These are widely employed, and the best known form consists of farmyard manure or the refuse of the stable. This manure contains nitrogen, phosphoric acid and potash in such a state that they are easily formed into a soluble food. One ton of farmyard manure contains about 10 or 12 lbs. of nitrogen, 4 to 9 lbs. of phosphoric acid and from 9 to 15 lbs. of potash. The nitrogen is chiefly present in the form of ammonia, which causes the characteristic smell of farmyard manure ; this compound is very volatile, and on this account a great proportion of this valuable food is lost when the manure is exposed to the atmosphere.
Farmyard manures should be used to the extent of three or four barrow-loads per square rod ; it may be spread over the surface and thoroughly dug into the soil a spade deep, but the best practice is to work it into the second layer or spit when the ground is being trenched. In strong soils this should be done in early winter so that the " breaking-down " action of the food compounds may commence early; in light, sandy lands it must be done in spring. It is unwise to place fresh stable manure in the ground at any time as it is liable to injure the roots ; it should have been previously sweetened by stacking and turning.
The compost heap provides the gardener with a convenient source of valuable plant food. It should be formed in an out-of-the way corner of the garden and built up from time to time of plant refuse mixed with quicklime ; to these may be added dead leaves. In the course of time the vegetable matter decomposes and provides a valuable manure for use during the next trenching operation.
A similar, yet quicker, method of manuring the garden is provided by " digging in " a quickly-grown green crop ; thus, in autumn, Turnips may be sown on a spare plot and then in the following spring dug deeply into the ground.