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Using grid soil sampling for manure distribution

Randy Pepin, Extension Educator
May 25, 2013

Grid Soil Sampling is rapidly becoming an accepted standard practice for crop farmers who depend on commercial fertilizer to balance their nutrient needs. GPS controlled variable rate fertilizer applicators have made adapting the results of Grid Soil Sampling along with other mapping technologies attainable for most farms.

So what is Grid Soil Sampling? With traditional composite sampling, we take a dozen or more core soil samples across a whole field, mix them, and get one lab analysis for the whole field. When Grid Soil Sampling, we typically grid the field into 2.5 acre cells, take a dozen or more core soil samples within each cell, and combine them for analysis of that cell. The soil test result information is mapped so we can visually see the variation for each nutrient across the entire field. Grid Soil Sampling is normally done about every four years. There are expenses; "crop farmers" usually see an excellent return on both commercial fertilizer savings and crop yield increases; similar returns can be realized by using grid soil results to determine where to place manure in a field.

One thing we have learned from Grid Soil Sampling, nutrient levels seem to vary greatly across fields. This variation is due to varying soil types, elevation changes, past management practices, and the many complexities of soil science. Add manure application to a field's history and you have another element in the soil variability; especially if part of the history is the typical historical practice of spreading most of the manure close to the livestock housing site.

What about our manure? Manure "is what it is", meaning the NPK and other nutrients are fixed, not giving us the flexibility of increasing or decreasing any one of the nutrients independent of the others. Variable rate spreading with most manure spreaders and drag line injection is mostly limited to varying the speed of the tractor; however, liquid tanker spreaders with GPS controlled variable rate pumps are available. So with those limitations, why bother? Why not continue to apply the manure at one rate across the entire field?

So why would a livestock farmer bother with Grid Soil Sampling? Why not just spread the manure across the whole field in the fields that are convenient? There are two major reasons, farm profitability and environmental concerns. With escalating prices of commercial fertilizer in recent years, more farmers are recognizing the high economic value of manure. To gain the most economic value from manure, we need to be applying it in the areas of fields that need as many of the nutrients in the manure as possible. This will not only save us money in purchasing less commercial fertilizer, but the discovery of the field areas needing the extra nutrients via Grid Soil Sampling, will most likely result in higher yields in those areas of the field.

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 online.

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