Box 7GThe Rothamsted experiment (UK)

The so-called Rothamsted experiment began more than a century ago. In 1870, researchers at Rothamsted built drainage gauges to measure the amount of water draining from soil which had received no fertilisers since 1868. From 1877, the scientists also measured the concentrations of nitrates in the drainage water. During the first seven years of these measurements, the soil leached an average of 45 kilograms of nitrogen per hectare as nitrate each year. Only 3 to 5 kilograms of this nitrogen was brought in by rain; almost all of the remainder must have come from the organic matter in the soil. Over a longer period, between 1870 and 1915, the difference between the amount of nitrogen leached as nitrate and the amount supplied in rain was close to the amount of organic nitrogen lost from the soil.

The data from the drain gauges raise two important points: firstly, although no crops were grown and no manure was applied, the amount of nitrate leaching from the soil matches some recent estimates of the nitrate leached by soils carrying fully fertilised crops of winter wheat. Secondly, the leaching continues for a very long time; calculations have shown that it took 41 years to decline to half its initial rate. This implies that there is a large pool of decomposable organic nitrogen in the soil, which is broken down very slowly. Researchers at Rothamsted believe that nitrate released from decomposition of soil organic matter is responsible for more of the nitrate in groundwater than are direct losses from chemical fertilisers. The research at Rothamsted Experimental Station thus shows that the blame for nitrate pollution should not fall only on farmers and use of excessive amounts of nitrogenous fertiliser. The problem is complex and still little is known about the dynamics of nitrogen through different components of soil organic matter. This is relevant to the question of 'set-aside' (taking agricultural land out of production); since soil left bare will leach nitrate, a good option would be to 'set aside' land under grass or forestry.

Source: Addiscott, 1988