Soil organic matter is a very heterogeneous mixture of substances, ranging from plant and root fragments, through the living bodies of the soil organisms, to brown amorphous humic substances produced by their activity. These materials have very different rates of decomposition in the soil and very different effects on the soil tilth and nutrient status. The method of soil cultivation used affects the stabilizing power of a limited addition of plant material. The soil organic carbon content under a well fertilized arable rotation stabilizes at about 1% on many loam and clay soils. Incorporating a 1 year ley in the rotation or ploughing in all the straw produced may increase this by just under 0.1%, and using normal farm dressings of farmyard manure by just over 0.1%. These increases may be additive, but the increases are considerably smaller than this on some soils. Incorporating a 3 year grass ley in a 5 or 6 course rotation increases the carbon by between 0.15 and 0.3%, and there is some evidence that farmyard manure has a greater effect if used in a ley-arable than an all-arable rotation. A 3 year lucerne ley behaves more like a 1 year grass-clover ley except just after it has been ploughed out, though it increases the nitrifiable nitrogen in the soil for several years.