THE SOIL. 43 



Below 30 cms. there is no hourly change in temperature. 



AlK IN ITS RELATION TO THE SoiL. 



For animals, a well ventilated room is a necessary condi- 

 tion of health, and similarly for plants, good health can 

 only be maintained in a well ventilated soil. The air in 

 the soil is required for the respiration of the soil organisms 

 and the plant roots. The soil air is naturally richer in 

 carbon dioxide than the atmosphere, and to clear out this 

 gas and maintain a supply of oxygen a constant circulation 

 of the soil air is necessary. The spaces between the soil 

 particles are filled with either water or air and consequently 

 it is the open, well drained soils which are the best aerated. 

 In stiff clay soils, plants probably suffer more from a defi- 

 ciency of air than from any other cause. In such soils 

 their roots are unhealthy and inactive, and the weathering 

 of plant food is reduced to a minimum. The circulation 

 of the soil air is brought about by the different tempera- 

 tures of the different parts of the soil, by the differences 

 of temperature of the atmosphere and soil air, by the 

 movements of the soil water and by the diffusion of gases 

 due to the different compositions of the soil air and the 

 atmosphere. Certain crops, like rice, can practically be 

 grown in water as long as this water is kept aerated by 



