274 ROCKS AND SOIL 



voirs at the time there is an abundance of it, and then let 

 it run off slowly during the dry period. Where natural 

 streams are wanting, irrigation canals carry the water from 

 the reservoirs to the farms. The reservoir is made by the 

 building of a dam across the outlet of a lake, or at some 

 suitable place in a stream. The water flowing over the 

 dam forms an artificial waterfall, which is often used for 

 power (cf. 162). 



There are several methods of irrigation. One is to flood the land to 

 a certain depth, and then to allow the water to sink into the soil. The 

 flooding is repeated at intervals. Another way is to build pipes having 

 openings that throw sprays of water over the land; this is like rain. A 

 third way is to have furrows for the water, and to let it soak sidewise 

 into the growing area. 



For that form of irrigation which we call " watering the garden" 

 the following suggestion is excellent: "The hose should be used in a 

 manner to resemble rain. Give the garden a good soaking, and then 

 let it alone until your daily observation shows you that it has dried 

 out to a depth of from two to four inches from the top. Then repeat 

 the watering. The practice of frequent, shallow watering is wrong, 

 as it induces shallow rooting; when the watering is neglected, the roots 

 near the surface die, and the plants suffer. If a garden is properly 

 watered, the roots will go down some distance; if watering is too shal- 

 low, they will run along horizontally, just under the top of the ground. ' ' 



298. Fertility. The fertility of a soil is its crop-pro- 

 ducing power. This power depends upon the elements 

 in the soil, upon the physical condition of the soil, upon 

 the water supply, and upon other conditions. None of the 

 conditions named can make up for an unfavorable climate. 

 A soil that has all the conditions for growing corn in Iowa 

 may be barren, so far as corn is concerned, in Labrador. 



It seems likely that at least 10 chemical elements are 



