We may, in conclusion, sum up in a single paragraph the different ways in which lime acts. Its action is mechanical, chemical, and biological. It acts on the texture of the soil, rendering clay soils more friable, and exerting a certain binding effect on loose soils. It decomposes the minerals containing potash and other food-constituents, and renders them available for the plant's needs. It further decomposes organic matter, and promotes the important process of nitrification. It increases the power of a soil to fix such valuable food-constituents as ammonia and potash. It neutralises sourness, and prevents the formation of poisonous compounds in the soil. It increases the capillary condition of the soil, prevents fungoid diseases, and promotes the growth of the more nutritive herbage in pasture-land.
CHAPTER XXI.
INDIRECT MANURES—GYPSUM, SALT, ETC.
Gypsum.
In the previous chapter mention was made of gypsum as a compound of lime, but no reference to its action as a manure was made. In the past, gypsum was used extensively and highly valued. It was found to be of especial value for clover; and there is a story told of Benjamin Franklin which illustrates the very striking nature of its action on this crop. It is related that he once printed with gypsum the words "This has been plastered" on a field of clover, and that for a long time afterwards the legend was plainly discernible on account of the luxuriance of the clover on the parts of the field which had been thus treated.
Mode in which gypsum acts.
Despite the fact that gypsum is a most ancient manure, it is only of late years that we have come to understand the true nature of its action. For long it was believed that the reason of its striking effect in promoting clover was due to the fact that, as clover was a lime-loving plant, the action of gypsum was owing to the lime it contained. That, however, the action of gypsum is not due to the fact that it supplies lime to the plant, seems evident when it is stated that were this so, any other form of lime would have the same beneficial effect. It is well known, however, that this is not so. Besides, as we have already pointed out, lime is not a constituent which most soils lack, so far as the needs of the crop are concerned. There is a certain amount of truth in the old belief that gypsum enriches the soil in ammonia by fixing it from the air. The power that gypsum has as a fixer of ammonia has already been referred to in the chapter on Farmyard Manure; but in this case the gypsum is brought in contact with the ammonia. The origin of this old belief was due to a misconception as to the amount of ammonia in the atmosphere. No doubt gypsum greatly increases the power of a soil to absorb ammonia from the air; but the quantity of ammonia in the air is so very trifling, that its action in this respect is hardly worth considering. The true explanation of the action of gypsum is to be found in its effect on the double silicates, which it decomposes, the potash being set free. Its action is similar to that of other lime compounds, only more characteristic. As a manure, therefore, its action is indirect, and its true function is to oust the potash from its compounds. Its peculiarly favourable action on clover is due to the fact that clover specially benefits by potash, and that adding gypsum practically amounts to adding potash. Of course it should be borne in mind that the soil must contain potash compounds if gypsum is to have its full effect. Now, however, that potash salts suitable for manuring purposes are abundant, it may well be doubted whether it is not better to apply potash directly. Further, it must be borne in mind that gypsum is applied to the soil whenever it receives a dressing of superphosphate of lime, as gypsum is one of the products formed by treating insoluble phosphate of lime with sulphuric acid.
It is possible that gypsum may act as an oxidising agent in the soil, just as iron in the ferric condition does. It has a large quantity of oxygen in its composition, and under certain conditions may act as a carrier of oxygen to the lower layers of the soil. When it is used, it should be applied some months before the crop is sown.