PLANT INDUSTRIES 323 



a crop. 1 It now seems probable that certain drought-resisting 

 plants, as some kinds of wheat, 2 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. 3 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 larvae 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 then- 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. 



