INFLUENCE OF CATTLE TRAMPLING ON SOIL POROSITY UNDER ALTERNATE DRY AND PONDED CONDITIONS

The impact of cattle trampling on the porosity of a representative soil (Typic Natraquoll) of the flooding Pampa of Argentina was studied from 1984 to 1987. Where content, total porosity (TP), macroporosity (> 30 mum) and mean weight diameter of water-stable aggregates (MWD) were determined in undisturbed toposoil samples taken from adjacent continuously grazed (1.0 animal unit/ha/yr) and ungrazed (since 1975) area. It was expected that trampling would decrease macroporosity when the soil was ponded, and that the damaged macropores would regenerate during the subsequent soild drying. This was only partly verified. The soil varied in TP from 58 to 64% in the ungrazed area, and from 53 to 78% in the grazed area. This variation resulted mainly from shrink-swell processes. Trampling decreased soil macroporosity (mainly > 60 mum) from 8 to 5% and decreased MWD from 5.35 tto 4.58 mm under dry soil conditions. The damaged soil pores regenerated and aggregate stability recovered during the subsequent period of surface water pounding, when soil swelling increased macropores in the grazed area but not ion the ungrazed area. There was no evidence of poaching damage in this soil.