Organic Waste Recycling

Composting 137

products were found to be highest when C/N ratio of the mixture was 20 (Table
3.11) (Polprasert 1984). It is apparent the magnitude of increase in N, P and K
percentages in the composted products was related to the initial characteristics
of the composting materials and the percent loss of volatile solids after
composting.
Organic chemicals such as pesticides, PCB's and heavy metals may be
present in some wastes in sufficiently high concentrations to make the compost
undesirable for land application.

Soil and plant response

Composts can be considered as fertilizers rich in organic matter. The organic
matter is an excellent soil conditioner because it has been stabilized,
decomposes slowly and thus remains effective over a longer period of time.
Composts improve soils' physical properties as evidenced by increased water
content and water retention; enhanced aggregation; increased soil aeration, soil
permeability, and water infiltration; and decreased surface crusting. The greatest
improvement in soil physical properties occurs in sandy and clayed soils.

Table 3.9 Nutrient contents of nightsoil, water hyacinth and leaves (dry weight basis)

Raw materials Carbon (C )

(%)

Nitrogen (N)

(%)

Phosphorus (P)

(%)

C/N Ratio

Nightsoil 54.1 3.6 0.9 15.2
Water hyacinth 45 2.9 0.5 15.8
Leaves 49.9 0.8 0.2 60.8
From Polprasert (1984); reproduced by permission of Pergamon Books Ltd

Table 3.10 Percentage of raw materials required in preparing compost piles (dry weight
basis)

Initial C/N ratio

Raw materials 20 30 40

Nightsoil, (%) 14.4 10.9 7.3

Water hyacinth, (%) 57.4 25.4 10.9

Leaves, (%) 28.2 63.7 81.8

Water hyacinth: Nightsoil ratio 80:20 70:30 60:40

From Polprasert (1984); reproduced by permission of Pergamon Books Ltd