Previous seminars organised by the working party under the auspices of the EEC Concerted Action have been devoted to the evaluation of the nitrogen, phosphorus and organic matter content of sewage sludges, almost invariably over the twelve month period of application. Depending on the type of sludge they have been shown to be useful sources of nitrogen, phosphorus and organic matter for grass, cereals and in land restoration in terms of improving the physical properties of disturbed lands. However, distance from the treatment work severely limits the radius of operation. Whereas it is of value to farmers within reach of the works, the impact which it makes nationally on reducing the annual fertilizer bill is very small.
On grass/arable farms the benefits from sewage sludge can be complementary to those of animal slurries in terms of providing an organic manure with an improved balance of nitrogen, phosphorus and potassium for grass and arable crops.
This was the first seminar at which the experts from both the sewage sludge and animal manure sides came together to discuss their common problems and rightly so. Liquid sludges and animal slurries have much in common though there are differences which must be borne in mind. The main components of both products are nitrogen and organic matter, the availability of the nitrogen depending on treatment and composition of the organic matter.
The present seminar is devoted to the residual and longer term benefits of sewage sludges and farm slurries. There are papers which deal with the availability of soil nutrients from sludges and slurries treated in different ways, on the phosphate balance in soil and the soil ameliorating properties of these organic amendments. Changes on storage and mineralisation in soils after treatment with sludges stabilised by different methods are also discussed. Lastly, but certainly not least, is the important aspect of evaluating the composition of sludges and slurries. Variability in slurry composition often makes it desirable to obtain an ‘on the farm’ estimate of nutrient value if they are to be used efficiently without detriment to the environment.