agriculture, We thus understand how rain-water should be- 

 nefit the crops in more ways than one, and at the early season 

 more than at a later season. 



204. Well and canal water. But why should not spring, 

 well or canal water do more good than rain-water ? The former 

 contains more substances in solution than rain-water, not 

 only more CO 2 gas, but also saline substances of various kinds, 

 most of which are actually required for the growth of plants. 

 The danger in using irrigation water lies, not in the fact of 

 the possibility of this water being too poor in soluble sub- 

 stances, but of its being too rich in such substances. Spring 

 or well-water may look purer than river water, but the 

 latter may contain only about one part or less of solids in 

 solution in every 1000 parts, while the former may contain 

 as much as 2 or 3 parts in a hundred. The water of a low 

 and dirty pool may look very clean, but it contains a high 

 proportion of solids in solution. We have said before that 

 plants can take up nourishment only in a very dilute solution, 

 the dilution best adapted for nourishment of plants generally 

 being one part of solid food in solution in 1000 parts of water. 

 5 parts in 1000 may be taken as the extreme limit of endur- 

 ance for plants, while 2*5 to 3 parts of solids in solution in 

 1000 parts of water indicate the danger point, i.e., the degree 

 of solution at which results of the use of water becomes uncer- 

 tain, specially for leguminous crops and seedlings. The salts 

 in solution may be one or more of the following: Sodium 

 chloride, sodium sulphate, magnesium sulphate, calcium 

 chloride, magnesium chloride, sodium chloride, sodium 

 bicarbonate, calcium carbonate, calcium sulphate, and some 

 silicate, iron, and alumina compounds, also some nitrates and 

 borates. Of these salts, the calcium carbonate, calcium sul- 

 phate, silicon, iron and alumina compounds, do no harm when 

 they are present in large proportions in irrigation water, as 

 upon the evaporation of the water after it has been applied 

 on the land, these compounds crystallize out and donot collect 

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