The volume of air required for cooling is usually greater than that required for supplying oxygen. There is a gradient of temperature from the centre of the heap to the outside, so mixing the heap at intervals ensures that all the material is composted at the same rate. Water Microorganisms require water to survive and so a minimum moisture level is needed for optimum activity. The maximum water content depends on the physical structure of the material, since the spaces between the particles contain either water or air. The coarser the material, the larger are the spaces and the more water can be held without restricting aeration.
Water is lost from the heap as vapour so the moisture content falls over time.
It is difficult to add water because the heap tends to shed it before it can penetrate far, so it is preferable to add water during mixing. Under conditions of high moisture content, liquid may drain from the heap. This leachate may contain high levels of nutrients, so it should be collected rather than allow it to drain into the soil. The leachate can be used to re-wet the heap if necessary. As well as having the correct balance of nutrients, the mixture should also have the correct balance of physical properties.