Air-Drying Effect on Soil Reaction and Phosphorus Extractability from Upland-Lowland Tropical Soils as Related to Their Colloidal Stability

Abstract

Although air-drying is a universally accepted practice for preserving soils before analyses, it can irreversibly affect certain soil processes and, hence, results of soil analyses. Reports about such effects on extractable/available phosphorus (Pav) are conflicting and are scanty for low-Pav tropical soils. Moreover, little is known about its relative effects for well and poorly drained soils or about any underlying role of soil colloidal stability. In this study, soil samples from well-drained uplands and adjoining poorly drained lowlands in four locations in southeastern Nigeria were analysed field-moist and air-dried for pH and Pav. The soils are mostly medium-textured. A trend existed of drying-induced decreases in soil pH for both uplands (4.8-5.8) and lowlands (4.6-5.6); whereas this effect was consistently non-significant in the former (unless tested for irrespective of location, 5.35 vs 5.53), it was location-specific in the latter. Air-drying tended to increase and decrease Pav in upland soils (0.62-12.75 mg kg–1) and lowland soils (0.93-20.52 mg kg–1), respectively, with a reverse effect in one lowland with values exceeding 15 mg kg–1. However, these effects were non-significant except in one upland soil with evidence of appreciable organic matter. Correlations between soil pH and Pav were positive and non-significant, but strongest for field-moist lowland soils. Soil colloidal stability indices of water-dispersible clay and clay flocculation index proved useful in understanding the results for soil pH and Pav, respectively, with the effect of the former being more distinct than that of the latter. Air-drying is still recommended only for upland soils while we explore fully the interrelationships among drying-induced differences and colloidal stability for soils of contrasting drainage status.