The last way in which lime acts is what we have termed biological. By this we mean the important _role_ lime plays in promoting or retarding, as the case may be, the various kinds of fermentative action which go on so abundantly in all soils. The presence of carbonate of lime in the soil is a necessary condition for the process of nitrification. Lime is the base with which the nitric acid, when it is formed, combines; and as we have seen, when discussing nitrification, soils of a chalky nature are among those best suited to promote the natural formation of nitrates. This is one of the reasons for the beneficial effects produced by lime when applied to peaty soils. Not merely does it help to decompose the organic matter so abundant in such soils, but it also furnishes the base with which the nitric acid may combine when it is formed. But while the action of lime is to promote fermentation, it must not be forgotten that there may be cases in which its action is rather the reverse of this. Fermentation of organic matter goes on when there is a certain amount of alkalinity present; while, on the other hand, the presence of acidity seems to retard and check it. Too great an amount of alkalinity, however, would, in the first instance, retard fermentation as much as too great acidity. It has been claimed that the addition of caustic lime to fresh urine may act in this way; and if this were so, the addition of lime to farmyard manure might, to a certain extent, be defended. The experiment, however, would be a hazardous one and not to be recommended, as loss of ammonia would most likely ensue. _Action of Lime on Nitrogenous Organic Matter._ The action of lime on nitrogenous organic matter is of a very striking kind, and is by no means very clearly understood. As we have pointed out, it sometimes acts as an antiseptic or preservative; and this antiseptic or preservative action has been explained on the assumption that insoluble albuminates of lime are formed. Its action in such industries as calico-printing, where it has been used along with casein for fixing colouring matter; or in sugar-refining, where it is used for clarifying the sugar by precipitating the albuminous matter in solution in the saccharine liquor; or lastly, in purifying sewage,--has been cited in support of this theory. While, however, there may be circumstances in which lime, especially in its caustic form, acts as an antiseptic, its general tendency is to promote these fermentative changes, such as nitrification, so important to plant-life. An important use of lime in agriculture is in preventing the action of certain fungoid diseases, such as "rust," "smut," "finger-and-toe," &c., as well as in killing, as every horticulturist and farmer knows, slugs, &c. _Recapitulation._