Residual Macro- and Micronutrients Levels: Five Years of Five Rates of Compost In An Organic Field Trial.

Changes in soil plant available nutrients derived from compost application and their effects on plant development have been observed in previous experiments. However, little research has been conducted with emphases on residuality of soil plant available macro- and micro- nutrient levels after the application of varying levels of compost has been discontinued for a number of years, in organic farming systems. The objective of this study was to measure the residual effect of prior compost treatment on macro- and micro- nutrient levels in the soil. In 2000 at the WVU Certified Organic Research Farm in Morgantown (WV), a compost application study was initiated with the establishment of 40 plots, each 1.4 x 2.4 m. Five compost levels with four replications were randomly assigned and compost applications were performed in the spring; the compost rates were 0, 5, 10, 20 and 40 tons. The plots were limed individually as needed, and winter rye was planted at the end of the year.  The previously described management was continued until 2004.  After 2005, no crops were grown or compost was applied, and in 2009 the plots were rototilled for weed control, with no further management practice.

In 2010, after 5 years in fallow, an intensive soil sampling was performed individually in all the plots. Stratified cores samples were taken to determine the Mehlich III plant available nutrients (organic matter, pH, plant available P, N, K, Ca, Mg, Zn, B, Cu, and Mn). Soil test results were then statistically evaluated using analysis of variance (ANOVA), simple linear correlation (Pearson correlation coefficient), and regression analyses.  Residual effects from prior compost application rates were observed in increasing amounts of organic matter (20.9 g/kg to 31.3 g/kg), P (14 to 149 ppm), Calcium, (1047 to 2372 ppm), pH (4.9 to 6.0), Magnesium (56 – 164 ppm), Zinc (2.2 to 12.6 ppm), Boron (0.50 to 0.98 ppm), Copper (1.26 to 2.12 ppm) and total percent Nitrogen (.14 to .23) as previous compost application rates were increased.  ANOVA analysis showed with 99% confidence level that all macro-nutrients levels of P, K, Ca, Mg, and N and micro-nutrient levels of Zn, B, and Cu were statistically and significantly related to the previously applied compost. Measured levels of Mn were not statistically related to compost application (p = .3564). Correlation analysis shows statistical significance (99% confidence) in levels of P, K, Ca, Mg, N, Zn, B, and Cu to organic matter and pH, but Mn was not statistically significantly correlated.  As shown in the statistical analysis, both macro- and micro- nutrient levels appear to be dependent on residual fertility in consequence of prior compost rate treatment. The variability of macro- and micro- nutrient contents among the residual treatments suggest that compost application rates, even after five years of fallow, had a significant impact on the amount of nutrients in the soil in an organic farming system.