THE BIOLOGICAL CONDITIONS IN THE SOIL 107 



pression is unfortunate, because it entirely disregards the water that 

 would be utilised if only it could travel quickly enough to the root, and 

 assumes that the whole of the untouched water is present in some state 

 other than the free state. Unfortunately wilting is so difficult to char- 

 acterise, and is affected by so many external circumstances, that in any 

 case it affords only a comparatively rough method of studying the 

 " availability " of the soil water for the plant. 



Micro-organisms require less moisture than plants, because they do 

 not pump out water into the air, and it often happens that the produc- 

 tion of nitrate and ammonia still continues in soils too dry to admit of 

 plant growth. 



Air Supply. 



The figures given in Table XLIII. show that about 10 per cent, of 

 the volume of a normally moist soil is occupied by air, but this volume 



TABLE XLIV. COMPOSITION OF THE AIR OF SOILS, PER CENT. BY VOLUME. 



(Atmospheric air contains 21 per cent, of oxygen and '03 per cent, of CO a .) 



is perpetually varying inversely as the amount of water varies. These 

 changes alone would lead to a renewal of the air supply in the soil, but 

 other factors, diffusion, changes in pressure, air movements, etc., come 

 in, making the gaseous interchange still more complete. At soil depths 

 reached by plant roots some 6 to 12 inches the soil air presents 

 no abnormal features : there is some accumulation of carbon dioxide, 



8* 



