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

The impact of cattle trampling on the porosity of a representative soi l (Typic Natraquoll) of the flooding Pampa of Argentina was studied fr om 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 contin uously grazed (1.0 animal unit/ha/yr) and ungrazed (since 1975) area. It was expected that trampling would decrease macroporosity when the s oil was ponded, and that the damaged macropores would regenerate durin g 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 p rocesses. Trampling decreased soil macroporosity (mainly > 60 mum) fro m 8 to 5% and decreased MWD from 5.35 tto 4.58 mm under dry soil condi tions. The damaged soil pores regenerated and aggregate stability reco vered during the subsequent period of surface water pounding, when soi l swelling increased macropores in the grazed area but not ion the ung razed area. There was no evidence of poaching damage in this soil.