68 ELEMENTARY BIOLOGY 



half the land area, outside of mountain and rock, which cannot 

 be cultivated, consists either of swamp land or of desert land. 

 Soil that is too wet is just as useless for farming as soil that 

 is too dry. But through the cooperation of farmers and engi- 

 neers and workers of all kinds it has been possible to reclaim 

 millions of acres of swamp land and millions of acres of desert 

 land, and to make it all usable for raising valuable crops. By 

 draining the swamps and by bringing water to the arid regions, 

 through miles of canals and ditches and pipes, soil containing 

 vast amounts of food-making salts has been added to the 

 national wealth. There is, of course, a limit to what man 

 may be able to accomplish in the way of reclaiming land, — for 

 example, in some of the Western dry regions the bringing of 

 water may not be practicable if the distance is too great. But 

 when we consider that at the present time more than half of 

 the great staple food crops of the world are raised on land 

 that is artificially irrigated, — in China, India, Egypt, Canada, 

 and other countries, — we can see that the possibilities in this 

 direction will probably not be exhausted in many generations, 

 99. Soil waste. The fertility of the Nile valley seems to 

 be inexhaustible. This is not due to the higher concentration 

 of usable salts in this soil than is found elsewhere ; indeed, 

 if the mineral matter were too highly concentrated, the plants 

 could not grow, as we can see when we try to water garden 

 plants with sea water. The richness of this soil is due to the 

 fact that the river is constantly bringing down into the valley 

 more and more material from the rocks in the mountains 

 where the river has its sources. In our own country every river 

 that empties into the sea carries away tons of usable minerals 

 which thus go to waste. In connection with some of the irriga- 

 tion projects in the Southwest, much water is lost during the 

 spring, and with the water a great quantity of valuable mineral 

 salts. Plans are being developed for saving this water in huge 

 reservoirs, some of which are already completed. In this way 

 it will be possible not only to irrigate larger areas but also to 



