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in the soil in a soluble form. The accumulation of the other 

 salts in solution may go on until the proportion of soluble salts 

 in the soil reaches the danger point. Herein lies the danger 

 of irrigating with well water or water from low cesspools 

 or canals which contains a high proportion of undesirable 

 solids in solution. There is another side of the question. Some 

 soils contain a high proportion of these undesirable salts in 

 a soluble state, and when to such soils water surcharged with 

 the same salts is applied, the proportion readily reaches the 

 danger point. Soils containing a large excess of these salts 

 are usar, i.e., altogether barren and unfit for cropping, but 

 soils not containing such excess but only a high proportion, 

 may be rendered usar by injudicious irrigation. What the 

 proportion of solids in solution in a soil is, and what in the 

 water meant for irrigation, may be determined, and roughly 

 speaking half the quantity in each case may be assumed to 

 -be made up of undesirable salts. From these data deduction 

 may be arrived at as to the suitability of the particular, water 

 for the particular soil, remembering always the principle that 

 plants require to be supplied with food at a dilution of about 

 i part in 1,000 parts of water. 



205. Evaporation. The question of evaporation then 

 comes in, which is further complicated by the fact that eva- 

 poration is much slower from land under crop than from bare 

 land, and is different at different seasons, and the whole 

 question, of evaporation is of minor importance when one takes 

 into consideration the loss by percolation in certain soils. But 

 leaving all side issues out of consideration, and assuming that a 

 tank 30 ft. deep loses by evaporation 15 ft. of water in course 

 of the year, it would be obviously an advantage to have 

 irrigation from such a deep tank than from one, say, 20 ft. 

 or 18 ft. deep. Just as the 30 ft. tank would lose by evapora- 

 tion 15 ft., so would the 20 ft. or 18 ft. tank. Now the 

 remaining quantity of the water in the tank would be more 

 or less rich in solids, and the residual 5 ft. or 3 ft. of water 



