INTRODUCTION Soil phosphorus (P) is one of the most important nutrients after nitrogen controlling plant root proliferation and biomass growth in tropical agroecosystems where highly weathered soils exist. These soils are rich in P but limit the availability in occluded form (Nishigaki et al., 2018). Soil characteristics affect the P bioavailability and stability. Excess accumulation of P in soils can hamper the availability and increase losses through runoff (Yan et al., 2020).P seasonal utilization efficiency is just 15-20% (Dwivedi et al., 2017) which is obviously very low in India resulting in high accumulations in soil and lower availability. The dominant form of P in soils depends on the parent material and the soil physicochemical properties. Calcium (Ca) P as primary minerals is the most dominant form at the beginning of soil formation when the soils are young, and after subsequent weathering the P released is transformed into less-labile pools associated with the (Al) and (Fe) hydroxides. Therefore, Al-P and Fe-P are the main P forms in highly weathered soils of tropical areas, while Ca-P is the dominant form in younger soils. Furthermore, the use of chemical fertilisers like superphosphate may increase the amount of Ca-P in surface soils on arable fields (Wright 2009). Organic matter in soil also enhances P availability by blocking the Al and Fe fixation sites in soil complexes. Labile, inorganic, and organic fractions of P pools give an indication of the stability of P in soil. The solubility of phosphorus is greatest between pH 4.5 and 7, and the dominant species are then H2PO4-. When the pH is outside the range of greatest solubility, phosphorus will form insoluble precipitates with available iron/ aluminium and calcium/fluoride respectively. At low pH aluminium iron becomes soluble, and the concentration of these cations in soil solution increases. If there are large enough amounts of orthophosphate in the solution, the soil water may become supersaturated with regards to the solubility product of AlPO4 and FePO4, and precipitate phosphate out of the solution. At pH above the region of greatest solubility calcium carbonate becomes soluble, and form Ca3(PO4)2 precipitates with phosphate. The availability of P depends on the soil pH as it governs the occurrence and abundance of those metal cations that are prone to precipitate with P ions in the soil solution, namely Ca, Fe and Al. Hence, in neutral to alkaline soils, P ions rather precipitate as dicalcium or octa calcium phosphates, hydroxyl apatite and eventually least soluble apatites (Hinsinger, 2011). The aim of the study was to extract the P forms soil using the sequential extraction method, and to identify the factors responsible to the formation of P pools in soils. Moreover, evaluating Olsen-P and P fractions can predict the bioavailability of P fractions by giving quantitative information on the dynamic process of the P replenishing ability (Yan et al., 2020). MATERIAL AND METHODS Rock phosphate. Three rock phosphates (RP) were used for the study, out of which two Udaipur grade I with 26% P2O5 and II with 27.48% P2O5 were used and one is from Jhabua (M.P) with 11.68% P2O5. Udaipur RP (URP) was obtained from Rajasthan State Mines and Minerals Ltd, Udaipur, Rajasthan, India; and Jhabua RP (JRP) was obtained from Jhabua, M.P., India. All the three RPs were used for the incubation study. Analysis of physico-chemical properties of soil of study area Available Phosphorus. The available P in soil was extracted using the Olsen’s reagent (0.5 M NaHCO3) (Olsen et al., 1954) for the neutral soil. The P content in the extracts was determined using the ascorbic acid blue colour method (Watanabe and Olsen, 1965). The intensity of the blue color developed was determined by a spectrophotometer at 660 nm wavelength. Fractionation of Inorganic P. Soil inorganic P was fractionated into various fractions by modified P fractionation scheme by Kuo (1996). The procedure is depicted through the flow chart (Fig. 1). Saloid means loosely-bound P, occluded P means P occluded in Fe/ Al oxides while residual P means P from the soil residue remaining after the last acid extraction which was digested with H2SO4/ H2O2 at 360°C (Soremia et al., 2017). All the inorganic P fractions was determined as per the ascorbic acid blue colour method as described above. The maximum colour intensity developed after 10 min and was measured using a spectrophotometer at 660 nm wavelength.The fractions of P i.e., Saloid-P, Al-P, Fe-P and Ca-P were analyzed in soils. Results showed the P pools in abundance. From analysis of soil result was: Ca-P> Res P> Al-P> Fe-P> Saloid-P which value was 39.4>34.4>20.9>13.6>10.5 