[Advances in Soil Science] Advances in Soil Science Volume 16 || Chemistry of Phosphorus Transformations in Soil

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  • Chemistry of Phosphorus Transformationsin Soil

    S.K. Sanyal and S.K. De Datta

    I. Introduction 2II. Physicochemical Processes Governing Phosphorus Concentrationin Soil Solution 2A. Sorption-Desorption 2B. Precipitation-Dissolution 30

    III. Reactions of Phosphorus Fertilizers in Soil 34A. Reactions of Soluble Fertilizers . . . . . . . 34B. Phosphate Rocks as Fertilizers 37C. Partially Acidulated Phosphate Rocks 55

    IV. Chemistry of Phosphorus Transformations in Submerged Soil .. 61A., Physicochemical Changes on Flooding that Affect PhosphorusAvailability 61

    B. Soil Test for Phosphorus in Flooded Soils 70V. Soil Organic Phosphorus 72A. Chemical Nature of Soil Organic Phosphorus 72B. Changes in Soil Organic Phosphorus Due to Cultivation 78C. Biological Transformations of Soil Phosphorus 80

    VI. Phosphorus Management Options 88A. General Observations 88B. Phosphorus Management Practices for Lowland Rice 89

    VII. Unresolved Challenges 93References 94

    1991 by Springer-Verlag New York Inc.Advances in Soil Science, Volume 16

  • 2I. Introduction

    S.K. Sanyal and S.K. De Datta

    Phosphorus (P) is essential for plants and animals because of its role in vitallife processes, such as in photosynthesis in plants and energy transformationsin all forms of life. It also has a significant role in sustaining and building upsoil fertility, particularly under intensive systems of agriculture.Soils are known to vary widely in their capacities to supply P to cropsbecause only a small fraction of the total P in soil is in a form available tocrops. Thus, unless the soil contains adequate amount of plant-available P,or is supplied with readily available-(inorganic)-P fertilizers, crop growth willsuffer.A large proportion of P depleted by the agricultural crops comes fromthe native P content of the soil, and this should be replenished primarilythrough inorganic P fertilizers. Furthermore, unlike nitrogen (N), which canbe recycled to the soil by fixation from air, P once removed from the soilby the crop or by erosion, runoff, or leaching cannot be replenished exceptfrom external sources.There is also concern for a rapid depletion of high-quality phosphaterock (PR) ores for production of soluble fertilizers, which is further compli-cated by the high manufacturing costs involved. Reactive phosphate rocksand their partially acidulated products are therefore often used as direct-application fertilizer materials, mainly in acid soils.The dynamics of P transformations in soils, and the fixation and releasecharacteristics of P are reported in this chapter. The importance of variousphysicochemical processes in governing P concentration in soil solution hasbeen highlighted. In particular, the distinct nature of P transformation pro-cesses in flooded soils as compared with those in upland soils has beendiscussed. An area of more recent investigations is the transformation oforganic P and the buildup ofmicrobial biomass P in soil (Tate, 1984; Stewartand Tiessen, 1987). Recent developments in methodology have made it pos-sible to estimate better the pool size of organic P, and its contribution toavailable P in the soil. Key processes of interaction of P with carbon (C), N,and sulfur (S) have also been identified, and incorporated into models of Pcycles (Parton et aI., 1988). Research data have been generated to validatethese models of P dynamics in a cropping system for medium- or long-termfertility trials in various environments.

    II. Physicochemical Processes Governing PhosphorusConcentration in Soil Solution

    A. Sorption-Desorption

    Phosphorus (P) adsorption by soils is a widely researched subject. This isprimarily because of the widespread P deficiency reported for agriculturalsoils, and the fact that P adsorption by soils is a process mainly responsible

