October 29, 1920] 



SCIENCE 



399 



and taken fully into account before the arid 

 regions can develop to their full fruition. 



It is difficult to give exact figures, but it 

 seems probable that when all possible econo- 

 mies are put into operation the irrigated area 

 of the United States can be enlarged to about 

 four times the present area. It is largely 

 through the agronomist, assisted by the irri- 

 gation engineer, that this enlargement can be 

 brought about. 



After all possible sources of irrigation 

 water are fully utilized there will be many 

 millions of acres of arid land that can not be 

 served. The only possible chance for pro- 

 ducing crops on this land is through the 

 methods of dry-farming, which means that 

 every process is directed toward moisture con- 

 servation. 



Dry-farming is essentially a branch of 

 agronomy. It is based on a system of tillage 

 that will store in the soil the moisture of one 

 or two years till it is needed by crops. Its 

 success depends on the selection of crops that 

 can endure the rigors of drouth and the breed- 

 ing of special drouth-resistant varieties. 



Probably a larger area can be added to the 

 present productive land by the conquest of 

 drouth than by any other means, but drouth 

 is a relentless enemy of crop production and 

 its successful conquest will call for all the 

 ingenuity of students of soils and crops. Part 

 of the preliminary work has already been 

 done, so that one now sees grain fields where 

 only sagebrush was found a few years ago; 

 but there still remain many diificulties to be 

 overcome before all these vast areas can be 

 made to serve the needs of man. 



In humid sections great tracts of land are 

 covered with swamps and produce no im- 

 portant human food. When reclaimed these 

 lands are often exceedingly fertile. The 

 drainage of some of the larger swamps offers 

 rather serious engineering difficulties, but 

 these can in most cases be overcome. The 

 drained swamp with its peaty residue calls 

 for special methods of management and fer- 

 tilizing, but since agronomists are seeking 

 problems to solve they will not be discouraged 



by the difficulties encoTuitered in changing a 

 drained swamp into a fertile field. 



Somewhat related to the drainage of the 

 swamp comes the reclamation of alkali land 

 since it is largely through drainage that alkali 

 is overcome. 



In all arid parts of the world the soil is 

 likely to contain such an excess of soluble 

 salts that crops can not be raised. This con- 

 dition becomes more acute under irrigation. 

 At the present time in the United States 

 there are millions of acres of land that fail 

 to produce g'ood crops chiefly because they are 

 impregnated with salts. In some of the west- 

 ern states alkali is considered to offer one of 

 the most important and difficult problems 

 ailecting agTiculture. It will be met by 

 drainage, by s]>ecial soil treatment, by breed- 

 ing more resistant crops, and in other ways 

 that agronomists may devise. The problem 

 is now waiting; its solution will mean more 

 food for the world. 



Since 1840 when Liebig explained how 

 crops feed great progress has been made in 

 increasing the productivity of the soil. Be- 

 fore the role of mineral matter in the growth 

 of plants was understood, all sorts of theories 

 were advanced concerning the food used by 

 plants and as a result many inconsistent prac- 

 tises of fertilizing the soil grew up. 



When the real basis of plant nutrition was 

 determined, the beginning of a rational use 

 of fertilizers was at hand. This has resiilted 

 in increasing very materially the crop-yields 

 of many soils. 



Just how much the acre-yield can be in- 

 creased is uncertain, but we are sure that by 

 the proper use of fertilizers, by rotation and 

 by better tillage methods the present cidti- 

 vated area may be made to produce very much 

 more than it is now producing, but the acre- 

 yield can not be increased indefinitely. 



Last year in his presidential address before 

 this society, Dr. Lipman ably discussed the 

 nitrogen problem in its relation to increased 

 food production. Each element entering into 

 commercial fertilizers might have been dis- 

 cussed by him with equal interest, so many 

 are the problems surrounding the supplying 



