66 SOILS AND MANURES 



increases. Finally the rate becomes very slow and the 

 quantity very small. 



The maximum height to which water will rise in clay 

 soils is about 4 feet. In very fine sandy soils the water 

 rises to nearly the same height and very much faster. The 

 presence of colloidal hydrates in clay tends to block the 

 passage of the water. In soils of coarser texture the water 

 rises faster, but riot so high, and the quantity raised to 

 any given height is much smaller. The presence of humus 

 in the soil greatly increases its capillary power. 



In estimating the importance of capillarity as a means 

 of bringing moisture to the upper soil, not only the charac- 

 ter of the material, but also the supply of underground 

 water must be taken into account. When the soil is 

 saturated at a depth not exceeding the range of its capil- 

 lary action, a large quantity may be raised to the surface 

 in this way. This condition is, however, of very rare occur- 

 rence, and it is probable that, under ordinary circum- 

 stances, the quantity of water raised to the surface is not 

 very great. Boots, however, penetrate into the soil, in 

 some cases to a considerable depth, and water may be brought 

 within their reach by capillary action, though it cannot 

 bo raised to the surface. 



Percolation. Under ordinary conditions the force of 

 surface tension is opposed to that of gravity, and the rate 

 of percolation is therefore diminished by the same cir- 

 cumstances that increase the power of retention, viz., the 

 smallness and closeness of the particles. When more 

 water than the soil can retain accumulates in the neigh- 

 bourhood of the drains, the excess passes into them and is 

 discharged. The rate of percolation is measured by the 

 quantity of water that drains from a saturated soil in a 

 given time. In gravels and coarse sands it is extremely 

 rapid and very little water is retained. The opposite 

 extreme is found in soils of finest texture in which the 



