No. 545] PRESEXT PROBLEMS IX SOIL PHYSICS 



295 



below the soil surface. But, as will shortly appear in 

 some detail, to appreciate the problems before us it will 

 be necessary, not only to deal with the internal condi- 

 tions of the root system together with the external ones 

 of the soil, but also to bear constantly in mind certain 

 relations which obtain above the soil. I shall begin with 

 a brief treatment of the water relations of the plant, 

 with special reference to the physical conditions of its 

 subterranean environment. 



In order that the water content requisite for the 

 various physiological processes may be maintained, the 

 condition must obviously be fulfilled, that the ratio of the 

 rate of water income to that of water removal must never 

 fall below unity. Now, the removal of free water from 

 the physiological system of the plant occurs in three 

 general ways: (1) the fixation of water by growth, etc., 

 (2) the excretion of liquid water at the periphery and (3) 

 the loss of water vapor by transpiration. The first two 

 of these are usually negligible, and the prime aerial con- 

 dition of plant activity — as far as the water relation is 

 concerned — is the rate of loss of water vapor. This loss 

 is to a variable extent controlled by conditions within 

 and without the plant, but we do not need to give these 

 attention now. The main point for us to bear in mind is 

 that, for the activities of the majority of terrestrial 

 plants, it is requisite that the entrance of water tli rough 

 the roots must equal its rate of exit through the leaves 

 and other aerial parts. 



Of course water will not, in general, enter through the 

 roots faster than it is removed from the plant body or 

 fixed therein by growth and metabolism, and the critical 

 consideration in respect to the soil water relation is not 

 the actual rate at which water is entering (this depend- 

 ing upon the internal conditions of the plant as well as 

 upon the soil), but the maximum possible rate at which it 

 may enter if the prerequisite internal conditions arise. 

 In this respect, then, that soil is best suited to continued 

 physiological activity, which possesses the highest power 



