g6 Fruitgrowing under Irrigation 



ages been gradually formed by the decay and 

 weathering of the earth's surface rocks and soil 

 particles, many of which contain traces of the 

 elements of chlorine and magnesium. In regions 

 where there is a heavy rainfall these salts are dis- 

 solved by the water in soaking through the soil, 

 and are carried with them into streams and rivers, 

 which in their turn deposit them into the sea. 



In countries of low rainfall, however, the case is 

 different, as the rain is very seldom sufficient to 

 penetrate the soil to any great depth and to carry 

 the soluble salts with it into streams that would 

 ultimately discharge them into the sea. Con- 

 sequently in arid regions, through the gradual 

 weathering of the surface rocks and the chemical 

 changes therefrom resulting, salts of various kinds 

 tend slowly to increase in quantity; or where 

 chemical activity has come to a standstill the salts 

 that have been formed will remain in the soil. 



Such rains as fall in arid regions, in penetrating 

 the soil, dissolve some of these salts, and carry them 

 downward. Through the influence of evaporation, 

 however, caused by the intense heat usual to such 

 regions especially during the summer the salt- 

 impregnated water returns to the surface of the 

 ground, where the water evaporates and the salts 

 remain as a residue. Subsequent rains will again 

 tend to wash these salts downwards, and evapora- 

 tion again draw them upwards, there thus being a 

 constant movement downwards and upwards of the 

 soluble salts in the soil. 



When irrigation is practised upon the soils of 

 such regions, similar phenomena take place, with 

 the exception of land resting upon an extensive bed 

 of sand or gravel, which, having perfect under- 

 drainage, allows for a considerable amount of the 

 water and such salts as are dissolved in it to pass 



