Agriculture Reference
In-Depth Information
soils have a greater capillary rise because of their higher percentages of smaller pores;
thus, the effects of differences in texture on the composition of the soil atmosphere are
clearly interlinked with those of structure and soil moisture.
Soil structure (see Section I.1.3, below) has a major effect on soil aeration; a structure
favourable for plant growth allows rapid infiltration and drainage and adequate aeration
through the larger, air-filled pores while still permitting sufficient moisture to be retained for
plant requirements. The clearly negative effects of excessive cultivation and compaction
in reducing soil aeration are mediated through the modification of soil structure.
The spatial distributions of soil gases are notably heterogeneous. Steep vertical
gradients occur towards the soil surface, although the nature of these gradients will
depend on the properties of individual soils. Local concentrations of organic matter in
the soil may, by providing a substrate for microbial respiration, result in locally-elevated
carbon dioxide levels; declining concentration gradients also occur away from actively-
respiring roots.
1.2.2
CARBON DIOXIDE
Carbon dioxide is a gas of major importance in soils and is produced by the respiration
of plant roots, micro-organisms and, to a lesser extent, the soil fauna. This compound
