Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems

Even though organic management practices are intended to enhance soil performance by altering the quantity or quality of soil organic matter (SOM), there is no consensus on how to measure or manage SOM status. We investigated the veracity of common perceptions about SOM quantity in organically and conventionally managed soils by evaluating the relative responsiveness to organic management of particulate organic matter (POM) and the Illinois Soil N Test (IL‐N), which has been proposed as a direct measure of labile N. Soil samples were obtained from nine farming systems trials in the USA. Soil organic C (SOC), total N (TN), POM‐C, POM‐N, and IL‐N were compared among manure + legume‐based organic, legume‐based organic, and conventional farming systems. The organic systems had higher SOC and TN concentrations than conventional systems whether or not manure was applied. The POM‐C, POM‐N, and IL‐N concentrations did not differ between manure + legume‐ and legume‐based organic systems. The amount of N recovered in POM and IL‐N was similar. Organic management enriched soil POM‐C and ‐N by 30 to 40% relative to the conventional control and this level of enrichment was two to four times greater than that in any other fraction. The IL‐N fraction was not a good measure of labile N as it was less enriched than POM and included recalcitrant components. This is evidenced by the strong correlation between IL‐N and SOC, TN, climate and textural characteristics. Particulate organic matter provided clearer evidence of SOM and labile N accrual under organic management. Direct links between POM status and soil N supply and physical condition are being pursued to help farmers manage biologically based fertility.