Submitted to: Crop Management at www.cropmanagement.org
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2002
Publication Date: 12/15/2002
Citation: CLAY, D.E., KITCHEN, N.R., CARLSON, C.G., KLEINJAN, J.L., TJENTLAND, W.A. COLLECTING REPRESENTATIVE SOIL SAMPLES FOR N AND P FERTILIZER RECOMMENDATIONS. CROP MANAGEMENT DOI:10.1094/CM-2002-1216-01-ma. 2002. Interpretive Summary: For many decades, soil sampling has been routinely used by farmers for assessing the soil's ability to supply nutrients for crop growth. A soil sample once analyzed in the laboratory is the basis for fertilizer recommendations. Yet, obtaining a soil sample to represent the nutrient status of the field or sub-field region is probably the single most challenging step in getting reliable fertilizer recommendations. When plowing and discing were more commonly used, added nutrients from manures and fertilizers were mixed so that it took fewer soil sub-samples to get a sample to approximate the average. However now, many farmers have adopted no-till practices to reduce erosion, which has essentially eliminated soil mixing. Also, crop production on land previously used as pastures, feedlots, or farmsteads will likely have very different soil fertility that may last for decades. This research and interpretation was conducted to show how past management practices greatly alter nutrient variability and to provide guidance for soil sampling under these conditions. Evidence for old homesteads and animal confinement can be seen in old USDA-Natural Resources Conservation Service aerial photographs collected during the 1950's and 1960's. If you don't separately sample those areas where animals were confined from the rest of the field, then soil sampling will not be a reliable tool for determining fertilizer needs. When possible, avoid sampling guess rows as they may contain none or two fertilizer bands. In a reduced tillage system where nutrients were band applied, keep records on how and where they were applied. The results of this study will benefit farmers and crop consultants by helping them understand the degree of error introduced when soil sampling is not targeted to minimize variability created by past land use.
Technical Abstract: Soil fertilizer recommendations in modern crop production rely on laboratory analysis of representative soil samples. Regardless on where the samples were collected (grid points, management zones, or whole fields) the accuracy and precision of the fertilizer recommendation can be improved by considering the factors that influence nutrient variability. As each producer's crop production enterprise varies, it is recommended that producers select approaches that are suited for their operation. The objectives of this research and analysis were to discuss how management influences nutrient variability and to provide insight into how to design a soil sampling protocol that provide good fertilizer recommendations. The accuracy of the fertilizer recommendation was improved by increasing number of individual cores included in a composite sample. In tilled fields where N and P fertilizers were broadcast, a good sampling strategy was to randomly collect between 15 to 30 individual soil cores from each sampling zone. A composite sample should always contain at least 15 individual cores. Sample areas where animals were confined separately from the rest of the field. Evidence for old homesteads and animal confinement can be seen in old USDA-Natural Resources Conservation Service aerial photographs collected during the 1950's and 1960's. When possible, avoid sampling guess rows as they may contain none or two fertilizer bands. In a reduced tillage system where nutrients were band applied, keep records on how, when, and where they were applied. Specific guidelines for sampling residual bands are described further.