**5. Rock phosphate (an indigenous and economical source of P)**

Phosphate rock or the RP is a non-renewable alternative natural source of P. It serves as raw material for the so-called chemical phosphatic fertilizers. It has been found on every continent of the world, however, quality may differ depending on the percentage of P present in it. Distribution of RP on the earth is given in Figure 1 [54].

In literature, there have been many reports showing effectiveness of directly applied RP being less expensive compared to the chemical phosphatic fertilizers like SSP or TSP. It has also been found to be effective for the perennial crops [55]. It has been directly applied in the soil of Nigeria in place of very expensive phosphatic fertilizers [50, 56]. The demand for the direct application of RP is increasing day by day as it would help reduce pollution and the burden on manufacturing industry for the high demand of chemical phosphatic fertilizers, and this would ultimately serve as a cheaper source of P. It has been found that RP results in increasing

**Figure 1.** Rock phosphate (RP) reserves around the world.

sible factor causing this fixation was the high activity of hydroxy-aluminum at high pH which has strong attraction with phosphate compared to hydroxyl. This attraction was found enough to displace hydroxyl from the hydroxy-aluminum-phosphate attraction. Due to this, increase in phosphate buffer capacity and the amount of phosphate required to attain the desired level of P in the equilibrium solution was noted [51]. In order to study the mechanisms behind the fixation of P in the soils, another study was conducted which showed that P adsorption by the amphoteric soil surface decrease with increasing pH from 4.0–7.0. But in soils high in ex‐ changeable Al, increasing pH results in the formation of highly reactive adsorbing surfaces for P as Al-ions precipitate and insoluble polyhydroxy-Al cation species. So if acidic soils are reacted with lime without intervening air drying, this will result in the adsorption of more P in the soils. Alternatively, it was found that the application of lime to the acidic soils after intervening air-drying results in decreasing the P adsorption in the soil as clear through

As mentioned earlier, precipitation, and adsorption of P with the soil colloids are the main responsible mechanisms/reactions for the removal of P from the soil solution. The former is induced by the presence of Ca2+ ion in the soil solution while latter depends on the chemical properties of the soil colloids. There is high probability of having the precipitates of calcium phosphate in the soils rich in exchangeable cations. It has also been found that calcareous soils

If we look at the manufacturing process of phosphatic fertilizers, we would found that the major factor responsible for their high prices is the use of very high energy in their manufac‐ turing process. Coming to the initial step of the process is the raw material that is used for the manufacture of most of the phosphatic fertilizers, the phosphate rock or RP that is a naturally occurring mineral source of P. One strategy could be the use of this raw material, the cheaper source of P, directly in the field and getting the benefit of phosphatic fertilizers. To cope with this alarming situation, there is a need to find alternative cheaper sources of P, so that we could

have a sustainable agriculture to feed out the ever growing population of the world.

Phosphate rock or the RP is a non-renewable alternative natural source of P. It serves as raw material for the so-called chemical phosphatic fertilizers. It has been found on every continent of the world, however, quality may differ depending on the percentage of P present in it.

In literature, there have been many reports showing effectiveness of directly applied RP being less expensive compared to the chemical phosphatic fertilizers like SSP or TSP. It has also been found to be effective for the perennial crops [55]. It has been directly applied in the soil of Nigeria in place of very expensive phosphatic fertilizers [50, 56]. The demand for the direct application of RP is increasing day by day as it would help reduce pollution and the burden on manufacturing industry for the high demand of chemical phosphatic fertilizers, and this would ultimately serve as a cheaper source of P. It has been found that RP results in increasing

**5. Rock phosphate (an indigenous and economical source of P)**

Distribution of RP on the earth is given in Figure 1 [54].

were poor in plant available P compared to the limed acid soils [53].

isotherm studies [52].

114 Organic Fertilizers - From Basic Concepts to Applied Outcomes

the relative agronomic efficiencies of the crops grown on P deficient soils. It has sedimentary origin and its direct application might be feasible due to the presence of somewhat open, roughly consolidated aggregates of microcrystals with large surface area [57].

In a study, the direct application ortho rock phosphate (ORP) as a P source increased the crop yield and was comparatively found to be more effective than chemical phosphatic fertilizers [58]. Another greenhouse experiment was conducted to assess the relative effectiveness of directly applying Togo and Egypt RP sources and these sources were compared with TSP, using Lucerne as a test crop. From the results, it was found that the application levels of 60 and 43 kg P ha−1 were the effective levels of Togo and Egypt RP, respectively. At these levels, the Egyptian and Togo RP were 92 and 64% as effective as that of TSP, respectively [59]. The direct application of RP to vertisols, oxisols, and ultisols, having pH less than 7, has reported to yield similar results as that caused by TSP. Moreover, the initial and residual effects of RP on volcanic ash soils like that of vertisols, oxisols, and ultisols are less compared to the TSP. It has also been found that soils with pH >7, like in andepts, should be taken with care when applying RP directly to the soils as these soils have greater tendency of P sorption [59–61].

It has been found that most of the phosphate rocks or RPs yield better results in the acidic soils like that of oxisols and ultisols compared to alkaline and neutral soils like andepts with high pH, high P sorption capacity, low cation exchange capacity (CEC), low rainfall, low organic matter, low microbial activity, etc. Moreover, RP has significant proportion of isomorphic substitution in the crystal lattice and variable proportion of impurities and accessory minerals. Thus, it has been found to be more beneficial if applied in acidic soil conditions as compared to the neutral and alkaline conditions [57].

One strategy for the direct application of RP on andepts soils might be the application of partially acidulated RP by using 20% H2SO4. It has been reported to increase the plant response from applied RP from 0–16% to 59–77% relative agronomic efficiency (RAE) on Andepts [61]. However, the application of inorganic acid like 20% H2SO4 could have detrimental effects on the soil microbiota. Moreover, some other procedures like mixing with elemental sulfur, partial acidulation with an acid, thermal alteration, combination with chemical phosphatic fertilizers, preparation of RP enriched compost, and dry compaction with water-soluble chemical phosphatic fertilizers [62–64] have been reported to increase the efficiency of RP in increasing the availability of P for the crops. However, these procedures are labor intensive, costly, and inappropriate to be practiced at large scale. Due to these problems, there is a growing interest in manipulation certain biological procedure like the application of PSMs. Composting have been proposed, which are discussed in the next sections separately.
