Challenges for Making Intensive Soil Sampling and Vrt Pay. Ongoing Iowa Studies with Phosphorus

Soil fertility evaluation and management can be greatly improved with the use of precision agriculture technologies. Differential global positioning systems (DGPS), yield monitors, various forms of remote sensing, geographical information system (GIS) computer software, and variable rate technologies are available for use to producers. Intensive soil sampling, crop scouting, and other practices complete the new technological package. Soil testing is a diagnostic tool especially adapted for site-specific management and, at the same time, DGPS and GIS can greatly improve soil testing. The spatial variation of plant nutrients over a field makes soil sampling one of the most important sources of error in soil testing. A very small amount of soil needs to represent many tons of soil and large areas. Intensive soil sampling, soil test mapping, and fertilizer application with variable-rate technology can improve the efficacy of fertilization and liming compared with the conventional practice of collecting soil samples from large areas and applying a uniform single fertilizer rate over each field. Although variable-rate fertilization can be used on the basis of sampling areas identified according to soil types, landscape, or previous management, many believe that it should be based on grid sampling. The conventional sampling by soil map unit may not be appropriate for precision agriculture because available soil survey maps may not have the required precision and likely high nutrient variation within units.