Soil organic matter (SOM) is a key indicator of soil fertility, yet variability among analytical methods limits its reliable interpretation, particularly in tropical soils. This study evaluated four SOM determination methods—Walkley–Black (WB), loss on ignition (LOI), colorimetric (COL), and hydrogen peroxide (HP)—and examined their relationships with maize growth under contrasting parent materials in southwestern Nigeria. Soil samples were analyzed for organic matter content and subsequently used in a screenhouse to to assess plant height, stem girth, and dry matter yield of maize. The result indicated that the organic matter determined was in the order of HP > LOI > COL > WB. Parent material affected the organic matter content, as soils from sedimentary parent material had higher organic matter than soils from the basement complex. A strong, significant relationship was observed among these methods as their R2 ranged from 0.9028 to 0.9686, though COL and LOI correlated best with each other. The regression equations are WB = 0.9041 (COL), WB = 0.8187 (LOI), COL = 0.9141 (LOI), HP = 5.0597 (LOI), WB = 0.1555 (HP), and COL = 0.1667 (HP). Colorimetric method correlated very significantly with all agronomic parameters during the vegetative stage (2 – 6 weeks after planting), whereas WB correlated best with the agronomic parameters at the reproductive stage (8WAP). The order of relationship between organic matter methods and dry matter was WB > HP > COL > LOI; a similar order was observed for plant height. It is therefore concluded that the Walkley Black method could be used as a reliable index for predicting crop response in tropical soils.
Odelana et al. (Fri,) studied this question.