GRASSES FOR SPECIAL CONDITIONS 197 
DRY LANDS 
Some of the cultivated grasses are noted for drouth- 
resistance, and are of great value in regions of com- 
paratively light rainfall. The most notable of these is 
brome-grass. In the South, Johnson grass is of some 
value as a drouth-resister, as is also Bermuda grass. 
These have already been discussed. There are many 
wild grasses that thrive in regions where the rainfall 
is too slight for farming, and several million acres of 
such grasses are annually cut for hay inthe West. An 
enumeration of them would be out of place in a treatise 
on farm grasses. 
ALKALI SOILS 
All soils contain more or less soluble mineral mat- 
ter. Soil consists mostly of small rock particles com- 
posed chiefly of quartz, but containing small quantities 
of numerous substances. As these rock particles disin- 
tegrate under the action of moisture, air, heat, and cold, 
small portions of soluble substances are set free in the 
soil. In humid climates these substances are washed 
out and carried off by streams to the ocean. This is 
the origin of the saltness of sea-water. This process 
has been going on ever since rain began to fall on the 
hot crust of the newly formed earth, and the ocean has 
thus become the storehouse of vast quantities of solu- 
ble material formerly constituting part of the rock and 
soil of the earth's surface. : 
In arid climates, where the rainfall is insufficient to 
saturate the soil down to the water-table below, and 
\-here most of the rain that falls is evaporated from the 
