Organic Fertilizer from Biogas Plants
Organic substances in fertilizers
While there are suitable inorganic substitutes for the nutrients nitrogen, potassium and
phosphorous from organic fertilizer, there is no artificial substitute for other substances such
as protein, cellulose, lignin, etc.. They all contribute to increasing a soil’s permeability and
hygroscopicity while preventing erosion and improving agricultural conditions in general.
Organic substances also constitute the basis for the development of the microorganisms
responsible for converting soil nutrients into a form that can be readily incorporated by plants.
Nutrients and soil organisms
Due to the decomposition and breakdown of parts of its organic content, digested sludge
provides fast-acting nutrients that easily enter into the soil solution, thus becoming
immediately available to the plants. They simultaneously serve as primary nutrients for the
development of soil organisms, e.g. the replenishment of microorganisms lost through
exposure to air in the course of spreading the sludge over the fields. They also nourish
actinomycetes (ray fungi) that act as organic digesting specialists in the digested sludge.
(Preconditions: adequate aeration and moderate moisture).
Reduction of soil erosion
The humic matter and humic acids present in the sludge contribute to a more rapid
humification, which in turn helps reduce the rate of erosion (due to rain and dry scatter) while
increasing the nutrient supply, hygroscopicity, etc. The humic content is especially important
in low-humus tropical soils. The relatively high proportion of stable organic building blocks
such as lignin and certain cellulose compounds contributes to an unusually high formation
rate of stable humus (particularly in the presence of argillaceous matter). The amount of
stable humus formed with digested sludge amounts to twice the amount that can be
achieved with decayed dung. It has also been shown that earthworm activity is stimulated
more by fertilizing with sludge than with barnyard dung.
Digested sludge decelerated the irreversible bonding of soil nutrients with the aid of its ion-
exchanger contents in combination with the formation of organomineral compounds. At the
same time, the buffering capacity of the soil increases, and temperature fluctuations are
better compensated.
Reduction of nitrogen washout
The elevated ammonium content of digested sludge helps reduce the rate of nitrogen
washout as compared to fertilizers containing substantial amounts of more water-soluable
nitrates and nitrites (dung, compost). Soil nitrogen in nitrate or nitrite form is also subject to
higher denifrication losses than is ammonium, which first requires nitrification in order to
assume a denitrificable form. It takes longer for ammonium to seep into deeper soil strata, in
part because it is more easily adsorbed by argillaceous bonds. However, some of the
ammonium becomes fixed in a non-interchangeable form in the intermediate layers of clay
minerals. All aspects considered, it is a proven fact, that ammonium constitutes the more
valuable form of nitrogen for plant nutrition. Certainly, the N-efficiency of digested sludge
may be regarded as comparable to that of chemical fertilizers.
In addition to supplying nutrients, sludge also improves soil quality by providing organic
mass. The porosity, pore-size distribution and stability of soil aggregates are becoming
increasingly important as standards of evaluation in soil-quality analyses.
Effects on crops
Crop yields are generally aknowledged to be higher following fertilization with digested
sludge. Most vegetable crops such as potatos, radishes, carrots, cabbage, onions, garlic,
etc., and many types of fruit (oranges, apples, guaves, mangos, etc.), sugar cane, rice and
jute appear to react favorably to sludge fertilization.
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