160 



Physics of the Soil. 



in the table of (160), when the eight-foot columns were 

 completely full of water at the start, took place at a much 

 slower rate than from the sands, as indicated in (188), the 

 rates being 



The rates in these cases were such that more water per- 

 colated from the three coarsest sands during the first 30 

 minutes than from the clay loam in as many days ; and yet 

 the loam contained at the start the largest amount of water. 

 It is clear from these differences in the rate of percolation 

 why the sand could not be productive under ordinary con- 

 ditions of rainfall, no matter how much plant food it might 

 contain. It is clear also that fineness or closeness of tex- 

 ture is one of the most important qualities of a good soil, 

 for without this the water drains away so rapidly that, 

 with the ordinary intervals between rains, not enough could 

 be retained for the needs of crops. 



190, Percolation Through Dry Soil. When soils have be- 

 come relatively dry, as happens especially during the mid- 

 dle and later summer, water does not percolate into them 

 as readily as it does in the spring when the pores are more 

 nearly filled. When the volume of air in the soil is large, 

 and when the films of water surrounding the soil grains are 

 very thin, the fiow downward past the air is very slow. 

 It is on this account, in part, that the lighter rains are less 

 effective in midsummer than they are in the spring, the 

 water being retained close to the surface where it is quickly 

 lost by evaporation. 



