## 3 Results and Discussion
### Soil Inorganic Phosphorus Speciation
The difference in the distribution of the P fraction was due to the different solubility of the fertilizers as they had different parent materials. The TSP treatments showed a higher Al-P content compared with the RPs treatments (Figure 2 and Table 6), suggesting that a portion of the P of the TSP might have been adsorbed to the allophane and Al oxides of the soil at a faster rate than it was converted to other fractions. This observation is consistent with a report saying that Al-P varies with the rate of fertilization, high sesquioxides content, and low pH [47]. Thus, the relative increase in Al-P recovery following the use of TSP in this present study suggests the strong effect of fertilization on Al-P. The lower Al-P content recorded with RP treatments compared with TSP (Figure 2 and Table 6) was because rock phosphates are inherently composed of calcium apatite, which considerably increases Ca-P in soils [48]. As soil pH increased (Table 5), dissolution of RP was impeded by the 90 DAI. Moreover, the absence of a plant-soil association in this study could not significantly affect an exchange-induced mechanism for which CZ is noted of inducing RP dissolution. The higher Ca-P content in the RP treatments compared with TSP  
Figure 2: Percentages of inorganic P speciation at 90 days after incubation and 49 days after seeding. Sol-P is loosely soluble P, Al–P is aluminum bound P, Fe–P is iron bound P, Ca–P is calcium bound P, Red-P is reductant P, and Ocol-P is occluded P. T0—Soil alone, T1—triple superphosphate (TSP) at 100%, T2—TSP 75% + Clinoptiolite zeolite (CZ), C1—Christmas Island Rock phosphate (CIRP) at 100%, C2—CIRP 75% + CZ, E1—Egypt Rock phosphate (ERP) at 100%, E2—ERP 75% + CZ. treatments suggests that the undissolved RPs dissolved slowly with time (Tables 6 and 7). The lower Ca-P of the CIRP treatments indicates that the CIRP solubilizes faster than ERP. Regardless of the rate of P application, the reduction of Ca-P with time could be related to the recovering of Sol-P in the TSP treatments as previously discussed. The afore-stated results are in agreement with the observation of [PERSON] et al. [48] that dissolution of water soluble fertilizer (TSP) in acid soils produces Al-P and Fe-P forms (active inorganic P fraction) as source (sink) of available P for plant uptake, whereas the RP application increases the Ca-P fraction in soils because of their incomplete dissolution. Table 6 shows inorganic P distribution at 90 DAI. The higher recovery of Sol-P in TSP treatments compared with those of the rock phosphates was due to the solubility of acidulated TSP. The higher content of Sol-P in T1 was due to the higher rate of TSP (normal fertilization which is 100% TSP), whereas the reduction of Sol-P in T2 is related to a reduction of TSP application by 25% of the recommended rate. The use of higher P rates reduced P fixation sites such that the P buffering capacity might have been exceeded. This partly explains the higher amount of Sol-P recovery following the application of TSP [49]. On the contrary, the RP treatments regardless of the fertilizer rate and CZ application showed no significant effect on Sol-P. The significant increase in soil pH and Ca\({}^{2+}\) (Table 5) is related to the liming effect of the CZ and rock phosphate structure (Table 3), a process that reduces soil acidity by minimizing Al hydrolysis in acid soils (Figure 1c). A recent study revealed that zeolites are more effective in increasing soil pH than organic amendments [50]. Irrespective of the incubation and pot studies, loosely-soluble P was higher in the soil with TSP (Tables 6 and 7). This observation can be attributed to the higher solubility of TSP than with rock phosphates. Although Sol-P was lower in the RPs than in TSP and at 49 DAS, the Sol-P of E2 and C2 were comparable to those of 100% of ERP (E1) and CIRP (C1). This observation is consistent with a report that in high P fixing Ultisols, Al-P and Fe-P dominate following application of TSP (highly soluble P), whereas Ca-P dominates in soils upon RP application [48; 49; 50; 51].