Sodium Hyposulphite-Soluble Iron Oxide and Water Retention by Soils

During the attempted explanation of differences in the water retention-extraction pressure curves of certain soils, estimates were made of their free iron oxide content by the sodium hyposulphite method. The close relationship observed between iron oxide content and volume of water retained within the 1/10 < 15 atmosphere range led to examination, with a larger group of soils, of the effect upon water retention of iron oxide removal. Positive associations were found between content of iron oxide and of water retained (w) at both the ⅓ and 15 atmosphere pressures but the amount retained at 15 atmospheres increases more rapidly with increases in iron oxide content than does the ⅓ atmosphere water. The proportional amount of (w⅓ − w₁₅), expressed as a fraction of the w⅓ water, therefore diminishes with increase of iron oxide. Removal of the iron oxide changes the situation, pronounced increases being brought about in the absolute and proportional percentages of (w⅓ − w₁₅). Consideration has been given to the effects of the oxide solvent upon the silicate clay minerals, the fact that removal of the iron oxide destroys iron oxide clay particles and that the measurements were made with loose bodies, rather than cores, of soil. It is concluded that the presence of free iron oxide is associated with proportionally high volumes of pores of size smaller than that equivalent to the 15-atmosphere extraction pressure and that, when present, iron oxide may commonly be expected to diminish the amount of (w⅓ − w₁₅) water in the soil.