Water-Content of Plants 53 



proportions of these general constituents. In addition a 

 fertile soil must contain a considerable amount of humus 

 derived from the disintegration of plant and animal re- 

 mains. 



35. Soil texture and water-holding capacity. The 

 water-holding capacity and the capacity for delivering 

 water to the plant will depend, therefore, to a very con- 

 siderable extent upon the mechanical constitution. In 

 general, a fertile soil should consist of relatively fine par- 

 ticles, since the water-holding capacity and the amount 

 of food-materials will be, up to a certain point, a factor 

 of the fineness of the particles. On the other hand, fine- 

 ness of particles at or approaching the point of satura- 

 tion of the soil may cause exclusion of air to such an 

 extent that the finer the soil the less the amount of air 

 present. In this connection, however, it should be re- 

 membered that under ordinary circumstances the pore 

 space of a clay soil may be 50 per cent of its volume, while 

 that of a coarse sand uniform in texture may not exceed 

 30 per cent. 



One cubic foot of an ordinary loam would cover a rela- 

 tively enormous area if spread out as a single layer of 

 particles. It is calculated that the contents of one cubic 

 foot would cover about one acre. Some sort of mental 

 picture of the absorbing system and its possibilities may 

 be had by recalling, in conjunction with these indications 

 respecting soil particles, the extent of the root-system as 

 clothed with root-hairs. The root-system of a mature 

 sunflower may almost completely permeate a cubic yard 

 of soil. 



It is necessary to make a comparison of some main soil 



