Phosphate Fixation in Soils—A Critical Review

Discussion and SummaryWhen phosphates are applied to soils they readily change to less soluble forms, the availability of which varies greatly depending upon the degree of fixation. It is difficult to distinguish between fixation by adsorption and chemical precipitation, especially in view of recent results obtained by Stout (26). He reports that the crystal structure of kaolinite changed to a new crystal structure after fixing phosphate. This would seem to indicate chemical precipitation, but since he also reports that the phosphate was exchanged for hydroxyl ions it would indicate a type of adsorption. Most of the contributors who support the theory of phosphate fixation by adsorption have tried to show that the fixed phosphate is in an exchangeable form and that it can be replaced by other anions. A number of workers have shown that the hydroxyl ion is quite effective in replacing or liberating adsorbed phosphate, but in some instances it is likely that the fixing complex was partly dissolved due to increased alkalinity. The citrate ion has been used and found to be somewhat advantageous in this respect but this, too, may be due to its dissolving effect since citric acid quite readily dissolves iron phosphate. Steele (25) claims that the oxalate ion helped to reduce phosphate retention, but Metzger (15) reports that the oxalate removed only very small amounts of phosphate and that there were no indications of an equivalent replacement. The silicate anion has also been proposed as being capable of replacing phosphate. This has been offered as an explanation for the increase in available phosphate in soils and for the favorable effect to crop growth when sodium silicate has been applied to some phosphate-deficient soils. However, the evidence thus far reviewed indicates that the silicate may dissolve rather than actually replace the native phosphate in soils. While it is difficult to prove that phosphate is adsorptively fixed in an exchangeable form, it is evident that phosphate may be retained in soils, even when freed of soluble chemical materials which would otherwise cause precipitation. Most of the contributors who support this theory of phosphate fixation worked with purified colloids or specially prepared soils and used water as the extracting medium; hence, the importance of such fixation under actual field conditions is not well established. Until recently a number of workers have inferred that soluble iron and aluminum in acid soils are the main agents which render phosphate slowly soluble and difficulty available to plants. While this is a factor in fixation, it does not seem probable that it is solely responsible because free iron and aluminum ions are not present in the soil solution to any appreciable extent except in very acid or alkaline soils. Furthermore, a number of contributors have shown that freshly precipitated iron and aluminum phosphates are quite readily available to plants (10). On the other hand, certain mineral phosphates of these bases are known to be only slightly soluble and difficulty available. Both Ford (3) and Heck (6) feel that such phosphates may be formed in soils by a reaction of soluble phosphate with hydrated oxides of iron and aluminum even though the latter are not in solution. It seems likely that the phosphate forms complex addition compounds with the hydrated iron and aluminum oxides because the speed at which such fixation takes place precludes the possibility of a gradual solution of iron and aluminum and their reprecipitation as phosphates. Certain hydrated iron oxides seem to reduce phosphate availability more than iron hydroxide. Practically all acid soils contain considerable amounts of hydrated iron oxide and, since it is distributed as a thin film over the soil particles, it is in an ideal condition to react quickly with soluble phosphate. Aluminum oxide seems to be of lesser importance because it is usually present in smaller amounts. Fixation by hydrated iron on the surface of soil particles may be of considerable importance, especially in lateritic and podozolized soils (B horizon). Lime on acid soils seems to reduce this type of fixation, not because it lessens the activity or the amount of the hydrated iron oxide present but mainly because of the competition of calcium and the low solubility of the calcium phosphate which is formed, since both di- and tricalcium phosphates have a low water solubility. Lack of phosphate availability in alkaline calcareous soils seems to be due to a carbonate-phosphate complex rather than simple tricalcium phosphate (12). Fluorophosphate or fluorapatite may also form under such conditions because fluorine is apt to be present in underground waters of calcareous soils (22) and it reacts quite readily with tricalcium phosphate when alkaline, to form fluorophosphate. Since superphosphate also contains active fluorine care should be exercised in producing non-acid forming fertilizers because fluorophosphate also forms under these conditions (9). While it is slowly soluble, its availability to plants in alkaline and acid soils has not been well established.