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RESEARCH ARTICLE

Variation in soil strength, bulk density and gravel concentration along a toposequence in Abeokuta, south-western Nigeria

F. K. Salako A C , P. O. Dada B , J. K. Adesodun A , F. A. Olowokere A and I. O. Adekunle A
+ Author Affiliations
- Author Affiliations

A Department of Soil Science and Land Management, University of Agriculture, PMB 2240, Abeokuta, Nigeria.

B Department of Agricultural Engineering, University of Agriculture, PMB 2240, Abeokuta, Nigeria.

C Corresponding author. Email: kfsalako@yahoo.ie or fsalako@ictp.it

Australian Journal of Soil Research 45(8) 643-650 https://doi.org/10.1071/SR07057
Submitted: 15 May 2007  Accepted: 18 October 2007   Published: 7 December 2007

Abstract

This study was carried out at Abeokuta, south-western Nigeria, to understand the variation in soil strength, gravel distribution, and bulk density along a toposequence. In 2003, a 120-m transect on a fallowed land was sampled at every 1 m for topsoil bulk density measurement by excavation (3278 cm3 pits), while soil strength was measured at every soil depth increment of 25 mm to 0.50 m depth. Total dry (ρt) and fine earth (<2 mm) (ρf) bulk densities were determined. Soil water content was also determined. Gravel was divided into classes of 2–4, 4–8, 8–16, and >16 mm. In 2006, four 100-m transects were considered; two each on adjacent fallowed and cultivated lands. Soil strength and water content were measured. The fine earth fraction of topsoil ranged from 62 to 90.6%. Gravel in the 2–4 mm class was dominant with a range of 0.8–35.7%. Thus, cores ≥50 mm could be used in the topsoil to obtain reliable estimates of bulk density.

Total bulk density (ρt) was reduced by 4–19% when corrected for gravel to obtain ρf. Soil strength of the lower slope was highest in 2003 (1981–4482 kPa) and lowest in 2006 (1546 kPa). In spite of the apparent significant influence of water content on soil strength, the relationship was weakly expressed by regression analysis, as only 35% of variation in soil strength was explained by water content at 0.10–0.15 m soil depth in 2003. No relationship was found in 2006; the cultivated segment had higher soil strength (2045 kPa) than the fallowed segment (1970 kPa) even though the water contents were similar. Also, only the 2–4 mm gravel significantly influenced ρt. Land use, soil depth, and slope position significantly affected soil strength. Root-limiting soil strength (>2000 kPa) would certainly be encountered below 0.20 m soil depth in the wet season irrespective of land use. Management of this gravelly landscape must be based on the heterogeneous nature of soil physical properties along the toposequence, and this could be made effective by grouping the soils according to slope position and taking interest in the few portions of the landscape with extreme values of gravel distribution and high soil strength.

Additional keywords: penetrometer resistance, rock fragments, fallow, cultivation, compaction.


Acknowledgments

The authors wish to thank Iro A., Oumarou A., Monday Joseph, Bello Ayoade, Bisi Sonuga, and Temitope. A. Adewole, formerly of the Department of Soil Science and Land Management, University of Agriculture, Abeokuta, for their technical and field support. The paper was written when the first author was on sabbatical leave in the Department of Environmental Sciences, University Ca’Foscari of Venice (UNIVE), Venice, Italy, under the Training in Italian Laboratory Programme (TRIL) of the International Centre for Theoretical Physics (ICTP), Trieste, Italy. He expresses his gratitude to Profs. G. M. Zuppi (UNIVE) and G. Furlan (TRIL-ICTP) for their encouragement and support.


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