244 SOIL PHYSICS AND MANAGEMENT 



per cent. As a general rule, the surface soils are dry in arid regions, 

 and this prevents, in a measure at least, a large loss of water, since 

 the movement oi' water through dry soil is very slow. Very little 

 evaporation takes place within the interstices of the soil itself, as 

 has been shown by the table on page 23'3. Buckingham has shown 

 that the amount of water lost by transfer upward along with the 

 air in the process of aeration amounts to no more than one inch 

 in six years. It is true, however, that coarse soils lose a larger 

 amount in this way than fine-grained ones, but the loss in either 

 case may be neglected. 



(d) Transpiration. All plants take water through the root 

 hairs and a very large part of it is transpired through the leaves. 

 The amount of water used in this way constitutes practically all that 

 is taken up by the plant except that used in building up tissues, 

 which generally amounts to only a small fraction of the total 

 amount. Transpiration varies with certain conditions, both of 

 weather and soil, and in general the factors that affect evaporation 

 from the soil affect transpiration from the plant (see page 188). 

 This applies to plants growing in humid regions as well as under 

 arid conditions. Transpiration varies inversely as the relative 

 humidity, directly with temperature, with wind velocity and direct 

 sunshine ; but it is decreased by a large amount of plant food mate- 

 rial dissolved in the soil moisture. Arid conditions are especially 

 favorable for transpiration. 



It must be remembered also that weeds, like useful plants, 

 transpire large amounts of water and may be one of the greatest 

 sources of loss unless the soil is kept free from them. Weeds have 

 no place on any farm, but more especially on a dry-land farm. 

 Eotmistrov 4 says, " Weeds are the bitterest enemy of field culture 

 and the best friend of drought." 



METHOD O-F PREVENTING LOSS OF WATER 



In dry-farm practice every means must be used for preventing 

 loss of moisture. Other crop factors sink into insignificance in com- 

 parison with this one. The moisture must be sufficient not only to 

 start the crop, but there must be enough stored in the soil to mature 

 it. The farmer knows that every pound of moisture taken from 

 the soil that does not go through the crop will lessen the yield. 



The loss of moisture by evaporation is prevented to some extent 

 by the crop itself. After the crop, becomes large enough to shade 

 the ground evaporation is greatly retarded. This is especially true 



