It is well known that certain mineral compounds, when they accumulate in the soil, are extremely harmful to vegetation. Common salt is one of those substances; carbonate of and sulphate of soda are also of this kind. On the other hand carbonates of lime and magnesia and the sulphate of lime are not, as a rule, harmful, but often beneficial. When a water supply is to be used for irrigation purposes, it is of the utmost consequence that a chemist be called in to determine whether or not the water has enough of objectionable mineral constituents to render it unsuitable for the proposed purposes. In this connection it must be borne in mind that the salts may accumulate, and do accumulate, in soils which are subjected to irrigation. Thus, if the water at first does not have a sufficien quantity of these objectionable salts to render it injurious, the accumulation of these salts in the soil in a few years may render it unfruitful. In such cases artificial drainage coupled with excess of water applied between crops will preserve the soil from injury.

Soap may also be used for softening hard water. To determine how much of the soap is necessary a solution of soap is made of a definite strength, usually in the proportion of 20 grains to a tablespoonful of rain or distilled water. This solution may be made in a larger quantity and a tablespoonful of it added to a pint of the water whose hardness is to be tested. After thorough shaking the water is set aside, and the foamy lather which has resulted should be permanent for at least five minutes. When the correct quantity of soap solution is determined by experiments of this kind, then the required quantity of soap can be added to large quantities of water.

For ordinary hard waters, where facilities for making the foregoing tests are not at hand, or the skill to make them is lacking, very good results may be obtained usually by assuming a certain average degree of hardness and proceeding as follows: Five hundred gallons of water are treated with a solution of five pounds of carbonate of soda in two or three gallons of water. Numerous so-called boiler compounds are offered for sale, but they consist essentially of some one of the ingredients just described.

In many cases water is sufficiently softened for bathing purposes by the addition of a proper quantity of borax.