36 THE FUTURE OF ARID LANDS 
good soil survey methods that have been tried out successfully 
in several countries (7, 8). We have learned to sample soils for 
laboratory work in relation to genetic soil horizons and geological 
layers and with regard to the marked influence upon soils only a 
few inches apart of both the kind of plants above them and of 
small differences in water relations. These improved techniques 
have made it possible to make the predictions from soil surveys 
more quantitative and to extend greatly the use of data from 
laboratories and small field plots. 
Reliable methods are now available for classifying soils accord- 
ing to defined kinds of soil that can be evaluated quantitatively 
as to productivity, both without irrigation and with different 
systems of management under irrigation. 
Laboratory methods are now available for characterizing the 
chemical properties and moisture relations of soil samples with 
an accuracy that permits us to predict changes in the physical 
and chemical properties of soils with irrigation (10). These new 
technical tools, developed mainly during the last fifteen years, 
make possible far better decisions between arable and nonarable 
soils and between different systems of soil-water management 
for specific crops. 
The results of modern researches make it possible to determine 
with reasonable precision the amount of moisture required at 
different depths for the best growth of important crops. We now 
have practical devices for measuring this moisture in the soil. 
The tolerance of various kinds of plants for different degrees of 
saltiness and alkalinity, under different sets of soil conditions, is 
becoming known with some precision. We have even, perhaps, the 
possibility of using these techniques for identifying more efficient 
plants to use as breeding stock. 
Suitable instruments for measuring soil moisture and for the 
control of irrigation water now make possible well-controlled field 
experiments for studying, on different kinds of soil, the interac- 
tions among moisture levels, fertility levels of the several plant 
nutrients, and plant spacing in order to give the data required 
for calculating the most economical combinations of inputs with 
variable costs for the individual items. 
