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

SOIL INDICATORSNumber <strong>and</strong> colour of soil mottles under <strong>cropping</strong> Assess the number, size <strong>and</strong> colour of mottles by comparing the sideof the soil profile, or a number of soil clods from the soil structuretest, with the three photographs opposite.Mottles are spots or blotches of different colour interspersed withthe dominant soil colour.The percentage chart below will help you determine the percent ofthe soil occupied by mottles.THE NUMBER, SIZE AND COLOUR of soil mottles provide a goodindication of how well the soil is aerated. Loss of structure reduces thenumber of macropores <strong>and</strong> coarse micropores that conduct air <strong>and</strong> water.With the loss of pores, oxygen in the soil is reduced, <strong>and</strong> carbon dioxidebuilds up. As oxygen depletion increases, orange, <strong>and</strong> ultimately grey,mottles form. A high proportion of medium <strong>and</strong> coarse grey mottlesindicates that the soil is waterlogged <strong>and</strong> starved of oxygen for asignificant part of the year. Poor aeration <strong>and</strong> the build-up of carbondioxide <strong>and</strong> methane reduce the uptake of water by plants, <strong>and</strong> induceearly wilting. They can also reduce the uptake of plant nutrients,particularly nitrogen, phosphorus <strong>and</strong> potassium, by wheat <strong>and</strong> maize.Poor aeration also retards the breakdown of stubble <strong>and</strong> other organicresidues, <strong>and</strong> can cause reactions that form chemicals toxic to plant roots.22 VISUAL SOIL ASSESSMENT: Volume 1