200 Nebraska Agricultural Exp. Station, Research Bui. 6. 
PART III. 
APPLICATION. 
The problem of conserving the water loss from the soil other 
than used by the plant does not concern us in this paper. It may 
be said, however, that because of the nature of the soil and the 
cultural treatments given it, much can be done toward conserving 
water for plant use (although this has been greatly over-emphasized 
— except the weed factor.) This is very important, since an abun- 
dance of soil moisture is so essential for crop production in all 
regions having only a moderately humid atmosphere. Much of 
the investigation and speculation which has been done concerning 
the economy of water in dry farming and irrigation has taken into 
consideration the relation of available water to crop yields, with- 
out determining the actual amount evaporated by the plant. 
It is possible that some means can be devised whereby the 
actual water transpired by crops under given conditions will be 
reduced. There are two chief points of attack, namely, adjusting 
the external factors to the needs of the plant and, second, adapting 
the plant to the conditions. 
The growth conditions of a cultivated crop may be either 
favorable or unfavorable for yields, with many intermediate 
degrees. Those factors which are most variable in any agricul- 
tural district are soil fertility, water supply, physical condition of 
the soil, and climate. There is also much variation in the plant 
characteristics of different crops and varieties. Each one of 
these may be a critical factor in crop production. A control 
of some one or more of these variable factors would naturally 
appear to give the greatest promise of controlling transpiration. 
The problem is somewhat different for irrigated and nonirri- 
gated conditions. Under irrigation the total amount of water 
may be adjusted to the requirements of the plant. Where the 
total available water supply is fixed by nature, no irrigation being 
possible, the problem is to produce the maximum crop (within 
profitable limits) on stated quantities of water. 
The optimum condition for any one growth factor is to be 
regarded as that which is associated with the highest yield. As 
any one important condition becomes less favorable, others being 
equal, the yield is reduced. In the case of many factors there is 
a rather wide range of optimum condition. 
SOIL FERTILITY. 
A broad interpretation of our present knowledge would be 
that for any given locality those soil conditions relating to 
available fertility which are conducive for optimum growth 
