Soil organic matter comes from plants and animals which fungi break down. This 'humus' helps the soil in terms of fertility, structure and water holding capacity.

Between 1980 and 1995 organic matter content levels generally decreased in agricultural top-soils by an average of 0.5%. There was a drop of 0.49% in the 904 arable or ley-arable soils re-sampled in the UK National Soil Inventory.

This doesn't sound very much until you realise that this 0.5% is from a total soil. Usually about 4-6% of the soil is organic matter. Taken as a percentage of the organic matter content, about 12 % of the total carbon was lost in 15 years.

Soil Organic Matter as <3.5%,3.5-7.0%, & >7% of total soil
Source: SSLRC, National Soil Inventory.
Coverage:England & Wales
MAFF (now DEFRA) project quoted in Application of National Sustainable Agriculture Indicators

The largest decreases in organic content have been on grasslands ploughed up for arable use, and on cultivated peaty or organic soils. Some of the decline in organic matter may have been caused by dilution following deeper ploughing over the last 15 years.

Despite this, targets for soil protection are not considered appropriate - partly because of diversity of content. More at "Towards Sustainable Agriculture: A Pilot Set of Indicators" (pdf)