Effect of high sewage sludge and pig slurry application

In a long term field trial with very high amounts of sewage sludge and pig slurry application (2 and 5 tons of organic matter per hectare and year) the nutrient content of the soil, especially phosphate, increased considerably. The pH-values of the soil decreased in the treatments where no fertilizer or high quantities of pig slurry were applied. In some cases too large amounts of nitrogen in pig slurry significantly decreased the yield. In case of high doses of sewage sludge and pig slurry, the levels of copper, zinc and cadmium in the soil increased remarkably during the trial period of more than twenty years. Levels of zinc and cadmium reached or even exceeded actual limits of Swiss legislation. We observed that the essential elements (copper and zinc) are mainly concentrated in the generative parts (grain), whereas cadmium is mainly found in the vege-tative parts (straw, leaves) of the plant. A lowering of the soil pH-value by a few tenths increased the solubility of the heavy metals in the soil. In some cases this leads to higher heavy metal contents in the plants. The crops grown on unfertilized acid soils can also be enriched with heavy metals. Challenges for the future are a well balanced fertilization adjusted to the plant uptake, adaptation of the livestock units to available agricultural surface and a further decrease of heavy metal limits for soil, sewage sludge and other wastes as well as animal feedingstuffs in the environmental legislation.