AMELIORATION OF LAND 85 



more distant parts of the area, where the 

 plants, being less abundantly supplied with 

 nutritive materials, grow less vigorously. 



In tropical countries, on the other hand, 

 irrigation is undertaken chiefly for the purpose 

 of supplying plants with the necessary water. 

 This being the object, water under these 

 circumstances may contain little if any 

 matter in solution or suspension ; and, as a 

 matter of fact, much of the water used for 

 irrigation in India, Australia, and other 

 tropical and semi-tropical countries, is pure 

 rain-water that has been collected and stored 

 in suitable ponds or " tanks," from which it 

 can be drawn for use during the dry season. 



The theory of irrigation may be shortly 

 stated as follows. The water necessary for 

 plant growth is supplied and, as has been 

 mentioned, this is the main object in dry 

 countries. Irrigation also regulates the 

 temperature of soil to some extent, tending to 

 make it cooler during summer and warmer 

 during winter. The presence of abundant 

 water in the soil enables certain chemical 

 processes to go on more rapidly than would 

 otherwise be the case, while the weathering 

 of stones is to some extent accelerated. 

 Irrigation also provides a means of clearing 

 the ground of certain noxious insects, and of 



