Effect of cultivation on physical speciation of humic substances and plant nutrients in aggregate fractions of crusting soil from Zimbabwe

The effects of clearing and cultivation of Rhodic Kandiustalf from Mazowe (Zimbabwe) were studied by quantifying humic substances and plant nutrients (available and exchangeable cations) in aggregate fractions. It was found that cultivation was associated with a relative depletion of plant nutrients in coarse aggregate fractions (i.e. a virtual migration of soil fertility towards microaggregate soil compartments). This effect was connected with the organic matter distribution patterns in the physical fractions: cultivation has led to a relative concentration of the colloidal fractions (humic acids and fulvic acids) in the microaggregates, accompanied by a selective depletion of the latter organic fraction. It is suggested that the increased physicochemical importance of the microaggregate fractions after cultivation may be an index for the disruption of the original soil structure leading to crust formation and it may also be related to the effect of ploughing on the humification processes and the mobility of the colloidal organic fractions.