From the earliest times agriculturists have been familiar with the idea that decomposition of vegetable and animal matter takes place in the soil, and that the process is intimately connected with soil fertility. By the middle of the nineteenth century three different ways were known in which the decomposition occurred. One had been since early times specially associated with soil fertility, in that it gave rise to humus, the black sticky substance in farmyard manure or in soil--which was supposed up to 1840 to be the special food of plants. No good account of the process or of the conditions in which it occurred is, however, given by the older writers.…
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