Phosphorus and Potassium Precision Fertilizer Placement.

There is a great deal of published research on P and K fertilizer placement methods and their impacts on crop yield and nutrient use efficiency, mainly focusing on band and broadcast methods. Information is more abundant for soils managed with tillage than with no-tillage. Differences between methods are soil and crop specific and few generalizations are possible. However, banding tends to be superior with high soil nutrient retention capacity, limited root systems or growth, and dry topsoil. Less research has investigated variable-rate technology (VRT) for nutrient application within fields having large soil-test or nutrient removal spatial variation. This technology now is readily available and affordable in many regions of the world. This presentation discusses P-K placement results and issues for corn and soybean production in Iowa, which approximately represents large areas of the Corn Belt. Field trials compared band (shallow or deep) and broadcast placement methods for granulated fertilizer with several tillage systems. Results for P showed small and inconsistent yield or nutrient use advantage for banding with any tillage system. Results for K often showed a deep-banding advantage with ridge-till and no-till systems, although the soil K stratification usually was less than for P. A humid climate, soil chemical and physical properties, and crop uptake differences for P and K explain these results. Therefore, decisions concerning P and K placement, in spite of theoretical considerations, should be based on local research, costs, and both environmental issues and practical considerations. Results of field-scale trials showed that using VRT coupled with dense grid or zone soil sampling approaches may not always increase crop yield, but does improve nutrient distribution and the efficacy of fertilizer use while reducing within-field soil-test variation. Higher crop and/or fertilizer prices with smaller increases for soil sampling and testing costs make VRT a practical and cost-effective way for increasing P and K use efficiency.