THE SOIL STRUCTURE 99 
and smoothing harrows are run along the hillsides 
for the same purpose and with results that are nearly 
always advantageous to the dry-farmer. Of neces- 
sity, each man must study his own farm in order to 
devise methods that will prevent the run-off. 
The structure of soils 
Before examining more closely the possibility of 
storing water in soils a brief review of the structure 
of soils is desirable. As previously explained, soil 
is essentially a mixture of disintegrated rock and 
the decomposing remains of plants. The rock par- 
ticles which constitute the major portion of soils 
vary greatly in size. The largest ones are often 500 
times the sizes of the smallest. The following table 
shows the limits of sizes and the names used to 
designate them : — 
NAMES AND S1zEs oF Sort ParticiEs 
NAME Aes pce =e NuMBER IN OnE Cusio [now 
Sand 0.5 —0.03 50-833 125,000-578,009,537 
Silt 0.03-0.001 833-25,000 578,009,537- 
15,625,000,000,000 
Clay Below 0.001 More than More than : 
25,000 15,625,000,000,000 
It will be observed that it would take 50 of the 
coarsest sand particles, and 25,000 of the finest. silt 
