Studies of Phosphate in Soils by Simultaneous Isotope Exchange Kinetics

Abstract

The estimation of available phosphate in soil by chemical extractants is essentially empirical and the amount of phosphorus removed from soil will depend largely on factors such as nature of extractant, the extraction time and the pH of the extracting solution. The best method is that which correlates well with the phosphate field trails. Very often, one method though applicable to one crop in one area, may however, fail to apply to the same crop in another region, having different climatic condition, soil minerals etc. The use of 32P-phosphate in estimating the amount of exchangeable phosphates does not involve the addition of acids, bases or complexing reagents and is therefore non-destructive in nature. Phosphates in soil that are exchangeable with 32P-phosphate in solution can be adequately regarded as being ultimately available to plants. The degree of availability of the exchangeable phosphates to the plant, will however, depend on the rate at which the phosphate can be released into solution. Mattingly (1957) commented that (1) so far little progress has been made in analysing the complex curve relating time and the percent of isotopic exchange with the different inorganic fractions of soil phosphorus; and that (2) in most works on the phosphate status of soils, the rate of release of phosphate has seldom been measured and it is possible that more detailed study of the rates of isotopic exchange may help to clarify the conditions under which fixed or precipitated phosphates become available for crop growth. A technique of Simultaneous Isotope Exchange Kinetics (SIEK) has been developed whereby the actual activity remaining in solution during the process of isotopic exchange between phosphates of the solid-liquid phases at equilibrium, can be measured and recorded continuously from the moment the 32P-phosphate was introduced to the time the exchange reaction reaches completion. The curve of count rate versus time can be analysed mathematically. The activity of the solution as a function of time can be expressed as the sum of a number of logarithmic terms each representing a different type or fraction of inorganic soil phosphate. The mathematical theory of the SIEK has been discussed in detail and the equations relating to the calculations of specific rate constants and the amounts of the different fractions of exchangeable phosphates, have been derived. The anion-exchange reactions involving phosphate are known to play an important role in the "phosphate retention" and the "phosphate availability" in soil. Investigations using the SIEK technique have been made by the author on the nature of anion-exchange, involving phosphate by the use of an anion-exchange resin, De-Acidite FF (a highly basic resin based upon cross-linked polystyrene containing quaternary ammonium groups) as a model. The advantages offered by the anion-exchange resins are many, namely, (1) It is a simply system, chemically pure and well understood. (2) It does not fix phosphate, and consequently all phosphate held by the resin is exchangeable. (3) All the positive sites on the resins are identical to each other. All the phosphate held ionically by the electrostatic force to the resins should be equally available for exchange. The specific rate constants appropriate to typical anion-exchange (ionic) have been determined. A criterion of ionic-exchange involving phosphate in soils has been proposed. Before studying complex soil systems, a wide selection of surfaces that are known to exist in soil and are of special interest in connection with the phosphate fixation or retention by soil, were investigated by the author using the SIEK technique; and they are as follows, (A) The Aluminium System: 1. Cation-exchange resins (Zeo-Karb 225-Al) 2. Gibbsite 3. Amorphous Al(OH)3 4. Illite (Al-form) 5. Montmorillonite (Al-form) 6. Halloysite (Al-form) 7. Kaolinite (Al-form) 8. Allophane (Al-form) (B) The Iron (Fe3+) System: 1. Cation-exchange resins (Zeo-Kerb 225-Fe) 2. Goethite 3. Amorphous Fe(OH)3 4. Illite (Fe-form) 5. Montmorillonite (Fe-form) 6. Halloysite (Fe-form) 7. Kaolinite (Fe-form) 8. Allophane (Fe-form) (C) The Calcium System: 1. Cation-exchange resins (Zeo-Karb 225-Ca) 2. Dicalcium Phosphate 3. Tricalcium Phosphate 4. Calcium Carbonate 5. Apatite 6. Illite (Ca-form) 7. Montmorillonite (Ca-form) 8. Halloysite (Ca-form) 9. Kaolinite (Ca-form) 10. Allophane (Ca-form) This SIEK work has shown that, (1) The technique has worked well on all the surfaces studied. (2) The curves of count rate versus time could all be resolved into a number of straight lines (1 to 4), representing different different types or fractions of exchangeable phosphates on the solid phase exchanging with solution phosphate at distinctly different rates. (3) Phosphate retention is an adsorption phenomenon rather than precipitation. (4) The specific rate constants for all the surfaces studied could be conveniently grouped into for distinct classes. The phosphate retention and the manner of distribution of the freshly retained phosphate on clay minerals, have been studied on quantitative basis. The rates of phosphate release (W) have been calculated and compared. Ten soil profiles (both top-soils and sub-soils) specially selected to represent different types of natural non-topdressed soils in New Zealand have been investigated. The soil samples collected for the International Soil Conference, 1962, were supplied by the courtesy of the Soil Bureau, D.S.I.R., Wellington. Also included in the studies were three allophanic agricultural soils supplied by the courtesy of the Ruakura Agricultural Research Centre, Department of Agriculture, Hamilton, New Zealand. It has been found that the SIEK technique can be applied equally well on complex soil systems. The specific rate constants, the amounts of all different fractions of inorganic phosphates and the rates of phosphate release of theses soils have been calculated, tabulated compared. The experimental results obtained by the SIEK technique are intended to serve as guides for all types of crops and have been found to correlate well with the pot trial (White Clover) results, conducted separately by both the Soil Bureau, D.S.I.R., Wellington and the Department of Agriculture, Hamilton, New Zealand. The SIEK technique has been extended to include the following studies: (A) The effects of fertilization on phosphate status in soil and the fate of added fertilizer. (B) The effects of liming on phosphate status in soil. (C) The effects of leaching on the phosphate status in soil. (D) The effects of ignition on the phosphate status in soil. The experimental results of the SIEK expressed in physico-chemical terms though self-explanatory to chemists, may, however, prove difficult to understand by farmers. In terms of common language, the experimental results for the natural soils are presented diagrammatically.