Again, it has been believed that soda is capable of replacing potash in the plant; but this does not seem to be the case to any extent. The view that soda is able to replace potash, it has been thought, is supported by the variation which exists in the proportion of soda and potash in different plants. It must be remembered, however, that it is highly probable that most plants contain a larger quantity of ash-constituents than is absolutely necessary for their healthy growth. Especially is this the case with such a necessary plant-food as potash, of which there is generally present, in all likelihood, an excess. The variation in the quantity of potash and soda present in many plants under different circumstances can scarcely, therefore, be regarded as furnishing a proof of the replacement of potash by soda. Incidentally we may mention, as a fact worthy of notice, that cultivated plants have more potash and less soda in their composition than wild plants. What has been said of soda may be held to apply equally to chlorine, as it seems to be chiefly in the form of common salt that soda enters the plant. The amount of salt, therefore, present in plants must be regarded as largely accidental and dependent on external circumstances, such as the nature of the soil, &c. _Salt of universal Occurrence._ But even were salt a necessary plant-food, its occurrence in the soil is already of sufficient abundance to obviate any necessity for its application. It may be said to be of almost universal occurrence. Even the air contains it in traces. That this is the case in the neighbourhood of the sea-coast is well known; but even in air far inland, accurate analysis of the air would probably demonstrate its presence in greater quantity than is commonly believed. It is a wise provision that plants absorb salt, for it increases their efficiency as food,--the function of salt as a constituent of animal food being of the very highest importance. It is an indispensable food-ingredient for animal life. With regard to ordinary farm-stock, the amount of salt which naturally occurs in their food is quite sufficient. In the case, however, of pastures in countries far removed from the sea, the custom of specially supplying stock with salt is common. This is done by placing a piece of rock-salt in the fields. _Special Sources of Salt._ The salt of commerce is obtained from various sources. Besides the sea, we have ample sources of salt in the large saline deposits found in many parts of Europe, especially in Austria, and in England in Cheshire. _The Action of Salt indirect._ From what has been said above, it is clear that the action of salt as a manure is indirect and not direct. What the nature of that indirect action is we shall now proceed to discuss.