197 



In water analysis the residue or solid matter either held in 

 suspension or in solution is expressed in so many grains per gallon, 

 or so many parts in 10,000 parts. We have seen in a previous 

 chapter when dealing with manures that the weight of one acre of 

 agricultural soil one foot deep is approximately 3,500,0001bs. It 

 has, moreover, been determined by chemical investigation that a 

 soil containing '1 per cent, of soluble salt is unsuitable for cropping, 

 and is only fit for growing salt-loving plants. 



The question, therefore, which the irrigationist has to solve is : 

 What amount of water containing a given quantity of salt will be 

 necessary to cause the accumulation into the ground of approxi- 

 mately '1 per cent, of salt (equivalent to 3,5001bs. to one acre one 

 foot deep). 



That question, indeed, can only be approximate, as a certain 

 proportion of that saline matter would be leached out of the ground 

 during the rainy season ; sandy soil could, besides stand more salt 

 than the more retentive soils, such as loam and clay, and drainage 

 would also delay to some extent the period when that dangerous 

 point of '1 per cent, of salt would be reached. When calculating 

 the amount of salt added to the soil by water containing a known 

 number of grains of that substance to the gallon, it must be borne 

 in mind that one gallon contains 7,000 grains. As 1 inch of water 

 over 1 acre is equivalent to 22,630 gallons (101 tons) it follows 

 that every grain of salt per gallon adds VoVo* Sjlbs. of salt to 

 the ground whenever that amount of water is used in irrigation. 



It would be unsafe to use extensively for the purpose of 

 irrigation a well or pond water containing more than 70 grains of 

 salt to the gallon. Such an amount would mean an addition of 

 about 2cwt. (22 7 Jibs.) of salt to the acre, for each one-inch 

 watering. Such a water used in the more arid regions, where rain- 

 fall is light, would soon accumulate in the surface of the ground an 

 amount of salt which would prove injurious to vegetation, and 10 

 irrigations of one inch each would add to the soil over a ton of 

 salt, which, added to the amount of that substance already in the 

 ground, would make it sterile. 



In the coastal districts, where the rainfall is more abundant, a 

 water containing 30 grains of salt to the gallon could be used 

 pretty freely for irrigation, provided the soil is of a light and porous 

 nature and is well drained. But if used on stiffer soil it would, in 

 in the course of two or three years, bring the amount of salt up to 

 danger point. 



A water containing 3 to 6 grains of common salt to the gallon 

 is often used for all domestic purposes. 



FACTORS WHICH INFLUENCE IRRIGATION. 



No cast-iron rule can be laid down regarding the amount of 

 water necessary for obtaining the best results from a given crop. 



