It has been long known that after long-continued cultivation, soils originally of neutral or slightly basic reaction become acid: and the liming of such lands is an ancient practice in Europe. The matter, however, received but scant attention until Wheeler and Hartwell, of the Rhode Island Experiment Station, demonstrated the almost universal acid condition of the older lands of that State, and the excellent effects produced by neutralization with lime, or even with the alkali carbonates.[38] The current neutralization of the humus-acids is unquestionably one of the cardinal advantages of calcareous lands; for such as contain only small amounts of lime carbonate will of course become acid more quickly under cultivation. [38] Reports of the Rhode Island Exp’t Station, 1895, and ff. _Humin Substances._—In the presence of only a moderate amount of moisture, therefore under the influence of a more or less rapid circulation of air, and in the presence of earthy carbonates (especially that of lime) to prevent the formation of acids, or to neutralize them as formed, the normal process of humification occurs; mainly under the influence of fungous instead of bacterial growths. The various molds take a prominent part in the conversion of the vegetable substance into black, neutral, insoluble humus compounds. Such fungous vegetation is always accompanied by the evolution of carbonic gas, and the resulting fungous tissues are markedly richer in nitrogen and carbon than the substance of the higher plants from which they were derived (see chapt. 9). Comparative analyses show that in the normal process of humification of vegetable substances, oxygen and hydrogen are eliminated in the form of water and carbonic dioxid, while at the same time there is an increase in the percentage of carbon, and generally also of nitrogen; the latter more particularly in the case of vegetable matter not very rich in that element. When once humification is complete, oxidation, especially under arid conditions, bears mainly upon the carbon and hydrogen, so that the nitrogen content may rise to very high figures; while another portion is ultimately wholly oxidized, with the formation of nitrates, under the influence of the nitrifying bacteria, this being the process chiefly efficient in the nutrition of vegetation with nitrogen.