact, the average on the right side is 15 ppm Bray 1-P; University of Minnesota recommends 45 pounds per acre of phosphorus in this area.
This is the area where we would apply the manure. So using the results of grid sampling, applying manure only to the right side of the field gives us a good economic return to the manure. It should also give us a yield bump, and we prevent the additional phosphorus escalation on the left side of the field. How much of a return? One could realize about a three-to-one return to the cost of grid sampling the whole field in the first year.
For additional information, see the University of Minnesota Extension Manure website where eight case studies demonstrating Grid Soil Sampling for manure distribution and a video presentation are posted: http://www1.extension.umn.edu/agriculture/manure-management-and-air-quality/