Soil porosity changes by compression
The larger soil pores (macropores) are the most easily compressed by cropping machinery, because they are usually full of air; the air is forced out of the soil when compacted. Smaller pores (mesopores) are less easily compressed, they are often full of water when the soil is moist; water cannot be compressed and is not easily squeezed out of the soil. When macropores are closed by compression they form more mesopores and allow increased water-holding. In sandy soils, which hold water poorly, this can lead to unexpected increases in crop growth and yield in dry seasons in some circumstances.
Macropores are large enough to be emptied of water by gravity; a condition usually found a day or two after heavy winter rain ('field capacity'). They play a role in transmitting air and water through the soil. Macropores include shrinkage cracks, burrows made by soil macrofauna (worms, ants and termites) and old root channels. Mesopores are small enough to retain water against the pull of gravity at field capacity, to be available to plants.
Compaction by wetting and drying
As soils absorb water and they drain and have water extracted from them, the forces developed between soil particles in the menisci (skins) which separate water from air can be large enough to draw soil particles together. You can see the same forces at work when you put a dry paintbrush under water then lift it out; the water films draw the bristles of the brush together.
With each cycle of wetting and drying, loose decompacted soils will be drawn together more by these forces known as 'effective stress'. Extreme wetting or flooding of soils and their collapse and drainage can lead to severe compaction and hard setting. This is exacerbated by chemical instability ('gypsum responsiveness') and can be minimised by inclusion of gypsum application as well as deep ripping.
Geological and pedological causes of compaction
Many of the ancient soils of Western Australia have undergone extreme natural forces in their development. Previous overburden forces, even from glaciers in some places have squeezed sediments and soils together into dense hard layers in the subsoil; extreme wetting and drying events have done this too.
Additionally, there has been opportunity for natural cements made in the soil from compounds of silica, iron and manganese to solidify soil layers in the subsoil; 'coffee rock' is a common example of this in the north-eastern wheatbelt.
Many such layers may not be easily or profitably altered by current soil management techniques and caution is advised when considering curative options for the compact layers they form.