Phosphorus fractions -their key role in soil management

Introduction

Phosphorus serves as a cornerstone for balanced fertilization in Indian agriculture.It plays a vital role in almost every biological process, being essential for energy generation, transfer, and storage through ATP and ADP, often referred to as the "energy currency" of living systems. In addition, phosphorus is a key constituent of cellular metabolism and serves as a structural element of nucleic acids, cell membranes, and other vital biomolecules (Wiedenhoeft, 2006). Phosphorus (P) is a vital nutrient for plant growth and plays a key role in the developmental processes of agricultural crops (Withers et al., 2008). In soils, nearly two-thirds of inorganic phosphorus and about one-third of organic phosphorus remain unavailable, particularly in soils with variable charges. The efficiency of phosphorus utilization by crops is typically low, as a substantial fraction becomes fixed with iron (Fe), aluminum (Al), and calcium (Ca), greatly restricting its availability to plants (McBeath et al., 2005) [11] . In fact, 70-90% of the phosphorus added to soil is immobilized, making it difficult for plants to absorb and utilize (Lei et al., 2004) [4] . It is therefore important to consider both organic and inorganic P fractions for soil P fertility evaluation. It can serve as an indicator for proper nutrient management. Shen et al. (2004) [24] reported that studying phosphorus fractions offers an effective way to evaluate soil P availability and to understand the interconversion of phosphorus among different soil pools. Therefore, the present study was undertaken to assess the distribution of P fractions across various soil orders. Phosphorus is the tenth most abundant element in the earth's crust. About 200 minerals occurring in nature, have been reported to contain P with apatite group of minerals being the most important. Soil holds phosphorus in both organic and inorganic forms. Understanding these forms is crucial to determine how plants absorb phosphorus and how much of it moves through the environment. Among the major plant nutrients, phosphorus is the least mobile. Generally P is present predominantly in the inorganic form, with organic P content varying from 20-80% of the total P in the soil. The inorganic phosphates in soil comprise the (i) Iron and Aluminium phosphates (ii) calcium phosphates. The former predominate in acidic soils while the later predominates in neutral and alkaline soils. The Al-P and Fe-P contents are generally higher in fertilized soils. Relatively less important inorganic P fractions include the occluded or the reductant soluble P. The soil organic P compounds may generally be classified into three gropus, (i) inositol phosphates, (ii) nucleic acids and (iii) phospholipids. Research on organic P suggests that the readily decomposable or soluble fractions of soil organic P is often important in supplying P to the plants in highly weathered soils. Mineralisation of organic P in soils is influenced by factors such as temperature, moisture, tillage etc. Of the total P content in soil a very small fraction is present in plant available form in the soil solution which generally do not exceed 0.1-0.2mg L -1 . [19] Step Reagent / Solution Conditions Extracted P Fraction Fractionation studies of Phosphorus in soil-Doddamani and Seshagiri Rao (1989) [2] investigated the forms of phosphorus in Karnataka soils and reported that Ca-P was the predominant fraction, contributing up to 19.5% of total P in Vertisols and up to 11.5% in Inceptisols. Fe-P accounted for 14.2%, 9.1%, 2.5%, and 2.4% of total P in Oxisols, Alfisols, Vertisols, and Inceptisols, respectively. The highest levels of reductant-soluble P were recorded in Oxisols, followed by Alfisols, Vertisols, and Inceptisols. Occluded-P and saloid-P contributed 2.9-3.0% and 0.5-0.6% of total soil P, respectively, across these soils.

