effect of calcium carbonate and ph on the kinetics of p fixation in different soils

effects of calcium carbonate and ph on the kinetics of p fixation were studied on four non-calcareous soils treated with different levels of pure caco3, and also on six soils which were adjusted for ph within the range of 5 to 9. soils were also treated with k2hpo4 at the rate of 45 mg p/kg before they were incubated for a period of 100 days at 25○c. soil samples were taken at certain predetermined times and the concentration of p measured through olsen method. it became evident that the reaction of p with soil was biphasic, composed of a set of fast reactions vs a set of slow ones. the rate of reduction in available p (olsen-p) was very fast during the first day, and then it gradually diminished for duration of 50 days, thereafter approaching zero in most of the soil samples. in addition, an increase in the level of caco3 content of the soils resulted in a significant (5% level) increase in the mean p recovery from the soils, however, the effect was not observed as the same in all the soils. it occurred that in one of the soils, an increase in caco3 up to 35% had no effect on p recovery. the effect of ph on p availability was not the same in different soils. in some soils an increase in the ph from 5.2 to 8.8 had no significant effect on p availability, while in others; the amount of olsen-p was either higher or lower at higher ph values. the different effects of ph on p availability was explained by the variable effects of a change in ph on such available-p related parameters as solubility of p minerals, charge characteristics of the soil surface, speciation of soluble p and change in concentration or activity of p species and as well the rate of organic p mineralization. it was finally proved that the net effect of these changes, as a result of a change in ph, could be different in various soils.