102 DRY-FARMING 
are some arid soils, notably gypsum soils, the par- 
ticles of which are so uniform in size that the pore- 
space is exceedingly small. Such soils are always 
difficult to prepare for agricultural purposes. 
It is the pore-space in soils that permits the stor- 
age of soil-moisture; and it is always important 
for the farmer so to maintain his soil that thé pore- 
space is large enough to give him the best results, 
not only for the storage of moisture, but for the 
growth and development of roots, and for the en- 
trance into the soil of air, germ life, and other forces 
that aid in making the soil fit for the habitation of 
plants. This can always be best accomplished, as 
will be shown hereafter, by deep plowing, when the 
soil is not too wet, the exposure of the plowed soil 
to the elements, the frequent cultivation of the soil 
through the growing season, and the admixture of 
organic matter. The natural soil structure at 
depths not reached by the plow evidently cannot be 
vitally changed by the farmer. 
Hygroscopic soil-water 
Under normal conditions, a certain amount of 
water isalways found in all things occurring naturally, 
soils included. Clinging to every tree, stone, or ani- 
mal tissue is a small quantity of moisture varying 
with the temperature, the amount of water in the 
air, and with other well-known factors. It is impos- 
