Spatial Variability and Soil Sampling in a Grazed Pasture

Spatial variability in soil tests for essential plant nutrients influences how well producers accurately sample their fields for fertilization and compliance with environmental regulations. The objective of this study was to determine the effect of spatial variability on soil tests in a pasture system. Soil samples were collected from a 6-ha and a manured 4-ha pasture (study sites 1 and 2, respectively). Samples were analyzed for soil pH, organic matter, and plant-available nitrogen, phosphorus, and potassium. Semivariance parameters from a geographical information system (GIS) were used to quantify spatial variability, and R computer program was used to optimize the number of soil subsamples sufficient to form a representative composite sample. Spatial variability was obtained in some of the soil properties, and approximately 22 soil subsamples were sufficient to form a representative composite sample in a pasture system.