Animal manure, leaf mould, spent mushroom compost, garden compost and green manure crops are all excellent additives when dug into the soil. Animal manures are probably the best because they contain useful quantities of nutrients as well. Animal manures and mushroom compost are readily available in bags and sometimes in bulk. Composted ‘green wastes’ are also readily available, or you can make your own compost.
Green manure crops are also a good source of organic matter but few home gardens have space to grow them nowadays. Peat moss is a good moisture-holding material but it contains negligible quantities of plant nutrients. There are concerns about the non-renewable nature of peat moss so cocofibre peat is an environmentally responsible substitute.
All organic materials will eventually decompose in soil and therefore must be renewed from time to time, especially in annual flower and vegetable beds that are continually cultivated. Clay soils benefit from organic matter too, because it improves their structure by binding clay particles into crumbs. This allows better air and water movement. By adding coarse sand to heavy soils you can make a permanent improvement in their texture. Spread the sand to a depth of 5–8 cm, then mix well into the topsoil to a depth of 15–20 cm.
Gypsum can be incorporated into a clay soil and in most cases will help the soil to function more effectively. Add gypsum at a rate of about 0.5–1 kg per square metre of soil. The crumb structure of clay and clay loam is destroyed if they are dug when too wet. Allow the soil to dry out for a day or two before digging. When cultivating any soil, only dig the topsoil. Do not dig so deeply as to bring subsoil (especially clay) to the surface.
