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01.09.2014 | Original Article | Ausgabe 6/2014

Environmental Earth Sciences 6/2014

Measured versus estimated total porosity along structure-stability gradients of coarse-textured tropical soils with low-activity clay

Zeitschrift:
Environmental Earth Sciences > Ausgabe 6/2014
Autoren:
Sunday E. Obalum, Martin E. Obi

Abstract

Soil total porosity is, rather than measured by water desorption method, more often estimated from bulk density (BD) and assumed particle density. Measured and estimated total porosities of even kaolinitic tropical soils (which have low tendency to expand upon wetting) usually differ by an extent that depends on soil structural stability, but such differences are scarcely documented. Seventy samples of coarse-textured soils under different fallow- and cultivation-management systems in the southeastern region of Nigeria were analyzed for texture, mean-weight diameter (MWD) of aggregates, BD and organic matter (OM) concentration. Soil total porosities measured by water desorption method were compared with those estimated from BDs (with particle density fixed at 2.70 g cm−3), after grouping the soils by structural stability, assessed by OM/(silt + clay) for 50 of the samples from fallowed plots (BD > 1.48 g cm−3) and MWD for the rest from cultivated plots (BD < 1.48 g cm−3). The fallowed plots showed a wider stability range than the cultivated plots. Irrespective of land use, structural stability tended to increase with decreasing soil BD. Measured total porosities were consistently higher than their estimated counterparts, with the differences closing up with increasing soil structural stability up till a mean BD of 1.41 ± 0.05 g cm−3 (corresponding to MWD of 2.66 ± 0.12 mm), beyond which the trend reversed. These results suggest that, as the soil structural stability increases, soil particle density decreases while entrapped air and transitory drainage of saturated samples at weighing increase. Estimating total porosity with a fixed particle density of 2.70 g cm−3 appears suitable only in highly stable soils, with BD of ≤1.40 ± 0.08 g cm−3 and/or MWD of ≥2.92 ± 0.05 mm [corresponding to OM/(silt + clay) of ≥16.38 ± 0.28 %].

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