352 



FORESTRY AND IRRIGATION 



August 



be used in irrigation, and where the 

 black alkali remains in the soil, to apply 

 the gypsum to the soil direct. 



Sodium carbonate, or black alkali 

 water, in contact with air, which always 

 contains carbonic acid gas, undergoes a 

 change, and is in part converted into 

 sodium bicarbonate by the absorption of 

 the gas. In this way the alkaline 

 character of water is sometimes much 

 arueliorated, for sodium bicarbonate is 

 neutral in reaction, and from all avail- 

 able information is no more harmful 

 than the other white alkali salts. In 

 equilibrium with atmospheric air, there 

 is a definite relation between the rela- 

 tive amount of carbonate and bicarbo- 

 nate found, the percentage of the latter 

 increasing with the amount of carbonic 

 acid gas present. Thus when water 

 containing sodium carbonate is applied 

 to a soil, a certain definite portion is 

 converted into the bicarbonate, a greater 

 portion than would be converted in 

 normal air, for carbonic acid gas is more 

 abundant in soil air than in the air above 

 the soil. 



Whenever water containing sodium 

 carbonate is used, every facility for 

 under-drainage should be supplied to 

 remove the excess of the salt, and should 

 the black alkali begin to accumulate to 

 the detriment of the growing crops, 

 gypsum in a powdered form should be 

 applied to the soil. 



The following table shows the amount 

 of sodium carbonate added to the soil 

 in an acre foot of water containing a 

 given amount of that salt, and the num- 

 ber of pounds of gypsum needed to 

 neutralize all of the sodium carbonate. 

 Part of the sodium carbonate may be 

 changed to the bicarbonate and another 

 part may be drained away, so that the 

 figures for gypsum represent the maxi- 

 mum amount ever required, and in prac- 

 tice some smaller amount may serve. 



carbonate 

 per 100,000. 



Parts sodiuii Pounds sodium Pounds of gypsum 



carbonate in needed to 



i acre foot neutralize the 



of water. sodium carbonate. 



. 5 136 221 



10 272 441 



15 408 662 



20 540 880 



25 680 1,100 



30 820 1,320 



35 960 1,540 



40 1,090 1,760 



The cost of gypsum varies so much 

 that it is difficult to give the cost of the 

 above applications. Ground gypsum or 

 land plaster for agricultural uses should 

 not cost much more than $2 per ton. 

 The average price varies from $2 to $3 

 per ton. 



At these prices, if it were necessary to 

 add the maximum amounts given above, 

 the use of water of higher concentra- 

 tion would be very expensive ; but it is 

 thought that it will never be necessary 

 to add but a third or half of this amount 

 in practice, and less if the under-drain- 

 age is good. 



Sodium bicarbonate is very frequently 

 found in irrigation waters. In itself it 

 is no more harmful than the other neu- 

 tral salts. Evaporation of the water 

 containing it, causes a portion of the 

 bicarbonate to revert to the carbonate, 

 and therefore waters containing sodium 

 bicarbonate should be held open to sus- 

 picion, just as are waters containing the 

 normal sodium carbonate. Where the 

 water contains less than 100 parts so- 

 dium bicarbonate per 100,000, the 

 amount of sodium carbonate formed 

 under natural conditions will never 

 amount to more than 20 per cent of the 

 whole. The danger is in allowing such 

 water to concentrate on the field, for the 

 percentage of sodium carbonate possible 

 increases with the concentration, and in 

 water containing 300 parts, as much as 

 40 per cent of it may revert to the car- 

 bonate. For this reason the dangers 

 from surface evaporation of waters con- 

 taining sodium carbonate and bicarbo- 

 nate are greater than in other waters, 

 for beside the actual concentration, 

 which in itself is harmful, there is 

 brought about by the concentration the 

 dangerous change to black alkali. Sur- 

 face cultivation and under-drainage are 

 therefore all the more desirable. So far 

 as our information goes, water contain- 

 ing more than 50 parts of sodium carbo- 

 nate per 100,000 should not be used per- 

 manently for irrigation purposes. If the 

 land to be irrigated contains gypsum, as 

 arid lands usually do, a larger per cent 

 of sodium carbonate is allowable ; and, 

 again, if gypsum be easily available for 

 agricultural purposes, or is on sale at a 

 reasonable price that is, not exceeding 



