The Importance of Soil Sampling Depth for Nutrient Application Recommendation in Kentucky.

Soil test nutrient concentrations vary with depth.  Therefore, sampling depth needs to be considered in routine soil sampling for the purpose of making fertilizer recommendations.  However, this information is limited in Kentucky.  The objective of this study was to determine the relationship between soil sampling depth and the nutrient values of soil test analyses. Samples were collected from a 0-5 cm depth, 0-10 cm depth, 0-15 cm depth, and 0-20 cm depth. Each field was grid sampled using a 0.25 acre grid. Phosphorus and K levels were determined using the Mehlich III method. The cation exchange capacity was measured using the ammonium extraction method. The results of the soil test analyses were used to determine the amount of P and K fertilizers that should be applied to each field in order to provide the needed nutrients for a soybean crop with a yield of 40 bushels per acre. The soil test analysis results were integrated into multiple fertilizer recommendation equations to analyze the significance of the results. Nutrient recommendations were made for each sampling depth.  The SAS program was used to analyze the data using an α 5%.  The results indicated that sampling depth significantly influenced P and K recommendation calculations for Kentucky’s soils.  More detailed results will be explained in the presentation.