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

SOIL INDICATORSNumber <strong>and</strong> colour of soil mottles under pasture 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 MOTTLES indicate how wellthe soil is drained <strong>and</strong> how well it is aerated. They are also an earlywarning of a decline in soil structure, <strong>and</strong> show whether the soil is beingdamaged by pugging. Loss of soil structure reduces the number ofchannels <strong>and</strong> pores in the soil that conduct air <strong>and</strong> water. This results ina deficiency of oxygen <strong>and</strong> a build-up of carbon dioxide. As oxygendepletion increases, orange <strong>and</strong> ultimately grey, mottles form. A highproportion of grey mottles indicates the soil is waterlogged <strong>and</strong> starvedof oxygen for much of the year. Poor aeration reduces the uptake ofwater <strong>and</strong> plant nutrients, particularly nitrogen, phosphorous <strong>and</strong>potassium. Poor aeration also retards the breakdown of organicresidues, <strong>and</strong> can induce chemical reactions that form chemicals toxicto plant roots.62 VISUAL SOIL ASSESSMENT: Volume 1