Carbon (C) to Nitrogen (N) Ratios for Various Kinds of Organic Materials

Material C:N Ratio

Urine 0.8

Activated sludge 6

Raw sewage sludge 11

Nonlegume vegetable wastes 11-12

Poultry manure 15

Cow manure 18

Mixed grasses 19

Horse manure 25

Potato tops 25

Straw from oats 48

Straw from wheat 128-150

Sawdust 200-500

Source: Adapted from Golueke, C. G. 1977. Biological Reclamation of Solid Wastes. Rodale Press, Emmaus, PA.

This kind of succession occurs when organic wastes are gathered up into heaps or piles so that an insulating effect emerges with conservation of heat and a rise in temperature. Heat is a by-product of the metabolic reactions of the microbes as they decompose the organic wastes. The pH also changes over the composting succession, beginning acid and becoming more alkaline over time. The first stage of composting succession is dominated by mesophilic microbes, primarily aerobic and anaerobic bacteria that metabolize simple carbohydrates such as sugars and starches. The optimal temperature for these organisms is about 35°C (95°F). Thermophilic (i.e., "heat loving") microbes dominate next and metabolize proteins and other nitrogenous materials. The optimal temperature for these organisms is about 60°C (140°F). No living organisms can exist above 70°C (175°F), so microbial metabolism stops and a cooling down phase follows the thermophilic stage. During this period acti-nomycetes and fungi increase in numbers and metabolize cellulose and other more resistant carbon compounds. The final stage shown in Figure 6.5 occurs with the formation of humus, which is the most valuable form of compost.

In a mechanized composting plant the complete succession sequence can take place in about one week, while in an open or windrow operation the sequence can require on the order of a month to complete. Unlike composting systems, in most natural ecosystems organic materials seldom accumulate under aerobic conditions to a sufficient extent for insulation to occur, so temperature remains at ambient

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