| dc.description.abstract | A significant portion of soil N and P occurs in organic forms that are not available for crop uptake unless mineralised ( Stevenson, 1986; Tisdale et al., 1990). Slowed nitrification rates (Weber and Gainey, 1962) and P fixation (Stevenson, 1986) greatly hinder the conversion of organic N and P to their mineral forms in acid soils. Other factors include soil temperature, moisture, pH, fertiliser additions and the C/N ratio of the organic material (Dalal, 1977; Jansson and Persson, 1982; Hendrickson, 1985).
Organic N and P mineralisation in acid soils are stimulated mainly through liming (Dalal, 1977; Kamprath and Foy, 1985; Hue, 1989) and/or P fertiliser application (Dalal, 1977; Evans, 1985). Liming raises soil pH, thereby creating favourable conditions for microbial growth, especially nitrifyers and actinomycetes (Anderson and Domesch, 1980), and decreases the solubility of Al- and Fe-hydroxides but increases the solubility of Al- and Fe-phosphates. After P application, there is competition between inorganic and organic P compounds for soil sorption sites resulting in a substantial increase in dissolved organic P (Evans, 1985).
Laboratory incubation experiments are a convenient way of quantifying and studying the N mineralisation processes (Bremner, 1965a; Keeney, 1982). Bremner (1965b) and Keeney (1982) found that incubation of soil under favourable conditions provides a rational measure of N availability. This is because the agents responsible for release of mineral N during incubation are the same ones which avail N from the organic soil pool for crop growth during the growing season. Lathwell et al. (1972) also found that N produced during incubation was highly correlated with N released to crops in the field. This method (Laboratory incubation) is however, unsatisfactory because either several simultaneous occuring processes are measured (in situ net mineralisation rate) or they establish potential nitrification rates rather than actual rates (Woldendorp and Laanbroek, 1989). There is scanty literature on the behaviour of P mineralisation under laboratory incubation. Nevertheless, the factors controlling P mineralisation are more or less the same as those of N (Vaughan and Malcolm, 1985).
The purpose of this study, therefore, was to investigate the effect of various soil amendments on N and P mineralisation in an acid soil through laboratory incubation. | |
