PLANT INDUSTRIES 323 
a crop. It now seems probable that certain drought-resisting 
plants, as some kinds of wheat, will make it possible to use soils 
which have not been usable because of inadequate moisture. 
In regions where available water supplies exist, irrigation is 
practicable. Between 15,000,000 and 20,000,000 acres of land 
are said to be under irrigation in the United States, but this 
area is small compared with the large extent of our so-called 
desert lands, which only need water and proper cultivation to 
make them highly productive. 
304. Effect of living things in the soil. Microscopic plants 
and animals of many kinds and in great numbers live upon 
one another, upon plant roots, or upon dead organic matter in 
the soil. The roots of living plants, the molds, and the burrow- 
ing animals, such as the larve of insects and the earthworm, 
constantly take from, add to, or otherwise change the soil. 
Earthworms eat their way through it, and as they do so they 
make it more porous and excrete materials that add to the 
soil’s available organic matter. Certain groups of soil bacteria 
have already been discussed (sect. 191). The living things of 
the soil may be said to constitute an extensive and intricate 
group of plants and animals living close together and greatly 
affecting the nature of the material in which they live. Some 
of the products of the soil inhabitants are helpful to agricultural 
plants and some are harmful. 
305. Quality of soil and growth of plants. A comparison of 
plants of the same kind that have grown in different regions 
readily shows that soils differ widely in their ability to sup- 
port vegetation. Even the different parts of the root system 
of one plant illustrate this fact (fig. 238). Soils that are at 
one time fertile may lose that fertility, as may be seen in any 
farming region. Many studies are being made, to determine how 
fertility is lost and what will restore it, and while the question 
1‘ Dry Farming in Relation to Rainfall and Evaporation,” Bulletin 188, 
Bureau of Plant Industry, U.S. Dept. Agr., 1913. 
2 Native wheat which is thought to be primitive has been found growing 
wild in arid districts in Palestine. 
8 See Marshall, Microbiology, P. Blakiston’s Son & Co., Philadelphia. 
