Field Guide. cropping and pastoral grazing on flat to rolling country.

PLANT INDICATORSSurface ponding under <strong>cropping</strong> Assess the degree of surface ponding.Base your assessment on ponding present at the time, or your generalrecollection of the time ponded water took to disappear after a wetperiod, or after heavy rainfall in the winter (see opposite).THE LENGTH OF TIME WATER REMAINS PONDED on the surfaceindicates the rate of infiltration into the soil, <strong>and</strong> the time the soil remainssaturated. Prolonged waterlogging depletes oxygen <strong>and</strong> causes carbondioxide to build up. Anaerobic conditions develop <strong>and</strong> induce a seriesof chemical <strong>and</strong> biochemical reduction reactions that produce byproductstoxic to plant roots. These include hydrogen sulphide <strong>and</strong>ferrous sulphide produced by the reduction of sulphate <strong>and</strong> iron, <strong>and</strong>nitrite produced by denitrification. Organic substances can alsoanaerobically degrade in degraded soils, the most toxic by-products ofwhich include ethylene, <strong>and</strong> acetic, butyric, <strong>and</strong> phenolic acids. In aword, the soil goes ‘sour’. Waterlogging delays cultivation because thelow load-bearing capacities of the soil increase its susceptibility todamage through deformation <strong>and</strong> excessive wheel slip. Sowing is alsodelayed because the seedbed is below the critical temperature for cropgermination.46 VISUAL SOIL ASSESSMENT: Volume 1