Rao and Krishnamurthy (2007) [21] reported that in the soils of Khamman district, Andhra Pradesh, the concentrations of different Pi fractions varied as follows: saloid-P ranged from 13.5 to 78.4 mg kg⁻¹, Al-P from 45.0 to 499 mg kg⁻¹, Fe-P from 6.2 to 47.9 mg kg⁻¹, and Ca-P from 6.9 to 438.2 mg kg⁻¹. Lungmuana et al., (2012) [9] investigated the distribution of phosphorus in rice-growing red and laterite soils of West Bengal and found that the average contribution of various Pi fractions to total P followed the order: Fe-P (18.5%) > Al-P (7.9%) > Ca-P (5.8%) > saloid-P (0.8%). Gupta et al., (1999) [3] observed that use of P alone and in blend with FYM extended the total phosphorus content and distinctive phosphorus pools. Dakhore reported that under unmanured cultivation, the utilization of inorganic P forms in sandy loam Entisols followed the order: Saloid-P > Al-P > Fe-P > Ca-P. Both forms of phosphorus were enhanced when nitrogen and potassium applications were reduced. Setia and Sharma (2007) [22] investigated that application of P @ 17.50 or 35 kg P ha-1 increased all the forms of P in 22 years of maize-wheat cycles. The relative abundance of P fractions was in the order of saloid-P < Fe-P < Al-P < Ca-P. Sukhvir Kaur (2015) [5] observed that the application of integrated fertilizers resulted in higher P fractions in very low and low gradient strips, which could be attributed to the acidic soil pH favoring the transformation of applied P into Al-P and Fe-P fractions. Jakasaniya and Trivedi (2004) [4] observed that with increasing rates of P application, the fractions of S-P, Al-P, Fe-P, and Ca-P increased across different soils, while Org-P accumulated as a result of sorghum cultivation in all soils. Nayak et al., (2015) [14] stated that red-P ranged from a minimum of 79 kg ha⁻¹ in the control plot to a maximum of 152.48 kg ha⁻¹ under the 150% NPK treatment. The application of graded doses of NPK fertilizer led to an increase in Red-P compared to the control. Chang et al., (2004) [1] reported that in acidic soils, the initially superficial and loosely bound phosphates associated with Fe and Al oxides, which are available to plants, gradually undergo reprecipitation and transform into highly crystalline Fe-and Al-P forms that are unavailable to plants.

Ojo et al. (2015) [17] stated that variations in soil P fractions are strongly influenced by soil type. Differences in total P content among soil orders arise from the combined effects of parent material, weathering, and other pedogenic processes.

A comparison between phosphorus fractions in different soil orders and cropping systems

