Soil acidification and the mobilisation of toxic metals caused by acid deposition and fertiliser application

The Classical and other long-term experiments at Rothamsted Experimental Station illustrate the causes and effects of soil acidification. One hundred and fifty years of precipitation measurements show how atmospheric deposition has increased, causing acidification of grassland and woodland soils. Where applied, ammonium fertilisers cause very rapid acidification unless their effects are offset by the application of lime. Acidification causes the mobilisation and removal by leaching of base cations to be replaced by aluminium, manganese or iron, the reduction of base saturation and, in the long-term, the reduction of cation exchange capacity by the weathering of clay minerals. Mobilised toxic metals are taken up by vegetation growing on the acidified soils. Some plots of the Park Grass Experiment have acidified to sufficiently to cause the release of aluminium to be taken up in hay in amounts toxic to cattle - a Chemical Time Bomb.