A field
test with the traditional rotation of paddy rice/upland crop (wheat) was carried out on a
paddysoil
derived from red earth to elucidate the effect of
organic manure on the phosphorus
adsorption-desorptionby soil and its P availability Soil samples were taken from different
treatments at rice harvesting stage andanalysed. The isothermal adsorption of P by the
samples fitted very well with Langmuir equation, and hence,the parameters in the equation,
i.e., maximum adsoaption (qm), constant related to bonding energy (k) andtheir product (k x qm)
could be used as a comprehensive index to characterize the potential P adsorptivityof the soil.
Organo-inorganic fertilization and organic manuring conld decrease qm and k, while mineral P
appli-cation had little effect on them. The isothermal desorption of P was significantly
correlated with initiallyadded and isothermally adsorbed P. Part of P added was fixed, which
represented the P fixation capacityof soil, and organic manuring could obviously lower the P
fixation. The content of soil available P had asignificant negative correlation with qm, k and
fixed P. It is concluded that organic manure could increase theP availability of paddy soil
derived from red earth by decreasing qm, k, maximum buffering capacity (MBC=k x qm) and
fixation capacity.