Mt. Pardo et al., EFFECT OF CULTIVATION ON PHYSICAL SPECIATION OF HUMIC SUBSTANCES AND PLANT NUTRIENTS IN AGGREGATE FRACTIONS OF CRUSTING SOIL FROM ZIMBABWE, Biology and fertility of soils, 25(1), 1997, pp. 95-102
The effects of clearing and cultivation of Rhodic Kandiustalf from Maz
owe (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 depletio
n of plant nutrients in coarse aggregate fractions (i.e. a virtual mig
ration of soil fertility towards microaggregate soil compartments). Th
is effect was connected with the organic matter distribution patterns
in the physical fractions: cultivation has led to a relative concentra
tion of the colloidal fractions (humic acids and fulvic acids) in the
microaggregates, accompanied by a selective depletion of the latter or
ganic fraction. It is suggested that the increased physicochemical imp
ortance of the microaggregate fractions after cultivation may be an in
dex for the disruption of the original soil structure leading to crust
formation and it may also be related to the effect of ploughing on th
e humification processes and the mobility of the colloidal organic fra
ctions.