Iron Solubility in Soils as Affected by pH and Free Iron Oxide Content

C of certain plants, particularly sorghums, corn, trees, and shrubs, is present in scattered areas of Kansas, especially in the western half of the state. Myers and Johnson (8) have shown in greenhouse and field tests that the chlorosis is due, at least in many cases, to an insufficient supply of iron for the plants. They found that the application to the soil of 100 pounds of ferrous sulfate per acre failed •to correct the chlorosis, but that a 3% solution of ferrous sulfate sprayed on the plants was effective. The present investigation was begun to study some of the factors that affect iron solubility in soils. It has long been recognized that the degree of acidity is an important factor controlling the solubility of iron in soils. Mann (7) found that moderate liming of two acid soils increased their content of water-soluble iron but that higher rates of lime decreased it. The increase in iron solubilities at moderate liming rates was attributed to a combination of iron with the organic matter brought into solution. Mann obtained water-soluble iron contents that varied from 4 to 11 ppm in one soil and from I to 3 ppm in the other. Sideris and Kraus (10), however, obtained water-soluble iron contents that ranged from only o.i ppm in soils of pH 3 used for the growth of pineapples to less than o.oi ppm in similar soils of pH6. Iron deficiencies have most often been found in alkaline or calcareous soils. However, as pointed out by Chapman (i), iron chlorosis often occurs on some calcareous soils and not on others. Chapman suggested that the ability of plants to get iron from alkaline soils may be due, in part, to contact feeding on iron-bearing minerals and that the presence or absence of iron chlorosis may be a function of the amount of potentially available iron present. In greenhouse sand cultures, Chapman found that the addition of calcium carbonate to cultures with small amounts of iron caused iron chlorosis of citrus seedlings and that the addition of magnetite corrected the chlorosis. Other investigations have suggested that the ability of plants to obtain iron from soils is related to the amount or form of iron present in the soils. Thorne (u) was able to extract more iron with 0.5% oxalic acid from areas of Hyrum clay loam that produced normal plants than from areas in which chlorosis occurred. Thorne and Wallace (12) extracted readily reducible iron from soils using ammonium acetate solutions of pH 5 containing 0.2% hydroquinone. The extracts of soils producing green plants contained significantly more iron than extracts of soils producing chlorotic plants.' The addition of ferrous and ferric iron in the form of chlorides increased the extractable iron but not all the added iron was extractable.