did not produce more than fields in which this ingredient was deficient. Although Liebig's theory of plant nutrition was essentially a mineral theory, yet he understood that organic matter (humus) decomposes in a soil, giving rise to carbonic acid and ammonia; and these gaseous bodies are also valuable plant foods. Liebig fully established the following important laws of husbandry:—
(1) A soil can be termed fertile only when it contains all the materials requisite for the nutrition of plants in the required quantity and in the proper form.
(2) With every crop a portion of these ingredients is removed. A part of this portion is again added from the inexhaustible store of the atmosphere; another part, however, is lost for ever if not replaced by man.
(3) The fertility of the soil remains unchanged if all the ingredients of a crop are given back to the land. Such a restitution is effected by manure.
(4) The manure produced in the course of husbandry is not sufficient to permanently maintain the fertility of a farm; it lacks the constituents which are annually exported in the shape of grain, hay, milk, and live stock.
These laws of Liebig form the basis of modern scientific agriculture.
Liebig was the first to treat bones with "Schwefelsäure" or sulphuric acid in order to make the calcium phosphates soluble (superphosphate); and the late Sir John B. Lawes was the first to commence the manufacture of superphosphate on a large scale, both from bones and mineral phosphates.
From these early beginnings an enormous industry
