SOIL SAMPLING AND NUTRIENT VARIABILITY IN DAIRY ANIMAL HOLDING AREAS1

Abstract

The variability of soil sampling and surface nutrient loading was investigated at Okeechobee, FL, on a Myakka fine sand (sandy, siliceous, hyperthermic Aeric Haplaquod) and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Haplaquod), where both sites were used as dairy animal holding areas. Soil pH and nutrient variability of P, K, Ca, Al, Fe, and Na were high across both sites. Soil nutrient variability was dependent on localized animal intensity and surface topography. Accumulation of total soil P in surface soils was related to the level of organic matter loading, but was also associated with other soil characteristics that influence the release of P such as total Al, Ca, and soil pH. The number of soil composite cores within animal holding areas necessary to obtain a specified confidence in soil test and laboratory results was determined. Depending upon the soil parameter measured, the number of samples required to obtain a high level of confidence (α = 0.10) with a 10% relative error in results ranged from 1 to 336 samples. Under guidelines generally recommended and used for soil sampling (≤20 samples per area), high relative errors (>50%) can be expected from soil test results due to high variability in diary animal holding areas. The soil testing methodology used in this study to generate point sources of data reflected the surface accumulation of selected nutrients in soil. But due to high spatial variability among point sources, traditional methods of sampling may not be of practical value for monitoring or determining accurate levels of nutrient loading.