sorghum or peanuts. Depending on the nutritive content of the digested slurry, 3-6 t of solid
substance per hectare will be required to cover the deficit. For supply with a moisture content
of 90%, the required quantity comes to 30-60 t per hectare and year. That roughly
corresponds to the annual capacity of a 6-8 m3 biogas plant.
Figure 44: Field experiments with sludge in Thailand
Photo: Kossmann (gtz/GATE)
Caustic effect on grassland
Digested sludge has much less caustic effect on grassland than does fresh liquid manure.
Effluent sludge is also very suitable for use as a "top-dressing" whenever its application is
deemed to have the best fertilizing effect.
Eutrophication
Serious ecological damage can be done by applying fertilizing sludge in excessive amounts
or at the wrong time, namely when the assimilative capacity of the plants is low. Nitrogen
"washout" can cause over-fertilization (eutrophication) of ground and surface water.
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