The ease with which bones when ground into a fine state of division putrefy, is evidenced by the fact that bone-flour has to be salted in order to enable it to keep. Another condition which determines the rate at which the fertilising matters in bones become available is the nature of the soil. Fermentation, as we have already seen, requires a plentiful supply of air, and a certain amount, but not too much, of moisture. Consequently bones act best in medium soils--soils which are "neither too light and dry, nor too close and wet." There can be no doubt that what gives to bones a peculiar value in the eyes of the farmer is the fact that they form a manure of a lasting character. They give what has been termed backbone to a soil. But the tendency of modern agricultural practice is to use quick-acting manures rather than slow. This has been admirably put by Professor Storer in the following words: "The old notion, that those manures are best which make themselves felt through a long series of years, is now recognised to be an error. The adage, that 'one cannot eat the cake and have the cake' is conspicuously true in agriculture; and just as it is the part of prudence in household or maritime economy to abstain from laying in at any one time more provisions than can be properly disposed of in a year or during a voyage, so should the farmer refrain from bringing to the land an unnecessary excess of plant-food. Such food is liable to spoil withal in the soil, as well as other kinds of provisions that are kept too long in store. A just proportion of food, properly prepared, is the point to be aimed at always."