Soil organic phosphates

Anderson, G. and Malcolm, R.E., 1974. The nature of alkali-soluble soil organic phosphates. J. Soil Sci., 25 282—297. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

J. Gerke, Orthophosphate and organic phosphate in the soil solution of four sandy soils—evidence for humic-Fe(Al) phosphate complexes. Commim. Soil Sci. Plant Anal. 25 601 (1992). 

N. G. Juma and M. A. Tabatabai, Hydrolysis of organic phosphates by corn and soybean roots. Plant Soil 707 31 (1988). 

Several different types of species are illustrated in Figure 6.1. The potassium cation (K+) at the top of the figure is separated from the soil surface by water molecules and would thus be considered an outer-sphere species. The potassium cation near the bottom of the figure is directly connected to the soil particle by an ionic charge and is therefore an inner-sphere species. Above this is an inner-sphere phosphate directly bonded to a soil surface aluminum. Also shown are potassium cations attached (inner sphere) to colloidal clay (CC) and colloidal soil organic matter (COM). Each of these is a different species. 

The condensed phosphates or polyphosphates are another important class of inorganic phosphates. In these compounds, two or more phosphate groups bond together via P - O - P bonds to form chains or in some cases cyclic species. In soils and waters polyphosphates generally account for only a small part of the total P content. However, these species are very reactive and in many places they are responsible for anthropogenic pollution of natural water, for instance, by detergents. The polyphosphates are formed also in reactions between orthophosphates of mineral fertilizers and soil organic matter and can be leached to surface waters (Kudeyarova and Bashkin, 1984 Kudeyarova, 1996). 

Sulphur (S) occurs in soils usually as sulphites, sulphates, sulphides and in organic compoimds. However, the most accessible form is sulphate (SO ). The turbidimetric procedure is widely used in the estimation of available S in the soil due to its rapidity. However, erroneous results are obtained in case the soil is rich in organic matter. Soil is shaken with a solution of monocalcium phosphate, containing 500 ppm P. The phosphate ions displaces the adsorbed sulphate. The calcium ions depresses the extraction of soil organic matter, thus eliminating contamination from extractable organic S. The method extract soluble plus a fraction of adsorbed The filtrate is then analysed for S by the turbidimetric procedure. In this method... 

Aluminium in soils is closely connected to soil acidity and is also discussed in the chapters on acid soils and ion-water reactions. The acidity of acid soils is due to the reactions of water with exchangeable Al3+ on the surface of soil particles. The strong Al-water reaction repels H+ from the water molecules iuto the soil solution. This can create soil acidities as low as pH 4.5. Stronger acidity means other H+-yielding reactions—organic acids from soil organic matter decay, sulfur and sulfide oxidation, phosphate fertilizers, ammonia oxidation, acid rain, and Fe- and Mn-water reactions—are active. 

Gel chromatography, which depends primarily on molecular weight differences, has been used (a) to separate mono-, di-, and tri-alkylated phosphates and (b) for the estimation of organic phosphates in soil. It is interesting to note a reversal of roles tri-n-octylphosphine oxide-treated cellulose has successfully separated gold, antimony, and thallium. ... 

In addition to inorganic orthophosphates, phosphorus is bound in organic phosphates (ester linkages) within humus, from which plant available phosphate is set free by microorganisms (mineralization). Examples are nucleic acids, inisitol hexaphosphates as the largest group, and phospholipids. In agricultural soils, the C P ratio is in the order of 50 1. 

Plant species differ widely in the extent of phosphorus absorption by their roots. Forest trees take up P from insoluble compounds with the help of mycorrhizas, whereas phosphorus is removed from soils by harvested crops ( 6 kg ha in agriculture), erosion and to a small extent by leaching (-0.1 kg ha ) and volatilization as phosphine PH3. In the case of erosion, colloids with their surface-bound P are transported into surface waters, and this leads to the eutrophication of aquatic ecosystems, for example in lakes. Leaching losses occur only in sand and peat soils, and in the case of organic phosphates. Under extreme redox situations - as in some paddy soils - phosphate is reduced to PH3 as a gaseous product. 

Phosphorus is necessary for all organisms as a constituent of nucleic acids, phospholipids and other organic phosphate compounds. Its frequent deficiency in soils must be supplemented with phosphate fertilizers. Phosphorus is liberated from organic matter during ammonification. Phosphate is also made available to plants as a result of the formation by microorganisms of organic acids which dissolve insoluble inorganic phosphate compounds in the soil. 

X-ray spectra of phosphorus in natural organic molecules. The P NEXAFS spectroscopic studies conducted on isolated humic substances and soil organic molecules indicate that the primary form of P is phosphate and phosphonate (Fig. 24 Myneni and Martinez 1999). When compared to humic substances from soil systems, the fluvial humic substances exhibit phosphonate as one of the important components. However, phosphonate constitutes only a minor fraction of total P in humic substances. Soil samples also exhibit features that correspond to polyphosphate. Another study conducted using NEXAFS spectroscopy at the P absorption edge suggested that marine sediments and humic materials do not exhibit phosphonate and the P NEXAFS spectra of these samples more closely resembled that of hydroxyapatite (Vairavamurthy 1999). 

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