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Walkley-Black Digestion Efficiency and Relationship to Loss on Ignition for Selected Sierra Leone Soils¹

ABSTRACT

Ten top soils sampled from six physiographic positions in the Njala Survey Area of Sierra Leone, West Africa, varied in organic carbon content from 1.02 to 7.82% and in clay content from 22 to 65%. Walkley-Black oxidation efficlency ranged from 93 to 100% and averaged 97%, using Wet Combustion as standard. Organic matter in these tropical soils is apparently in forms easily oxidized by the sulphuric acid-dichromate digestion.

Simple correlation between organic carbon (C) and loss on ignition (L.O.I.) was very strong, r = 0.997; partial correlations showed that when organic C was held constant, 31% of the variation in L.O.I. was associated with clay content variation, and when clay content was held constant, 98% of the variation in L.O.I. was associated with organic C.

Ignition loss of organic C was closely predicted by the linear relationship y = 0.505 x – 0.094, where x = % organic C. Prediction error (about 0.5%) was proportionately greater at lower levels of ignition loss.

¹ Contribution from the Agron. Dep. Njala Univ. College, Private Mail Bag, Freetown, Sierra Leone, West Africa.

² Senior Lecturer, Former B. Sc. Agriculture, Undergraduate Student, and Lecturer, respectively.

Received for publication January 9, 1981. Accepted for publication July 28, 1981.