CHAPTER X. 



SOIL AND SUBSOIL (Continued) 



THEIR RELATIONS TO VEGETATION. 



Physical Effects of the Percolation of Surface Waters. 

 The muddy water formed by the beating of rains on the soil 

 surface will, in penetrating the soil, carry with it the diffused 

 colloidal clay to a certain depth into the subsoil. We should 

 therefore expect that as a rule every subsoil will be more 

 clayey than its surface soil ; and this is found to be almost uni- 

 versally the case in the humid region. Subsoils are therefore 

 almost always less percious and more retentive of moisture, as 

 well as of plant-food substances in solution, than their surface 

 soils, unless these are very rich in humus ; and as the finest 

 particles are usually those richest in available plant-food, it 

 follows that subsoils will as a rule be found to contain larger 

 supplies of the latter than the surface soil. Common experi- 

 ence as well as comparative analysis confirm both of these in- 

 ferences so thoroughly, that it becomes unnecessary to adduce 

 examples in this place. 



On the other hand, the reverse, upward movement of moist- 

 ure caused by surface evaporation tends constantly to bring 

 any soluble salts contained in the soil mass nearer to the sur- 

 face, thus increasing the stock of easily available plant-food in 

 the surface soil. In extreme cases, especially in the arid 

 region, this accumulation of salts may become excessive, and 

 seriously injurious to plant growth. (See "Alkali Soils, 

 chapters 21, 22.) 



Chemical effects of Water-Percolation. The accumulation 

 of plant-food in the subsoil is not, however, due only to the 

 mechanically-carried particles, but also to the ingredients 

 carried in solution from the surface soil and redeposited in the 

 more retentive subsoils. Especially is this true of lime car- 

 bonate, which is dissolved by the carbonic acid formed chiefly 

 ii 161 



