Importance

Mottles are spots or blotches of different colour, generally grey or orange, interspersed with the dominant soil colour. The number, size and colour of soil mottles provide a good indication of how well the soil is aerated. Loss of structure reduces the number of macropores and coarse micropores that conduct air and water. With the loss of pores, oxygen in the soil is reduced and carbon dioxide builds up.

As oxygen depletion increases, orange, and ultimately grey mottles form. A high proportion of medium and coarse grey mottles indicate that the soil is waterlogged and starved of oxygen for a significant part of the year. Poor aeration and the build-up of carbon dioxide and methane reduce the uptake of water by plants and induce early wilting. Waterlogging can also reduce the uptake of nutrients, particularly nitrogen, phosphorous and potassium by wheat and maize.

Poor aeration retards the breakdown of stubble and other organic residues and can cause reactions that from chemicals that can be toxic to plant roots.

Assessment

Assess the number, size and colour of mottles by comparing the side of the soil profile, or a number of soil clods from the soil structure test, with the three photographs.

 
Good
Moderate
Poor

Scoring

Good condition: Score 2

Mottles are generally absent.

Moderate condition: Score 1

Soil has common (10-25%) fine and medium orange and grey mottles.

Poor condition: Score 0

Soil has abundant to profuse (>50%) medium and coarse orange and particularly grey mottles.