Gypsum, therefore, although it contains two necessary plant-constituents, lime and sulphuric acid, cannot be regarded as a direct manure; and as its action comes to be more fully understood, its use, which was never very abundant in this country, will probably decrease. We have already, in the chapter on Nitrification, referred to the action of gypsum in promoting nitrification. SALT. The action of salt as a manure presents a problem which is at once of the highest interest and surrounded with the greatest difficulties. In view of the large quantities now used for agricultural purposes, a somewhat detailed examination of the nature of its action is not out of place in a work such as the present. _Antiquity of the Use of Salt._ The recognition of the manurial functions of salt dates back to the very earliest times. Its use among the ancients is testified by numerous allusions in the Old Testament; while, according to Pliny, it was a well-known manure in Italy. The Persians and the Chinese seem also to have used it from time immemorial, the former more especially for date-trees. _Nature of its Action._ Despite, however, the great antiquity of its use, much difference of opinion seems always to have existed as to the exact method of its action, and as to its merits as a manure in promoting vegetable growth. It furnishes, in fact, a good example of the difficulty which exists in the case of many manures, whose action is chiefly indirect, of fully understanding their influence on the soil and on the crop. In fact, the action of salt is probably more complicated than that of any other manurial substance. _Salt not a necessary Plant-food._ We have already seen that neither sodium nor chlorine--the two constituent elements of salt--are in all probability absolutely necessary plant-foods. If they are necessary, the plant only requires them in minute quantities. Despite this fact, soda is an ash-constituent of nearly every plant, and in many cases one of the most abundant. In amount it is one of the most variable of all the ash-constituents, being present in some plants only in minute quantities, while in others it occurs in large quantities. Mangel and plants of the cabbage tribe may be cited as examples of plants containing large amounts of soda in their composition. But the plants which contain it in largest quantity are those which thrive on the sea-coast, and it has been thought that for them at least salt is a necessary manure. This, however, does not seem to be the case. In fact, the amount of soda in a plant seems to be largely a matter of accident. It may be added that the succulent portions of a plant are generally richest in soda. _Can Soda replace Potash?_