Over-use of organic fertilisers in agriculture could poison soils

Although controlled amounts of organic residues, sewage sludge and animal waste are a good choice for soil fertilisation, they can have damaging effects on soil biota when applied in excessive doses. In an effort to prevent these toxic impacts on soil, a team of researchers from the UAB’s Centre for Ecological Research and Forestry Applications (CREAF) has carried out a test that sets the maximum safe doses for organic fertilisers.

“We based this on bio-trials in the laboratory using soil-based organisms that are representative of agro-ecosystems, and which need to be protected: plants (Brassica rapa, Lolium perenne and Trifolium pratense), earthworms, annelids, collembola and micro-organisms,” the study’s lead author Xavier Domene told SINC.

The research, which has been published in the magazine Environmental Pollution, shows that the low levels of stability in the residues used is one of the main reasons for their damaging effects on plants and animals. “The rapid decomposition of the residue in the ground generates substances such as ammonia, which is the main cause of the toxic effects observed,” said Domene.

Finding a safe dose

The research group established a “safe dose” for each of the seven residues analysed (two kinds of dehydrated sewage sludge, two kinds of composted mud, two kinds of heat-dried mud, and one sample of heat-dried pig waste).

The researchers believe that using these residues in agricultural fields at levels below this cut-off limit would protect 95% of the species potentially present within an agro-ecosystem. The study goes on to explain that by comparing the safe dose with the amounts usually used it is possible to assess the potential impact on soil biota.

The European Union currently produces a great range of organic residues, using a variety of treatment technologies that minimise their volume and make them easier to handle. According to the researchers, “eco-toxicological criteria should also be included in legislation in order to prevent the environmental impact caused by the use of organic residues”.