Arable land holds phosphorus from the past

During the 1960s and 1970s, it was common in Sweden to ‘store-fertilise’, that is farmers fertilised the soil with a large amount of phosphorus fertiliser at a time, enough to cover the need of several years’ crops ahead*. This way of farming the land peaked in the beginning of the 1970s. Today, the soil is mainly fertilised by phosphorus from farmyard manure and legislation regulates how much land a farmer must have to spread the manure, to avoid over-fertilisation. But the phosphorus once added in excess is still present, bound to particles in the soil.

Risk of leakage
General effects of the stored phosphorus are difficult to assess. Losses of phosphorus from the soil to surrounding waters largely depend on soil conditions.

‘The risk of leakage is primarily linked to hydrological conditions in the soil and depends only to a smaller extent on the stored phosphorus. The physical and chemical properties of the soil also determine the risk of leakage. Phosphorus, bound to organic material or soil particles, may be carried by water through cracks in the soil,’ explains Barbro Ulén, who participates in SLU’s monitoring of losses of plant nutrients from agricultural land.

‘Swedish agriculture is developing towards an integration of conventional and ecological farming, with less processing of the soil and less use of fuels, fertilisers and pesticides,’ says Barbro Ulén.

*Nitrogen was never ‘store-fertilised’ in Sweden.