Organic carbon, texture, and quantitative color measurement relationships for cultivated soils in north central Iowa

The quantification of soil organic C (SOC) concentrations is becoming increasingly more desirable because of environmental and economic concerns regarding the reactivity of SOC with pesticides, fertilizers, and waste materials. The objectives of this study were to quantify soil color organic C relationships and to quantify soil particle-size organic C relationships for Ap horizons in north central Iowa. All of the 130 soils examined developed in glacigenic diamicton or local hillslope sediment derived from glacigenic diamicton. A Minolta CR-310 chroma meter was used to quantify the percentage of reflectance, and Munsell value and chroma for both air-dry and moist soils. Organic C concentration of the sample set ranged from 4.4 to 70.8 g kg(-1). Significant relationships were observed between organic C concentration and percentage of reflectance (r(2)=0.77 moist, r(2)=0.74 air-dry), Munsell value (r(2)=0.77 moist, r(2)=0.74 air-dry), Munsell chroma (r(2)=0.68 moist, r(2)=0.77 air-dry), the percentage of sand (r(2)=0.74), the percentage of clay (r(2)=0.71), and geometric mean particle diameter (GMPD) (r(2)=0.74). Logarithmic relationships existed for reflectance, Mansell value and chroma, and GMPD while linear relationships were observed for sand and clay contents. Chroma meter soil color measurements and particle-size data are useful predictors of organic C concentrations for Ap horizons in north central Iowa. Evidence from this study and the literature suggest that unique relationships exist for different soil landscapes.