Fig. 1. Influence of farming systems on soil organic matter fractions in surface soils (generally 0 to 25 cm) collected from nine long-term farming systems trials located across the USA. Shown are (a) soil organic C (SOC), (b) total N (TN), (c) particulate organic matter C (POM-C), and (d) particulate organic matter (POM-N), Illinois Soil N Test-N (IL-N), and exchangeable NH₄⁺–N concentrations of manure-based organic (MLO: eight sites), legume-based organic (LO: three sites), and conventional (CV: nine sites) farming systems. Different letters within a fraction show significant differences among farming systems at the p < 0.05 level. Statistical analyses for SOC, TN, IL-N, and POM fractions were conducted on log₁₀ transformed data to improve normality and equality of variances. Untransformed data are presented in the figure.