Development of a Loss On Ignition Procedure for Measurement of Soil Organic Matter in Kansas Soils and Comparison to Other SOM Tests.

Accurate and precise organic matter (OM) measurements are important to Kansas farmers as each one percent soil organic matter (SOM) is credited to supply 11.2 or 22.4 Kg N ha^-1, for winter and summer crops, respectively.  Walkley-Black (WB) measures easily oxidizable carbon (C), produces hazardous waste, and varies in total C recovery.  Loss-on-ignition (LOI) measures weight loss of soil after ignition, and does not produce hazardous waste.  Dry Combustion (DC) measures total C, but is expensive in material and labor. Objectives of this study are: 1) compare LOI, WB and DC for Kansas soils 2) evaluate method variances and 3) calculate percent C recovery of WB.  Sample size for LOI and WB was one gram, and 0.35 for DC C.  Additionally, a scooped 0.85 cm³ sample was also analyzed using WB.  Methods were compared with 98 samples in triplicate. Total C recovery by WB was also calculated.  Regression of gravimetric WB to LOI produces a linear equation WB = 0.89 (LOI) - 0.23 with a R² of 0.97.  Using 76 soils with pH < 7.1, LOI gives a linear equation to DC of LOI  = 1.71 (DC) + 0.37 with a R² of 0.98.  Gravimetric WB lends a regression to DC C of WB = 1.42 (DC C) + 0.21, with a R² of 0.97.  Standard deviations were highest with volumetric WB, 0.24, followed by LOI, 0.17, gravimetric WB, 0.13, and DC C, 0.04. Recovery of total C using WB ranges from 55 to 80 percent.