Seasonal variations in soil test results

Soil test results can vary for a multitude of reasons. Sampling errors, contamination, and analytical errors plus true differences caused by cropping and fertilizer application exist. Seasonal differences have also been reported. Six plots receiving variable P‐K fertilizers were sampled monthly over a 38 month period and analyzed for P, K, Ca, Mg, pH, OM, SO4‐S, B, and Zn. Results show that appreciable differences in test results can be associated with month of the year and time since nutrient application. Average differences between months as great as 18 lb/A P, 100 lb/A K, 56 lb/A Mg, 260 lb/A Ca, 0.6% OM, 4 ppm SO4‐S, 0.1 ppm B, 2 ppm Zn, and .4 pH units were observed on medium to fine textured soils in Ohio, Monthly differences could be observed with both fertilized and non‐fertilized soils. Standard deviations among months were greater with P‐K applications, but the coefficient of variation was less. Use of a moving average for measuring seasonal variations reduced both the standard deviation and coefficient of variation. Fall possibly appears to be the best time to take soil samples where fertilizers are applied in the spring. Timing of soil samples relative to season and fertilizer applications should be considered when evaluating results.