432 Journal of the Department of Agriculture. 



Now, the roots of plants and other living things in the soil 

 require air for the oxygen gas it contains. They use this oxygen 

 gas up ; it is therefore necessary that the soil air be renewed from 

 time to time. This is accomplished naturally by the movements of 

 air which take place between the atmosphere above ground and that 

 below ground. 



If the air (oxygen) supply in the soil be deficient, one would 

 expect the root system to suffer; one would expect to find a better 

 developed root system in the sandy as compared with the clayey soil. 

 Every farmer knows that this is so. A well aerated soil and a good 

 root system go together, as do also a poorly aerated soil and a feeble 

 root system. 



Much can be done by skilful tillage and management to improve 

 the air supply of the finest types of soil ; the skilful husbandman is 

 able to cause the finest particles of this class of soil to coalesce, the 

 compound particles so formed then behaving much like the coarser 

 sand particles. Calcium and magnesium carbonates are also able to 

 bring about this desirable coalescence of the clay particles; that is 

 another reason why " carbonates " figure in the mechanical analysis 

 of the soil ; the more carbonates the soil contains, the more clay it 

 may contain and yet be tillable. 



It will be obvious that the more extensive the root system, the 

 better able will the plant be to obtain the food it requires from the 

 soil. We shall see later that this means that less percentages of 

 food suffice in the case of sandy soils than are necessary in the case 

 of clayey soils. 



If an animal be shut up in an air-tight box, it would not die 

 immediately, but as soon as it had used up the oxygen of the air 

 in the box (or, to be correct, a part of it only) it would die from 

 suffocation. A plant will die from suff'ocation if the air supply 

 available to its roots is sufficiently restricted. 



This sometimes happens in practice when a growing crop is 

 submerged for too long a period, either in a natural way or by 

 irrigation. 



Not only is it necessary to avoid the condition described in the 

 preceding paragraph, it is also necessary for best results to keep the 

 water supply in the soil below a certain limit. 



According to King* ideal conditions exist in the soil when of 

 a cubic foot (= 100 per cent.) 44.24 per cent, consists of solid par- 

 ticles, 33.63 per cent, of air, and 22.13 per cent, of water. A larger 

 proportion of water than that means a deficient air supply. These 

 figures are not quoted with a view to suggesting that the farmer 

 can maintain such conditions in his soil, but rather to point out that 

 too much water is as much to be guarded against as a deficiency. 



Soils that in their natural state contained so much water (and 

 therefore so little air) as to be useless ft)r the growth of crops have 

 frequently been made highly productive through drainage which 

 removes and prevents the subsequent accummulation of excess water. 



We have seen that air supply is a very important factor in plant 

 growth, and further that it is influenced by the mechanical composi- 

 tion of the soil, tillage, and water supply. 



Since the air requirements of crops vary a good deal, the 

 mechanical analysis of the soil is to be regarded as an aid in deciding 



* F. H. King, " Soil Management." 



