SCIENCE IN MAN’S STRUGGLE ON ARID LANDS 35 
where the full effects of climate and living matter are recorded in 
the basic morphology of the soil. 
Since World War I researches on the soils of arid regions have 
begun to throw light on their genesis, and considerable work has 
been done recently in parts of Australia, North Africa, the western 
part of the United States, and elsewhere. Important studies had 
started before that time in what is now the Soviet Union. 
Because of the unique environment, the characteristics of soils 
in arid regions differ importantly from those of other regions. At 
the recent Desert Symposium in Israel I summarized these dif- 
ferences and their implications (3). I should emphasize, however, 
that in the detailed appraisal of the potentialities of arid soils 
we must continually recall that individual properties, such as 
texture and structure, have a different significance from those of 
soils of humid regions where much of the scientific research about 
soils has been conducted. 
Detailed examinations of soil characteristics in field and labora- 
tory are especially critical to predictions about the use of arid 
soils for irrigated crops because the soils are being studied in an 
obviously different environment from the one in which they will 
be used. The bringing of large quantities of irrigation water onto 
the soil amounts to giving it artificially a humid climate. Soil 
characteristics of little or no significance to native plants are very 
important to the productivity and stability of soils under irriga- 
tion. These include the porosity, texture, mineralogy, salinity, and 
reaction of deep layers. 
The combinations of soil characteristics that are most nearly 
ideal for irrigation are not necessarily those best for continued 
production of native forage without irrigation. For example, 
nearly level, deeply pervious, salt-free loams of good structure are 
nearly ideal for irrigation. A sloping soil consisting of about equal 
amounts of fine soil material and basaltic rock fragments to several 
feet would be useless for crops under irrigation; yet with the con- 
centration of moisture around the roots it would probably give 
double the yield of uncultivated forage plants. 
Despite the great gaps in our knowledge we have learned a 
good deal about arid soils in recent years. We now have reasonably 
