Abstract This study tested the hypothesis that the level of available P in volcanic ash soils is a rate‐limiting factor for organic matter mineralization. Additions of calcium phosphate to give levels of P of 1,200 and 2,400 ppm significantly increased the amount of N mineralized and CO 2 evolved as compared to soils not treated with P. Although the addition of glucose to these soils increased CO 2 evolved, there was significantly more CO 2 evolved in the presence of glucose and P than when glucose had been the only addition. Amendment of the soils with N did not have any significant affect on the rate of CO 2 evolution. Both N and P additions stimulated N mineralization. The P×N interaction was also significant for C and N mineralization. It was concluded that available P is a rate‐limiting factor for organic matter mineralization in Andepts.