430 Mineral Nutrition of Plants 



complicated by a hydration-dehydration equilibrium. The complex 

 hydrated oxide found in cool, moist soil has a much lower activity than 

 dehydrated divalent manganese. The evidence available is ample to 

 lend weight to the suggestion of Hoffer (6/) that a relatively high 

 accumulation of reduced iron and manganese in the soil under anaero- 

 bic conditions may well be toxic per se to plant roots over and above 

 any effect due strictly to inadequate aeration of the root surfaces. 



There is some evidence to indicate that the reduced forms of certain 

 organic components prevailing under the anaerobiosis of waterlogged 

 soils are specifically toxic to plants (106, ig, 94), and that these sub- 

 stances are readily oxidized and rendered harmless under aerobic soil 

 conditions. The concept that specific toxicity of certain organic sub- 

 stances in soil has an adverse effect in plant nutrition has been scoffed 

 at many times, but it has been adequately demonstrated by recent 

 investigations (10, 8, 50). Thorn and Smith (113) point out that the 

 anaerobic decomposition of organic matter in waterlogged soils fre- 

 quently produces hydrogen sulfide. This compound is very toxic to 

 roots. 



The presence of accumulations of carbon dioxide in the atmosphere 

 of waterlogged soils exerts a modulating influence over the activities 

 of Fe+++ and Fe++, and the consequent influence upon plant nutri- 

 tion, over and above the effect of the state of oxidation-reduction in the 

 system. Halvorson (55) points out that ferrous carbonate is very insolu- 

 ble as compared with ferrous bicarbonate which is readily soluble. 

 Thus, the activity of Fe++ in a given soil system is conditioned by 

 oxidation-reduction potential, pH, and partial pressure of carbon di- 

 oxide within the limitations of the amount and kind of ferruginous 

 mineral present. Halvorson's (55) analysis of the obtaining equilibria 

 indicates that anaerobic conditions in an alkaline soil in the presence 

 of a relatively high partial pressure of carbon dioxide may actually 

 bring about a reduction in solubility of iron as compared with the 

 aerobic state. On the other hand, increased partial pressure of carbon 

 dioxide under aerobic soil conditions is actually conducive to solubility 

 of ferric hydrate. 



There is considerable evidence to support Kliman's (6g) conclusion 

 that iron enters plants mostly in the reduced state. As a consequence, 



