The compaction susceptibility of soils in the Free State

[摘要] Crop production in the Free State is mainly performed on sandy soils. The compaction ofthese apedal soils can present a management problem which affects crop yields andfinancial returns. In addition, environmental problems such as soil erosion and siltation ofrivers 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 andto predict their compaction susceptibility. Additional objectives were to define criticaldensities beyond which crop growth would be impeded, to test the applicability of therelative bulk density concept proposed by Bennie & Van Antwerpen (1988) and topropose a procedure for the prediction and classification of the compactibility of the soilsin 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 variationwere submitted to a determination of the maximum bulk density (using the Proctor test),minimum bulk density, uniaxial compression test and penetrometer resistancemeasurements. A procedure for the determination of minimum bulk density wasdeveloped.Good quadratic relationships were obtained between the maximum bulk density and siltplus clay or organic matter content as loss on ignition. The minimum bulk density can bepredicted from the silt plus clay percentage or loss on ignition. When the actual bulkdensity and the silt plus clay or loss on ignition percentages are available, therelationships allow the estimation of maximum and minimum bulk densities. Themeasured 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 evaluatethe degree of denseness or looseness of the soil, and its suitability for differentapplications.It is proposed that compactibility be regarded as the difference between the maximumand minimum bulk densities, instead of simply the maximum bulk density. Differentclasses of compactibility were derived. The compression index of all the soils at thecritical water content showed good relationships with the silt plus clay, clay or organicmatter contents of the soils. Compressibility classes were also established. A combinationof the compactibility and compressibility of soils was used to determine its susceptibilityto compaction. Compaction susceptibility was classified into five groups varying fromvery 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 offorestry soils by Smith (1995), made it possible to derive equations with a more universalapplicability for the prediction of the maximum and minimum bulk densities, criticalwater content as well as compression index.The results from this investigation, when properly transferred in popular terminology setguidelines for the evaluation of the degree of compactness and compaction susceptibilityof soils. These guidelines will allow persons responsible for soil evaluation to draw moremeaningful conclusions from bulk density measurements.

[发布机构] University of the Free State