SCIENCE IN MAN’S STRUGGLE ON ARID LANDS Sy/ 
All these new results together make us realize that a large per- 
centage of the water now being delivered to irrigated lands is 
wasted, not counting losses through evaporation from reservoir 
pools and from poorly engineered canals. In other words, farmers 
could get a great deal more out of the water they are using, cer- 
tainly twice as much; and I think they will do better after the 
educational work catches up more nearly with the research results. 
As a concluding remark about irrigated soils, I should like to 
emphasize the great importance of maintaining high levels of plant 
nutrients. Very few irrigated soils give good yields of crops with- 
out abundant fertilization. Most of them need additional nitrogen 
since arid soils are low in organic matter; many need additional 
phosphorus; some need potassium fertilizer, especially after long 
cropping; and many need one or more of the minor nutrients. 
Since early times the horticulturist on irrigated land has been 
plagued with chlorosis, the loss of normal green color in the leaves 
of plants. We know that much of this trouble is due to deficiencies 
of zinc, iron, and other minor nutrients. Only within the last three 
or four years have the new chelates furnished an effective iron 
fertilizer that may go a long way toward correcting this trouble- 
some deficiency. 
Since the addition and control of water are expensive, econom1- 
cal production depends upon good husbandry in all other phases 
of the farming enterprise. 
Plant Distribution 
Plants may be assumed to be in themselves indicators of the 
combined effects of the growth factors in the spots where they are 
grown. Even on uncultivated soils, however, the influence of the 
type and condition of the vegetation as modified by use must be 
evaluated along with the basic soil and climate. 
Most plant scientists had formerly assumed soil (or the “edaphic 
factor”) to be one of the fixed features of the landscape along with 
climate and relief. Actually the plants themselves have a great 
deal to do with the soil under them. In arid lands, for example, 
some plants are salt collectors much more than others (6). Large 
differences exist in the physical and chemical properties of soils 
