The compaction susceptibility of soils in the Free State
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Date
1998-12
Authors
Nhantumbo, Alfredo Bernardino Julio Da Costa
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Crop production in the Free State is mainly performed on sandy soils. The compaction of
these apedal soils can present a management problem which affects crop yields and
financial returns. In addition, environmental problems such as soil erosion and siltation of
rivers may result.
The main objective of the study was to see whether the easily measured soil properties,
texture and organic matter, could be used to classify the compact ion degree of soils and
to predict their compaction susceptibility. Additional objectives were to define critical
densities beyond which crop growth would be impeded, to test the applicability of the
relative bulk density concept proposed by Bennie & Van Antwerpen (1988) and to
propose a procedure for the prediction and classification of the compactibility of the soils
in the Free State. Finally, the results were compared to the data from forestry soils
(Smith, 1995) for more humid conditions.
Twenty two (22) samples of selected soils covering a large range of soil texture variation
were submitted to a determination of the maximum bulk density (using the Proctor test),
minimum bulk density, uniaxial compression test and penetrometer resistance
measurements. A procedure for the determination of minimum bulk density was
developed.
Good quadratic relationships were obtained between the maximum bulk density and silt
plus clay or organic matter content as loss on ignition. The minimum bulk density can be
predicted from the silt plus clay percentage or loss on ignition. When the actual bulk
density and the silt plus clay or loss on ignition percentages are available, the
relationships allow the estimation of maximum and minimum bulk densities. The
measured bulk density can then be used to calculate the relative bulk density of a soil.
Threshold relative bulk density classes (5) were derived which can be used to evaluate
the degree of denseness or looseness of the soil, and its suitability for different
applications.
It is proposed that compactibility be regarded as the difference between the maximum
and minimum bulk densities, instead of simply the maximum bulk density. Different
classes of compactibility were derived. The compression index of all the soils at the
critical water content showed good relationships with the silt plus clay, clay or organic
matter contents of the soils. Compressibility classes were also established. A combination
of the compactibility and compressibility of soils was used to determine its susceptibility
to compaction. Compaction susceptibility was classified into five groups varying from
very low to very high degrees of susceptibility for apedal soils.
The data set from this study when combined with the data reported from the study of
forestry soils by Smith (1995), made it possible to derive equations with a more universal
applicability for the prediction of the maximum and minimum bulk densities, critical
water content as well as compression index.
The results from this investigation, when properly transferred in popular terminology set
guidelines for the evaluation of the degree of compactness and compaction susceptibility
of soils. These guidelines will allow persons responsible for soil evaluation to draw more
meaningful conclusions from bulk density measurements.
Description
Keywords
Soil compaction -- South Africa -- Free State, Dissertation (M.Sc.Agric. (Soil Science))--University of the Free State, 1998