  • Chemistry of Phosphorus Transformations in Soil 3

    for rendering soluble phosphate in soil solution unavailable to plants. Theadsorption process, which refers to surface P accumulation on soil compo-nents, may, in some cases, be accompanied by penetration of the adsorbed Pby diffusion into the adsorbent body, leading to further absorption of theadsorbed species. The general term sorption is sometimes used to denote bothof these processes taking place simultaneously.In acid soils, P adsorption is generally attributed to hydrous oxides ofiron (Fe) and aluminum (AI), and to (1: 1) layer lattic clays, particularly intropical soils with low pH. The possibility that some natural phosphatesof aluminum and/or iron (such as variscite and strengite) are formed inthese soils was discounted by some authors (Ryden and Pratt, 1980). Phos-phate adsorption in acid soils was also considered (Hsu, 1965) to be a specialcase of precipitation wherein aluminum (or iron) remained as the constituentof the original phase while reacting with P by the use of residual force on thesurface. Furthermore, a more fundamental approach to study P immobiliza-tion in acid soils suggested would be to examine the development of reactivealuminum hydroxides and iron oxides rather than the solubility of some Pcompounds. However, in recently fertilized soils, the local conditions of lowpH and high-phosphate concentrations in the vicinity of the P fertilizergranules are conducive to the dissolution ofclays and reprecipitation ofa widevariety of P compounds by reaction with soil components (Sample et aI., 1980;Ryden and Pratt, 1980). Such a process of crystallization of P compounds,having definite values of solubility products, is slow, and the attainment ofequilibrium is likely to be disturbed by P uptake by plants, diffusion, and moreimportantly, by rapid adsorption of the soluble P by surface-reactive alumi-num hydroxides and iron oxides, especially if the latter is predominantlyamorphous in nature. As a consequence, persistence of fertilizer reactionproducts in soils is unlikely. Details on P precipitation-dissolution equi-librium in soils will be discussed in a subsequent section (Section lIB).In neutral and alkaline soils, various forms of calcium phosphates arethe stable minerals that govern P concentration in soil solutions. In calcareoussoils, treatments with a high level of P tend to converge to a commonconcentration of P in solution of 1-2 ppm P after a long period of contact(AI-Khateeb et aI., 1986). This behavior can best be explained by the slowformation of a calcium (Ca)-P compound (Barrow, 1987). More direct evi-dence of formation ofcalcium phosphates on a calcite surface, reacting with Psolutions, was provided by Freeman and Rowell (1981) who demonstrated byscanning electron microscopy and X-ray diffraction technique the formationon a calcite surface of dicalcium phosphate (DCP) that slowly changed tooctacalcium phosphate.

    1. Equilibrium Systems

    a. Adsorption Isotherms

    The relationship between the amount of P adsorbed per unit weight of soil(x) and the equilibrium P concentration in solution (c) bathing the soil at

  • 4 S.K. Sanyal and S.K. De Datta



    >C 2.0......


    Figure 1. Phosphorus sorption isotherm data plotted according to the Langmuirisotherm. (Source: Bennoah, E.O. and D.K. Acquaye. Phosphate sorption characteris-tics of selected major Ghanaian Soils, Soil Science 148, 114-123. Williams &Wilkens 1989.)

    a constant temperature has been described by several adsorption isotherms.The main motivations for describing adsorption curves were to (1) identifythe soil constituents involved in adsorption (Adams et aI., 1987; Loganathanet aI., 1987), (2) predict the amount of fertilizer needs of soils to meet thedemand of plant uptake for an optimum yield (Fox and Kamprath, 1970; Fox1974; Fox and Kang, 1978; Roy and De Datta, 1985; Greenland and De Datta,1985; Klages et aI., 1988), and (3) study the nature of the adsorption processto learn more about the mechanism of the process (Barrow, 1984, 1987).Langmuir Adsorption Isotherm. There have been many attempts to fit theresults of P adsorption studies on soils, clays, and sediments to the simpleLangmuir equation (Bache and Williams, 1971; Sims and Ellis, 1983a; Vig andDev, 1984; Bennoah and Acquaye, 1989). A close fit to the simple Langmuirequation is generally obtained at low concentrations 15 mg/l) over a limitedrange. In fact, the adsorption maximum calculated for a lower concentrationrange is often exceeded at higher concentrations (Barrow, 1978; Harter, 1984).A typical plot of the simple Langmuir equation for P adsorption by soils isshown in Figure 1, which illustrates the rather narrow range of linearity ofthe plot. A curved relationship between (c/x) and c over a wide range ofconcentrations implies that the bonding energy is not a constant but rathera function of adsorption, and that there is no well-defined maximum. Aprobable reason for these deviations may be the migration of sorbed P tosubsurface layers and crystalline hydrous Fe oxides. The restriction to amonomolecular layer assumed in the Langmuir model also seems unlikely,especially at higher concentrations where some kind of surface structuremay start forming (Olsen and Khasawneh, 1980). In conclusion, the assump-tion of no lateral interaction among the sorbed P species and constant freeenergy of adsorption does not fit well with the present knowledge that thesorbed P species carries a charge and that surface charge and potential

  • Chemistry of Phosphorus Transformations in Soil 5

    decrease as more P is sorbed on oxide minerals or in the soil system (Sposito,1981; Kuo and McNeal, 1984).To account for such nonlinearity ofthe simple Langmuir plots, researchershave proposed several modifications to the simple Langmuir equation. Theequation proposed by Gunary (1970) includes an additional square-root termin concentration. In some cases, the data can best be fitted with two intersect-ing straight lines, and t