Brett Allen, Northern Plains Agricultural Research Lab, USDA-ARS, 1500 N Central Ave, Sidney, MT 59270, Antonio Mallarino, 3216 Agronomy Hall, Iowa State University, Iowa State University, Department of Agronomy, Ames, IA 50011-1010, and Jeremy G. Klatt, Iowa Department of Natural Resources, 2300 15th St. SW, Mason City, IA 50401.
Manure application contributes to crop-available soil P. However, high temporal and spatial variability of soil P in manured fields may hinder the value of soil P testing for agronomic and environmental purposes. To better understand soil P variation in manured fields, soil P was measured monthly for 1 yr following application of three rates of manure (from egg layers) to 0.4-ha research plots that had a 5-yr history of manure application incorporated into the soil in soybean residue in a corn-soybean rotation. Application rates were a control, 38 kg P ha-1, and 76 kg P ha-1. Each plot was divided into six equally-sized areas, where a composite soil sample consisting of 12 cores was taken to a 15-cm depth every sampling date. Soil-test P (Bray-1) spatial variability was very high, and was 49-168 and 87-192 mg kg-1 for the low and high manure rates, respectively. Temporal soil P variation was comparatively lower. Such a high soil-test P spatial variation was of little short-term relevance for crop yield because all values were above optimum levels (20 mg kg-1). However, data from these and other plots for P loss through subsurface drainage and surface runoff suggest that such a high soil P variation at those levels is very relevant for environmental P assessments and may seriously impair the value of soil testing to assess risk of P loss from manured fields. The results emphasize the need for effective and representative soil sampling methods for manured fields.
