HOW CAN WE MAKE INTENSIVE SOIL SAMPLING AND VARIABLE-RATE P AND K FERTILIZATION COST-EFFECTIVE? Antonio Mallarino, Associate Professor

Precision agriculture technologies have potential for improving soil fertility evaluation and nutrient management. Global positioning systems (GPS), yield monitors, various forms of remote sensing, geographical information system (GIS) software, and variable rate technology are available to producers. Intensive soil sampling, crop scouting, and other practices complete the new technological package. Soil testing is a diagnostic tool that adapts well to site-specific management because it can evaluate nutrient availability of areas of different sizes. However, the spatial variation of nutrients within fields makes soil sampling one of the most important sources of error in soil testing. Intensive soil sampling, soil test mapping, and variable-rate application of fertilizers or manure can improve the efficacy of the conventional practice of collecting soil samples from large areas and applying a uniform fertilizer rate over a field. Although variable-rate fertilization can be used on the basis of sampling areas identified according to soil map units, topography, and/or previous management, many believe 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 often there is high nutrient variation within soil map units. This presentation discusses soil sampling methodology and summarizes results of ongoing Iowa research. The research compares various soil sampling methods and fixed-rate versus variable-rate P or K fertilization conducted in various producers' fields. The research is not based on simulations based just on soil test data. It evaluates the effectiveness of intensive grid sampling schemes and the effectiveness of current variable rate technology.