Lack of organic matter in soil reduces water retention. Organic matter not only helps soil retain water, it also creates soil fertility. Organic matter is made from decomposing animal and plant residues and living microorganisms such as earthworms, beneficial bacteria, fungi, protozoa and small arthropods. Each teaspoon of productive soil contains billions of living organisms that help it retain water. "Stabilised organic matter acts like a sponge and can absorb six times its weight in water," says the University of Minnesota Extension's Soil Management Series. Home gardeners can improve soil's water retention by adding compost to the garden.
Dig compost into garden soil to a depth of 8 to 12 inches. One shovelful per square foot is adequate for a new vegetable or flower bed. Mature compost is dark, loose and crumbly.
Add a half shovelful of compost to each plant as a side dressing during the growing season. The compost decomposes slowly, fertilising and helping soil retain water.
Leave two-third of the harvested plants on the ground after vegetables or flowers are removed. Topsoil and water retention are protected by their slow decomposition. Soil improves yearly as layers of decomposing plants improve its structure, fertility and water retention.
Spread dry leaves, pine needles, shredded bark or newspaper over the garden in winter to improve water retention in spring. The materials decompose slowly, adding texture and structure to the soil. You can buy or make your own compost. Local recycling agencies conduct composting workshops and offer low-fee compost bins.