Effect of Soil Sampling Density and Landscape Features on Soil Test Phosphorus

Abstract

<abstract> <bold><sc>Abstract. </sc></bold>Organic and/or inorganic fertilizers are used to promote crop growth yet the rate of fertilizer application is often based on the nitrogen needs of the crop and not the phosphorus needs, especially when utilizing animal manures with low N-to-P ratios as compared to plant N:P uptake. Because of this, soil phosphorus levels in many areas of the world are in excess of crop needs. Excessive levels of phosphorus in water bodies such as streams and lakes can result in a condition known as eutrophication. With eutrophication, nutrients promote algal growth which in turn decreases dissolved oxygen levels in receiving water bodies. The issue of excessive nutrient inputs to waterbodies is of particular importance to Kentucky due to its large agricultural base and the Commonwealth’s link to hypoxia in the Gulf of Mexico. Nutrient management is one way to minimize the impacts of phosphorus on waterways; however, the success of nutrient management is dependent in part on soil sampling strategies. This study examined the effect of soil sampling density (15 × 15 m, 15 × 30 m, and 15 × 60 m) on Soil Test Phosphorus (STP) in three subplots. Results indicated that mean STP values for the subplots did not change with sampling densities; however, maximum STP levels displayed large variations. Depending on the sampling density used and the method of data aggregation (mean vs. maximum), recommended nutrient application rates varied from phosphorus-based to nitrogen-based. Also of importance was the recognition that maximum STP values were driven by a shallow limestone rock layer, which was not visible at the time of sampling. This limestone rock layer, and hence these highest STP values, were immediately upgradient of a sinkhole from which collected soil samples also exhibited higher STP values. When developing nutrient management plans, particularly in karst landscapes where the potential for groundwater contamination and hence surface water contamination is higher, additional consideration should be given to the influence of geologic materials on soil nutrient levels and greater sampling densities around preferential flow paths.