Temporal Variation in Soil Test Values in Mississippi.

Nutrient management based on recent soil tests is integral to economical and environmentally sound crop management. Based on historical information, soil test results vary between spring and fall sampling dates due to climate conditions during the growing season and at sampling time. Soil test-based phosphate (P) and potassium (K) recommendations offered for soybeans based on samples analyzed by the Mississippi State University Extension Service Soil Testing Laboratory (MSU ES STL) over a fifteen year period found, based on soil region, 10 to 61% of the soybean samples were medium or lower, thus triggering at least maintenance recommendations. The cost of providing inorganic fertilizer increased significantly in 2008 and have maintained that level. More modern information on soil sampling timing and the influence of soil moisture is needed to optimize soil plant nutrient recommendations. Georeferenced sites for monthly fertility and soil moisture sampling were established at nine locations in cooperation with Experiment Station and Extension staff in April 2011. Other parameters measured at initiation were bulk density and particle size. Monthly soil samples for fertility and moisture are collected at eight continuing sites as various factors precluded sample collection from some sites on occasion. The standard procedures of MSU ES STL are used to analyze each sample after sub-sampling for moisture content. Variation in P and K through April-March fiscal years of the project is sinusoidal in nature, and ranges up to 40% from levels of the first sample.