82 



THE TEXTURE OF THE SOIL 



[chap. 



while the upper layers hold an amount dependent on 

 their distance from the saturated zone, and on the 

 extent of surface exposed by the particles. 



The accompanying diagram (Fig. 7) expresses the 

 results of an experiment of King's, where columns of 

 sand and soil, 8 feet and 7 feet long respectively, were 

 saturated and then allowed to drain till they parted 

 with no further water, which required a period of sixty 

 days in the case of the soil columns, and of more 

 than two years for the sand. The tubes were then 

 cut up, and the proportion of water in the sand or 

 soil in successive 3-inch lengths of the tubes was 

 determined. 



It will be seen that the sands retain very little water 

 by surface tension in the upper layers, whereas the clay 

 loam, with the enormous area its particles expose, holds 

 practically the same proportion throughout. 



If we also consider the following table, showing the 

 time taken by the same sands and soils to part with 

 their water, the difference of the texture of the soils will 

 be even more evident : 



The downward movement of rain water through 

 soils is known as " percolation," and is distinguished from 

 " flow " by the fact that the water is supposed to have 

 free surfaces, so that surface tension comes into play. 



