Soil phosphorus variability in pastures: Implications for sampling and environmental management strategies

Soil phosphorus (P) is an increasingly important consideration in the development of P-based nutrient management strategies. The objectives of this study were to (I) obtain baseline information on soil P variability in pastures amended with animal waste, (ii) examine if current sampling recommendations related to the number of sub-samples adequately reduce uncertainty to acceptable limits, and (III) examine the implications of uncertainty in soil P estimates on implementing a soil P threshold of 150 mg kg(-1). Grid soil samples were collected from 12 pastures. Soil P was determined using Mehlich 3 extractant and an inductively coupled argon plasma spectrometer. The arithmetic mean of soil P ranged from 7 mg kg(-1) in a pasture never amended with animal manure to 437 mg kg(-1) in a pasture that had been annually treated long term with poultry litter. Variance of soil P generally increased with mean soil P. The mean standard deviation of all pastures was one-third of the 150 mg kg(-1) threshold. This study points out that smaller variances associated with mean soil P values that approach, but do not exceed, the threshold can influence estimates of soil P. In turn, management decisions could inappropriately change. When a uniform acceptance criteria (within 15 mg kg(-1)) with respect to measured means was used, the required minimum number of subsamples increased with measured standard deviation. The results of this study imply that following soil-sampling recommendations Is critical to obtaining trustworthy measures of central tendency, especially in pastures approaching but not exceeding the 150 mg kg(-1) threshold.