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.
Fertilisation with slurry: Transportation of slurry by a modified wheelbarrow and buckets
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. In contrast, crops such as wheat, oilseed, cotton and baccra react less favorably. Sludge is a good fertilizer for pastures and meadows. The available data vary widely, because the fertilizing effect is not only plant-specific, but also dependent on the climate and type of soil. Information is still extensively lacking on the degree of reciprocity between soil fertility, type of soil and the effect of fertilizers (particularly N-fertilizers) in arid and semi-arid climates. Thus, no definitive information can be offered to date. Nor, for the same reason, is it possible to offer an economic comparison of the cost of chemical fertilizers vs. biogas sludge. The only undisputed fact that can be stated is that biogas sludge is better from an ecological point of view.