kg ha-1 and percent contribution 30.71> 26.91> 16.29> 10.60> 8.18 of total-P, respectively. RESULTS AND DISCUSSION Among the various forms, Ca-P fraction was dominant fraction in the soil which ranged from 40.41 to 42.61 kgha-1, Compared to other P fractions Saliod-P was present in a small quantity which ranged from 11.02 to 13.81 kgha-1, the contribution of Fe-P in phosphorus fractions is about 29.28 to 31.48 kgha-1 followed by Al-P which ranged from 17.78 to 19.86 kgha-1. The dominance of different inorganic P fractions in soil followed the order: Ca-P> Fe-P > Al-P> Saloid-P. Among the various treatments, FYM has maximum contribution in Saloid (13.34 kg ha-1), Al-P (19.29 kg ha-1), Fe-P (29.65 kg ha-1) and Ca-P (40.82 kg ha-1) fraction. Effect of RPs and organics on P pools in soils at the end of incubation experiment. The data pertaining to distribution of different pools of phosphorus is given in Table 2. The alkaline nature (soil reaction) of soils tends to precipitate the available phosphorus as Ca-P and least as saloid-P similarly reported by Sarkar (2002). Similar findings were reported by Kanwar and Garewal (1990), they reported that the Ca-P fraction increased with addition of P fertilizer as well as increase in pH, on ageing Ca-P increased up to 15-30 day. The result is in conformity with findings of Chang and Tomar (2007), Trivedi et al. (2010). The dominance of different inorganic P fractions in soils followed the order: Ca-P> Fe-P > Al-P> Saloid-P. Among the various treatments, FYM has maximum contribution in Saloid, Fe P, Al P and Ca P fraction. The content of the Ca-P ranks highest which was an indication of the fact that Ca-P form contributed to the major source of P in black soil as reported by Garg and Milkha (2010). The organic fraction generally constitutes 20-80% of P in surface horizon (Brady and Weil, 2002). The process of soluble phosphate anion being bound with soil constituent so, that they become insoluble and unavailable to the plants. Several mechanisms like precipitation, dissolution, adsorption-desorption, immobilization and mineralization reactions occurred (Kanwar and Reddy, 2003). Variation in P fraction is the pedogenic manifestation (Walker and Syres 1976), stage of soil development (Smeck 1976) and age of soil. As the soil mature the Ca-P gets transformed into Al-P and sesquioxide bond phosphorus with maturity of soils. Changes in the P pools in soils are significantly affected by soil type. In general, total P content is low in strongly weathered soils orders and high in young soil. In acid soils among the different forms of P, Fe-P dominates. In India Alfisols with pH ranging from 5.5 to 6.8 the order of soil P distribution was Fe-P> Ca-P> Al-P. The presence of clay, Al, Fe and sesquioxides is responsible for fixation of applied P (Doddamani and Seshagiri Rao, 1996). The availabilities of P also depend on soil pH, type of clay minerals, Fe, Al hydroxides and calcium carbonates (Kaistha et al. 1997). Therefore the response of phosphorus in different soils varies and the crop response varies. But, in case of slightly acidic soils, applied phosphorus gradually reacts with Fe and Al compounds present in the soil and consequently, gets transformed into relatively insoluble compounds (variscite and strengite), which are hardly available to plants. Reddy et al. (2000) reported that the Vertisols of Bhopal of which about 18% is tied-up in organic form. Sarkar (2002) concluded that the saloid-P was very low in comparison to all other forms of inorganic soil P and varied from 1.5 to 2.8 mg kg-1 which might be due to high P-fixing capacity of chotanagpur plateau soils. Lakshminarayana (2007) suggested that minimum amount of P is in the saloid fraction (S-P), whereas maximum amount is in reductant soluble form (RS-P) in the soil and such high values may be attributed to the acidic to neutral soil reaction and high content of Fe2O3. Sarkar et al. (2014) revealed that the mean relative abundance followed the order: RS-P> Fe-P> occluded-P> Ca-P> Al-P> S-P. The soil is fairly rich in total P reserve but the available P status is low, the overall contribution of the inorganic soil P fractions to the total soil P is 57.75%. Singh et al. (2015) indicated that the release of P in alluvial and black soil was of higher degree of magnitude than that of red soil. Thakur et al. (1975) observed that in rice culture at Jabalpur (Madhya Pradesh), the Fe-P fraction formed the major portion of native inorganic soil P fractions, Al-P Fe-P and Ca-P fractions increased with the addition of P fertilizers. Goswami and Sahrawat, (1982.) indicated that Ca-P was the dominant form followed by Fe-P with very low amount of Al-P.