As all plants contained certain organic acids, and as these organic acids were nearly always found in a neutral state--_i.e._, in combination with bases, such as potash, soda, lime, and magnesia--the plant must be in a position to take up sufficient of these alkaline bases to neutralise these acids. Hence the necessity of these mineral constituents in the soil. According to him, however, the exact nature of the bases was a point of not so much importance. He assumed, in short, as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual replaceability amongst the bases than can be now admitted. Passing on to a consideration of the difference of the mineral composition of different soils, he attributes this to the difference in the rocks forming the soils. "Weathering" is the great agent at work in rendering available the otherwise locked-up stores of fertility. He attributes the benefits of fallow exclusively to the increased supply of these incombustible compounds which were thus rendered available to the plant. Treating of this subject, he says: "From the preceding part of this chapter" (in which he has been explaining weathering) "it will be seen that fallow is that period of culture when the land is exposed to progressive disintegration by the action of the weather, for the purpose of liberating a certain quantity of alkalies and silica, to be absorbed by future plants." _His Theory of Manures._ Treating of manures, he showed how the most important constituents of manures were _potash_ and _phosphates_. In the first edition of his work he also insisted on the value of _nitrogen_ in manures, condemning the want of precautions, in the treatment of animal manures, against loss of nitrogen. In the later editions of his work he seems to have receded from that opinion, and considered that there was no necessity for supplying nitrogen in manures, since the ammonia washed down in rain was a sufficient source of all the nitrogen the plant required. It was here that Liebig went astray, first in denying the importance of supplying nitrogen as a manure; and secondly, in overestimating the amount of ammonia washed down in rain, which has subsequently been shown to be entirely inadequate to supply plants with the whole of their nitrogen.[15] _His Theory of Rotation of Crops._ In explaining the benefits of the rotation of crops, Liebig propounded a very ingenious theory, but one which was largely of a speculative nature, and which has since been shown to be unfounded on any scientific basis. It was to the effect that one kind of crop excreted matters which were especially favourable to another kind of crop. He did not say whether he considered such excretion positively injurious to the crop which excreted them; but he inferred that what was excreted by the crop was what was not required, and what could, therefore, be of little benefit to a crop of the same nature following it.