temperatures in the pile, there are other factors involved in nutrient
retention. Research has shown nitrogen conservation rates are reduced by
aerating composting material by turning the pile, increasing pH through the
addition of lime or ashes, high concentrations of ammonia N in the.initial
raw material, and inadequate moisture content (Scott 1952: 176-181;
Handrick
1978a: 8; Raven et al. 1978: 552; Gotaas 1956: 93-96).
As already mentioned, the slow release of nutrients is one major advantage of organic matter fertilizers including composted human excrement. Only 15-50% of the nitrogen in mature compost is released and available to plants annually (Beeby: 1995: 154). Mitchell (1974: 144) argues that unless the
C:N
ratio of composted material applied as fertilizer is below 20:1, it will be unlikely that nitrogen is available for the use of growing plants. Obeng and Wright (1987:51) maintain that typically, in the first year of compost
application to arable land, only 10% of nutrients are available. Rajgopal et al. (1981:
22)
assert that only about 15-30% of organic nitrogen contained in composted sewage sludge applied to land is mineralized in the first year after application, while 3% of the residual organic nitrogen is released in each of the 2nd, 3rd, and 4th years respectively. Wild (1993: 156) also argues that farm compost can supply all the nutrient needs of crops, with about 30% of the nitrogen, 50% of the phosphorus, and all the potassium becoming available in the first year of application.Human excreta composts can impact positively on soils and plants in other ways besides directly supplying major and minor nutrients, in the same way as described for organic matter in section 4.2. These include improving soil physical properties (bulk density, soil structure and aggreg�tion), hydraulic conductivity, microbial activity and health, and the prevention of surface crusting (Halbach et al. 1994:
22;
Grant and Long 1981: 97-126). The formation of humus is also significant since 'all the processes operative and interacting in the soil are reflected in the quantity and quality of the humic compounds formed' (Pauli 1967: 64)26. Humus is a complex combination of stable molecules providing large surface areas for holding nutrients very resistant to microbial attack, enabling a very long occupation time in the soil. However, immature composts can be unstable and detrimental to growing plants and soil.4.4 Compost maturity
As already discussed in section 4.2, immature and unstable composts contain very dynamic microbial populations which immobilize nitrogen in the interim as an energy source for catabolization of carbon and for cell building. This immobilization can limit the availability of nitrogen when immature composts or incompletely degraded organic matter are added to soils or are used as a fertilizer for plants. High microbial activity resulting in supstantial
26 There is an exa!llent and comprehensive analysis of humus and its relationship to soil fertility in
(Pauli 1967: 17-75).
microbial oxygen demand in unstable composts and soils can also create oxygen deficiencies in the plant root zones. There are other reasons associated with phytotoxic27 influences for determining the maturity or stability status of human excreta composts before they are utilized as a fertilizer. Stability of a compost is usually related to the degree of microbial activity while maturity is normally connected with factors likely to limit plant ' growth such as phytotoxicity. Nevertheless, stability and maturity are
customarily considered together since microorganism catabolizing organic compounds in unstable compost also release phytotoxic substances (Hue and Liu 1995: 8; Chen and Inbar 1993: 555; Fahy 1992: 7). High nitrogen losses such as the production of excessive amounts of ammonia can also result from the activities of microorganisms in unstable or immature composts. Surplus amounts of ammonia is the most common phytotoxin detected in composts which can detrimentally affect plants and roots, as well as creating conditions amenable to plant pathogens (Fahy 1993: 5). During organic matter decomposition, other toxins can also be released and produced. The presence of considerable amounts of acetic, propionic and butyric acids in immature composts and 'unknown biochemical composition in
decomposing organic matter ... and the microbial kinetics occurring during composting' have been suggested as the reason for the toxicity that
sometimes occurs in soils amended with immature compost (Anid
1986: 46).
Salt and pH at toxic levels, ethylene oxide concentration, accumulations of benzoic and phenylacetic acid, elevated trace elements, and other phytotoxic substances released by composts before the humidification process is complete can also inhibit plant growth and damage plants (Chen and Inbar1993: 555).
Therefore methods to determine the degree of compost maturity are important so that a judgement can made about when to incorporate compost into soils. This is especially critical when applying compost to young growing plants and sensitive crops. Composted human excreta used
as a mulch around established trees and other established plants should create no problems to plants even if the compost is still immature. With experience it is also possible, by physical examination and smell, to fairly reliably determine the maturity status of toilet compost. Nevertheless, many procedures have been suggested and investigated to try and assess compost maturity. While pH level and C:N ratio are generally regarded as important indicators of the level of compost maturity, other meth�ds that
'E1 Phytotoxins are plant as o�anic chemicals emanating from decomposing plant residues and