A series of experiments was conducted to study the behavior of the phosphorus added to the soils having the high phosphorus fixing capacity derived from volcanic ash in Cheju Island. Soil samples were taken from different depths of 0-10, 10-30, and 30-50㎝ in six citrus orchards where heavy application of phosphate fertilizer has been practised. Various forms of phosphorus were determined and phosphorus adsorption experiments were performed. The results obtained can be summarized as follows:
1. The content of inorganic phosphorus fractions determined by the method of Chang and Jackson was: water soluble P$lt;Al-P$lt;occluded Fe-P$lt;Fe-P$lt;Ca-P$lt;occluded Al-P. The phosphorus combined with Al (Al-P+occluded Al-P) consisted of the largest portion in the inorganic soil phosphorus, and Ca-P was found to be one of the main components in the soil phosphorus.
2. Difference among the percent phosphorus in the fractions against total inorganic phosphorus along the soil depth was insignificant.
3. Total phosphorus, Bray No. 1-P, and all the inorganic phosphorus fractions decreased with the soil depth. The Bray No. 1-P was highly correlated with water soluble P, total P, and Ca-P.
4. The P absorption coefficient was inversely correlated with the contents of clay, Bray No. 1-P, and total phosphorus, while the coefficient was positively correlated with organic carbon content.
5. The amount of phosphorus adsorbed at the equilibrium concentration at which each 50% of added phosphorus was distributed into both solid phase and liquid phase was calculated using the Freundlich adsorption isotherm equation. The calculated value was used as a parameter to compare the phosphorus fixing capacities among the soils.
6. The magnitude of the parameter was highly correlated with the P absorption coefficient. However, the parameter was more sensitive in showing the difference in the phosphorus fixing capacity than the P absorption coefficient.
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