1 904 



FORESTRY AND IRRIGATION 



that quite a margin existed between 

 the limits set by the two authorities. 

 Since then, in 1903, the writer pub- 

 lished a short account of the use of 

 saline waters in Algeria, where water 

 with as high as 816 parts soluble matter 

 was in successful use. 



The interest in this subject warrants 

 a more extended statement of the sub- 

 ject than has been made. Inquiries 

 have been received from many quarters 

 for more explicit information, and to an- 

 swer these this article has been written. 



In considering the fitness of a water 

 for irrigation purposes there are a num- 

 ber of factors to be taken into consid- 

 eration besides the amount and chem- 

 ical character of the salts which it holds 

 in solution. The most important of 

 these factors are: the soil to be irrigated, 

 its texture, depth, and amount of alkali 

 salts present in it ; the abundance of 

 the water for irrigation, and the char- 

 acter of crops to be raised. 



Certain salts, such as those of iron or 

 copper, are very poisonous to plants, 

 and very small quantities in water are 

 sufficient to condemn such a water as 

 dangerous or unfit for irrigation. For- 

 tunately, the occurrence of such waters 

 is rare, but waters pumped from mines 

 or from waste, from mills and smelting 

 works, is sometimes dangerous, and 

 where trouble is experienced with water 

 in mining districts where mills or mines 

 are upstream, then metals should be 

 looked for in the water. 



LIST OF SALTS. 



The salts most commonly found, 

 however, are sodium chloride, or com- 

 mon salt; sodium sulphate, or Glau- 

 bers salt; sodium bicarbonate, or bak- 

 ing soda; sodium carbonate, or washing 

 soda; magnesium sulphate, or Epsom 

 salts; magnesium chloride, or bittern; 

 calcium carbonate, or limestone, and 

 calcium sulphate, or gypsum. This, 

 though apparently a formidable list of 

 salts, is much simplified when consid- 

 ered as regards the effect upon plant 

 growth. The two salts of calcium are 

 beneficial to plant growth, and are so 

 slightly soluble that they can not exist 

 in a water in harmful quantity, so need 

 not be considered in an analysis. The 

 remainder of the salts are divided into 



two groups black alkali, or sodium 

 carbonate, and white alkali, or the 

 other salts. 



Of all the salts the sodium carbonate, 

 or black alkali, is by far the most harm- 

 ful. In ordinary irrigation practice 

 plants will stand at least five times as 

 much white alkali as black, and in irriga- 

 tion waters the same relation will hold. 



Of the white alkali salts, sodium 

 chloride is generally the most destruc- 

 tive, its effect being approximately 

 twice as harmful to vegetation as the 

 rest of the salts. Of the other salts 

 little can be said, except that they are 

 the least harmful of those ordinarily 

 found in irrigation waters, and that 

 there is very little difference in their 

 relative poisonous effect on plants. 



Sodium carbonate has two effects on 

 growing plants in the soil first, the 

 ordinary effect of any material in solu- 

 tion, whether that be an osmotic effect 

 upon the cells of the roots (or whether 

 it be due to the action of irons on these 

 cells is a question still in dispute), and, 

 second, a caustic action upon the vege- 

 table tissues of the growing plant. This 

 caustic action corrodes and dissolves the 

 delicate portions of the plant below 

 ground, and frequently immediately at 

 the surface girdles the plant. By the 

 application of gypsum or land plaster to 

 a soil containing black alkali this caustic 

 nature is neutralized and a chemical re- 

 action takes place between the gypsum 

 and sodium carbonate, forming one of 

 the less harmful white alkali salts, 

 sodium sulphate, and a relatively in- 

 soluble compound, calcium carbonate. 

 This reaction can be utilized in the treat- 

 ment of soils containing black alkali, 

 but the high price of gypsum has not 

 permitted this to be carried on to any 

 great extent. Ordinarily under-drain- 

 age is cheaper. The same reaction can 

 be utilized where waters containing 

 sodium carbonate are used. 



It has been recommended that water 

 containing sodium carbonate in solution 

 should be run through cribs filled with 

 crushed gypsum. The reaction is, how- 

 ever, exceedingly slow. It would gener- 

 ally require several hours or days' con- 

 tact to thoroughly neutralize the alka- 

 linity. For this reason it is generally 

 better, where black alkali waters are to 



