2. Soil air, a gaseous phase of the soil.

The process of soil aeration in one of the most important determination of soil productivity. Plant roots absorb oxygen and release carbon dioxide in the process of respiration. Soil microorganisms also respire and under conditions of restricted aeration might compete with the root of higher plants. Some students of soils say that aeration has became now a major limiting factor to the attainment of maximal productivity. Anaerobic conditions induce a series of reduction reactions, for example, denitrification: NO₃^-  NO₂^- N₂O N_2.

2.1. Soil air composition and properties.

In a well-aerated soil, the composition of air is close to that of the «open» atmosphere, as the oxygen consumed in the soil is readily replaced from the atmosphere by diffusion. Not so in a poorly aerated soil. The CO₂ concentration of the atmosphere is about 0.03%. In the soil it frequently reaches levels which are ten or ever one-hundred times greater. Since the O₂ concentration of air is normally about 20%, it would seem that even

A hundred fold increase of CO₂ concentration, from 0.03 to 3% can only diminish the O₂ concentration to about 17%. However, even before they begin to suffer from lack of oxygen, some plants may suffer from excessive concentration of CO₂ and other gases in both gaseous and aqueous phases . Soil air is characterized by a high level of relative humidity, which very often approaches 100% except at the soil surface during prolonged dry spells.

Volume fraction of soil air (aeration porosity) is its important property. Maximum volume fraction of soil air is equal to the total porosity of the soil. Since the two twin fluids-water and air-compete for the same pure space, the fraction of soil air is equal to aeration porosity:

AP = TP – W ∙ d ,

where TP is the total and AP – aeration porosity in % by volume; W is the field wetness of the soil in % by weight, and d is the bulk density of the soil, g∙cm^-3. AP should not be less than 15 % by volume in mineral soils.

Another important property is the conductivity or permeability of the soil to air. It depends not only on porosity but on the geometry and character ( size,etc.) of air-conducting pores. Air conductivity determines the rate of exchange of soil air with atmospheric. At high wetness values, soils often contain isolated pockets of occluded air. At times, even a thin surface crust can form a bottleneck limiting aeration. We are thus led to the necessity of characterizing aeration in more dynamic terms.

There are two essential mechanisms of soil air flow: (1) convection and (2) diffusion. In the case of convection, also called mass flow, the moving force consists of a gradient of total gas pressure. The entire mass of air streams from a zone of higher pressure to one of lower pressure. In the case of diffusion, the moving force consists of a gradient of total gases pressure. The entire mass of air streams from a zone of higher pressure to one of lower pressure. In the case of diffusion, the moving force is a gradient of partial pressure (or concentration) of any constituent member of the variable gas mixture which we call air. The diffusion process, considered to be the dominated one in the soil, can be described by the Fick’s law:

q _d=-D dc/dx ,

wherein q_d is the diffusive flux ( mass diffusing across a unit area per unit time ). D is the diffusion coefficient having the dimensions of area per time, C is concentration ( mass of diffusion substance per volume ), x is distance, and dc/dx is the concentration gradient.