of the soil air shows that it differs but little from the composition of the ordinary atmosphere. The content of carbon dioxide is higher—about 2%-3% as against 0.03% and the oxygen content is correspondingly lower. Studies of biological activity in the soil show that about seven litres of carbon dioxide are produced daily for each square metre of soil surface, and the low content of this gas found by actual analysis of soil air, indicates that it readily escapes from the soil and is replaced by oxygen from the atmosphere. The mechanism of this exchange has been much disputed. Many workers have ascribed it to the pumping action of barometric, temperature and moisture changes in the soil causing expansion and contraction of the air in the interstices, and to the action of wind. Careful estimates of the effects of these actions show that they are in sufficient to effect the exchange, which must therefore be due mainly to the phenomena of gaseous inter-diffusion. This process is continuous, whereas the other factors are intermittent.
Although the normal soil atmosphere closely resembles the or dinary air there is evidence of a second atmosphere in the soil, that is composed very largely of carbon dioxide. It is sometimes referred to as the dissolved atmosphere, as it is slowly evolved after the normal soil atmosphere has been evacuated by suitable means in the laboratory. It appears to come from the moisture in the soil, and from the colloidal material, and it is probable that some of it comes from the excessively minute interstices in the interior of compound particles. The significance of the dis solved atmosphere is that it points to the possibility of anaerobic conditions existing at places even in a soil that is in excellent tilth, a point of considerable importance in connection with the environment of soil organisms.
Much of the significance of this section of soil physics has become apparent in the preceding sections. The aim of cultivation is to provide a suitable seed bed for active germination and vigorous early growth of the seed ling plants, and to maintain the optimum conditions for subse quent plant growth. The basic operation is ploughing, and the tilth finally obtained is largely dependent on it. The heavier the soil the more important is proper ploughing. If the soil is too wet, the furrow slice suffers plastic deformation as it passes over the mouldboard, and as a result has a tendency to dry into large hard clods that resist the disintegrating action of the cultivators and harrows used in the next stages of seed bed preparation. When less moisture is present the furrow slice, although main taining its general coherence, becomes permeated by numerous cracks on passing over the mouldboard, and subsequent disinte gration of the soil by cultivators is much easier. Alternations of wet and dry spells are also of great service in assisting tilth for mation, and the shattering effect of frost on heavy land is well known. As a general rule it may be stated that no cultivation process should be carried out unless the moisture content has fallen to a value that permits air to enter the soil interstices.
(B. A. K.)