This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
It has already been shown (p. 108) that mineral foods alone are insufficient to supply all that is needed for plant life. Scientific investigation has proved that the soil, if properly cultivated, is teeming with life. Millions of minute organisms or bacteria are working away in the dark converting the minerals and metals of the earth into plant food, with the aid of the fresh air and the organic manures. This explains why tons of stable or farmyard manure and vegetable refuse of all kinds vanish after a time in the soil. It has been eaten away by the bacteria, and in the process a certain amount of heat and fermentation have been generated. This heat (more or less according to the condition of the manure applied), in conjunction with a proper amount of moisture and fresh air, dissolves some of the mineral substances, and brings them into such a condition that they are readily absorbed by the roots. So long as all the necessary factors are present the work goes on steadily; but if one or the other is absent or in a poor state the whole work is impeded. If lime, or manure, or both be absent no bacteria come into existence and no changes take place. Hence a soil without them would be as sterile as a heap of cinders or as the material in the roadway.