Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 10/15/2006
Publication Date: 1/26/2007
Citation: Clay, D.E., Kitchen, N.R., Carlson, C.G., Kleinjan, J.L. 2007. Using historical management areas to reduce soil sampling errors. In: Pierce, F.J., Clay, D. E., editors. GIS Applications in Agriculture. Boca Raton, FL: CRC Press. p. 49-64.
Interpretive Summary: For many decades, soil sampling has been routinely used by farmers to assess 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 a 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 have essentially eliminated soil mixing. Also, crop production on land previously used as pastures, feedlots, or farmsteads will likely have very different soil fertility that can be seen 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 aerial photographs collected during the 1950's and 1960's. We found that if you don't sample separately those areas where animals were confined from the rest of the field, then soil sampling results will likely be unreliable for determining fertilizer needs. We also found that in a reduced tillage system where nutrients were band applied, that only one sub-sample should be obtained from soil containing 'old bands'. The goal is to minimize variability created by past land use or practices in order to obtain meaningful fertilizer recommendations. The results of this study and review will benefit farmers and crop consultants by helping them understand the errors introduced when soil sampling is not targeted.
Technical Abstract: Soil fertilizer recommendations in modern crop production rely on laboratory analysis of representative soil samples. Regardless of how 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. 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 provides good fertilizer recommendations. We found that in most fields, the accuracy of fertilizer recommendations can be improved by increasing the number of individual cores included in a composite sample. In tilled fields where N and P fertilizers have been broadcast, a good sampling strategy was to randomly collect between 15 to 30 individual soil cores from each sampling zone. Sample-to-sample variability greatly increased when a composite sample contained < 15 individual cores. Areas where animals were confined in the past can have elevated P and K decades later. Using grid sample data sets from fields in South Dakota and Missouri, it was found that 3 to 10 times more soil cores were needed to adequately represent a field when historic feedlots or farmyards were not handled separately. Evidence for old homesteads and animal confinement can be seen in USDA aerial photographs collected during the 1950's and 1960's. These same photos can be used effectively to develop soil sampling zones. In a reduced tillage system where nutrients were band applied, we found that no more than one sub-sample per each composite sample should be taken to represent the residual fertilizer bands. These results benefit farmers and crop consultants by helping them understand the errors introduced when soil sampling is not targeted to account for historical practices.