Applications in Soil Amendment
Plant roots need oxygen to function. Waterlogged soils occur due to heavy rainfall or bad irrigation techniques. For example, when rain guns are used on unstable soils, sand, silt or limestone, soils run together creating a seal that is impervious to oxygen and carbon dioxide. In these cases, diffusion of gases through soil pores is so strongly inhibited that it fails to meet the needs of growing roots. Indeed, a slow oxygen influx was found to be the main reason of root injury.
In addition, flooding reduces the escape or oxidative breakdown of gases such as ethylene or carbon dioxide produced by roots and soil organisms. Accumulation of ethylene may slow root extension, and an excess of carbon dioxide in the soil can severely damage the roots of certain species. In addition, carbon dioxide can form bicarbonate ions that accentuate the effect of high lime in some soils leading to iron unavailability and chlorosis.
Typically, soils with ample supply of organic matter accelerate the development of these potentially harmful soil conditions due to the increased growth of microorganisms that decompose the organic matter leading to reduced oxygen availability in the soil. Crops growing at high temperatures or under a lot of sunshine also have very high oxygen demand at the roots.
These conditions also lead to soil chemistry changes that are potentially harmful to the roots.
In the absence of oxygen, microbial respiration can use alternate electron acceptors to sustain energy generation. This leads to a decrease in redox potential. For example nitrate is converted to nitrite, rendering nitrate unavailable to the plant. With an increase in the soil’s reducing environment, oxides of Mn 4+, and Fe3+ are reduced to the highly soluble Mn2+, and Fe2+ which might penetrate the roots and affect enzyme activity leading to membrane damage.
Injury can also occur from chemically reduced metabolites such as organic acids and respiratory intermediates such as acetaldehyde.
Under these conditions, the lack of enough oxygen reduces root growth resulting in stunted growth, poor yields, lack of seed germination, etc. Another issue is the development of mycorrhizas. They need well aerated soils and are inhibited by water logging and reducing conditions (Soil Conditions and Plant Growth 10th edition by E. W. Russell, Longman Group Limited 1973 p 253).