28 



BOTANY: PRINCIPLES AND PROBLEMS 



larity is dependent chiefly on the size of the soil particles; for the 

 smaller the particles, the larger will be their surface in proportion 

 to the spaces between them (Fig. 15), and thus the higher will be 

 the rise of water (Fig. 16). In ordinary soil this rise varies 

 roughly from two to twenty feet. It is evident, therefore, that 

 water which has percolated very far below the surface ordinarily 

 cannot be made available to plants again through capillary ascent. 



Fig. 16. — Rise of water by capillarity. In the two glass tubes at the left, the 

 rise of water is much higher in the narrower than in the wider one. In the two 

 chambers at the right, which are filled with spheres, the rise of water is much 

 higher where the spheres are smaller. 



In most soils there is a capillary movement of water toward 

 the soil surface, where it evaporates. If the particles at the 

 surface are very close together, as they are where the soil has 

 been packed down or where a crust has been formed, a very 

 efficient capillary system is produced there which connects the 

 soil surface with the deeper water-holding layers, and thus greatly 

 hastens the loss of water by drawing it up to a point where it may 

 be evaporated (Fig. 17). An important purpose of tillage is to 

 prevent such waste of water by breaking up the capillary system 

 at the surface and forming there a layer of loose, coarse material 

 called a 7nulch. 



Capillary movement of water is by no means always vertical 

 but may take place in all directions within a soil, just as ink 

 spreads in all directions in a piece of blotting paper. This move- 