Here V-Vertisols, A-Alfisols, I -Inceptisols

In (table 3) the distribution of phosphorus fractions varied across Vertisols (V), Alfisols (A), and Inceptisols (I). Calcium-bound P (Ca-P) was the dominant fraction in all soils, followed by Fe-P and Al-P, while Saloid-P contributed the least. Among soil orders, Alfisols (A1-A3) showed relatively higher Fe-P and Al-P contents compared to Vertisols and Inceptisols, indicating stronger P fixation by Fe and Al oxides. Vertisols (V1-V3) had higher Ca-P, reflecting their calcareous nature, whereas Inceptisols maintained more balanced fractions but comparatively lower Ca-P. Total P was highest in Alfisols (145.37 kg ha⁻¹ in A2) and lowest in Vertisols (122.32 kg ha⁻¹ in V2). Overall, the results highlight soil-order-specific differences in P fractionation, with Vertisols dominated by Ca-P, Alfisols by Fe-P and Al-P, and Inceptisols showing intermediate levels. In this (table 4) it was clearly observed that organic carbon content declined with increasing soil depth, while calcium carbonate content showed an upward trend across all cropping patterns. In the surface layer, organic carbon ranged from 3.2 to 4.0 g kg⁻¹, whereas in the subsurface layer it ranged from 2.4 to 3.1 g kg⁻¹. Calcium carbonate content varied between 3.90 and 5.00 mg kg⁻¹ in the respective layers. Overall, soils under all cropping patterns were characterized by low organic carbon status and classified as non-calcareous. It was observed that the organic P fraction was consistently higher in the surface layer than in the subsurface soils across all cropping patterns, mainly due to greater organic matter content in the surface horizon. Organic P ranged from 55 to 68 mg kg⁻¹ in surface soils and 37 to 48 mg kg⁻¹ in subsurface soils. Among the inorganic P fractions, Ca-P was the dominant form, followed by Fe-P, Al-P, and Saloid-P under all cropping systems. Saloid-P content varied from 20 to 40 mg kg⁻¹ in the surface layer and 13 to 21 mg kg⁻¹ in subsurface soils, with consistently higher values in surface horizons. The maximum saloid-P was recorded under the maizecauliflower sequence, followed by rice-cauliflower. Interestingly, Ca-P increased with soil depth across all patterns, with the highest Ca-P content under okra-potato and the lowest under rice-pea and maize-pea cropping sequences. Chang et al., (2004) [1] observed that the organic-P content declined up to 90 DAS but increased again at crop harvest across all treatments. This reduction could be attributed to microbial mineralization and crop uptake, while the subsequent increase at harvest was likely due to enhanced plant growth and the accumulation of organic matter in the soil, which contributed to higher organic-P levels. Patel et al. (2024) [18] reported that phosphorus application had a significant effect on Red-P levels, which ranged from 80.00 to 91.04 mg kg⁻¹ in 2020-21 and from 80.84 to 93.62 mg kg⁻¹ in 2021-22. Application of 90 kg P ha⁻¹ markedly increased Red-P compared to 30 and 0 kg P ha⁻¹. Across both years, Red-P content rose with higher phosphorus levels, reaching 91.04, 93.62, and 92.33 mg kg⁻¹ under 90 kg P ha⁻¹. Phosphorus fertilization at 30, 60, and 90 kg ha⁻¹ enhanced Red-P by 5.59%, 10.87%, and 14.10% over the control, respectively. The lowest mean Red-P levels (80.00, 80.84, and 80.92 mg kg⁻¹) were consistently recorded in P-omitted (P₀) plots. Majumdar et al., (2007) [12] reported that organic phosphorus (Org-P) contributed between 48.90 and 53.70 percent to total phosphorus (T-P). They also observed a significant increase in saloid-P (S-P), aluminum-bound P (Al-P), iron-bound P (Fe-P), and calcium-bound P (Ca-P), while reductant-soluble P and occluded-P fractions showed a decline. Nunes et al., (2020) [15] investigated the effects of long-term tillage and fertilizer practices on soil phosphorus fractions. They found that, compared to conventional tillage, no-tillage resulted in greater accumulation of inorganic P (Pi) in labile fractions, while organic P was more strongly associated with protective aggregates. Additionally, the use of reactive rock phosphate under no-tillage increased the Ca-P fraction compared to the use of triple superphosphate under conventional tillage, with this increase being more pronounced when fertilizers were applied by broadcasting. Mahawar et al. (2022) [13] reported that the application of FYM at 5 t ha⁻¹ significantly enhanced the proportion of the saloid-P fraction. It is important to note that saloid-P corresponds to the form of phosphorus that is readily available for plant uptake. Tiwari and Vats reported that the availability of phosphorus in soil improved significantly with the application of 100% NPK along with organic matter. The addition of organic manure enhanced available phosphorus through the involvement of cations such as Ca²⁺, Mg²⁺, Fe³⁺, and Al³⁺, which play a key role in phosphorus fixation. Yang et al. (2015) [7] , in their long-term study on phosphorus fertilizers, observed that the addition of organic matter markedly enhanced the residual fraction as well as other organic fractions (labile and partially labile), while no change was detected in the resistant form. In contrast, the application of mineral fertilizers, consistent with their findings, had no significant effect on any group of organic fractions. Majhi et al. (2003) [10] reported that the continuous inclusion or omission of certain secondary nutrients (S) and micronutrients (Zn and B) had no significant impact on grain yield. However, the results clearly indicated that the application of FYM had a notable positive effect on grain yield in both seasons. Nwoke et al. (2004) [16] observed variations in different inorganic-P fractions in soils under diverse management practices, with phosphorus depletion ranging from 33% to 129% over an 11-year period. Sharpley et al. (2004) [23] reported that continuous application of organic fertilizers increases soil organic phosphorus fractions. In our study, the variations were most evident in the upper two soil layers (0-20 and 20-40 cm), whereas the 40-60 cm layer generally showed the lowest values across most treatments and fractions. These differences were statistically significant, particularly in the manure-applied treatments (IV and V). Liao et al., (2020) [8] stated that crop roots release substantial amounts of organic compounds that help mobilize sparingly available phosphorus (P) in soils. This includes the exudation of protons, malate, and citrate into the rhizosphere, which solubilize insoluble P forms. Additionally, roots secrete acid phosphatase enzymes that hydrolyze organic phosphorus (Po) into inorganic phosphorus (Pi), making it available for plant uptake.

Conclusion

Phosphorus fractions play a crucial role in soil management, influencing soil fertility, plant nutrition and ecosystem health. Understanding the different phosphorus fractions and their dynamics is essential for optimizing fertilizer use, improving soil fertility, sustainable agriculture, environment protection. These studies have highlighted the importance of phosphorus fractionation in determining phosphorus availability, soil fertility and crop productivity. It is concluded that by increasing the level of phosphorus there is a significant increase in growth and yield parameters of the plants and even in long term experiments continued application of phosphorus leads to an increase in all fractions of phosphorus in soils. It also shows that different levels of N, P, K also affect the different fractions of phosphorus in soil.

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