Alkali and Seepage Problems 97 



away from that particular locality. Such conditions 

 of natural under-drainage are, however, rare. On 

 the great majority of soils the irrigation water, 

 acting in conjunction with the natural rainfall, 

 naturally penetrates further into the soil than is 

 the case with the rainfall alone, and consequently 

 more salts are dissolved. Through the influence of 

 heat a considerable portion of this salt-impregnated 

 water is drawn to the surface of the ground, where 

 through evaporation the salt is left deposited. The 

 amount of salts thus brought to the surface of 

 the ground through the influence of evaporation 

 is therefore, under otherwise similar conditions, 

 greater on irrigated than on non-irrigated land. 



. On irrigated land that has not perfect under- 

 drainage there is, therefore, always a tendency for 

 such soluble salts as are contained in the land to 

 accumulate in the surface layers of the soil. Salts 

 that are the most detrimental to plant life, such as 

 salts of chlorine and magnesia, may not cause any 

 damage to vegetation if they are fairly evenly 

 distributed throughout five to six feet of soil ; but 

 when concentrated in the first foot of soil may be of 

 such strength as to make plant life impossible. 



Hence the object of good irrigation practice must 

 be to prevent the accumulation of injurious salts in 

 the top layers of the soil ; or, where that has already 

 taken place, to get rid of the salts there accumu- 

 lated. 



DRAINAGE. 



As has previously been mentioned, accumula- 

 tion of surface salts does not take place on irrigated 

 land that has perfect under-drainage. Therefore 

 where the natural drainage of a piece of land is not 

 good, artificial drainage has to be reverted to if salt 

 trouble is to be avoided, or where that has already 



