Culture media, whether they be soils, nutrient solutions for higher plants, or bacteriological cultures, are subject to fluctuations, often of considerable magnitude, in their H-ion concentrations. Plant and animal fluids and contents of living cells also fluctuate in reaction. One phase of the problem upon which the writer is working involves changes in the reaction of soils, apparently due in part to the changes in the nitrogen cycle. It seemed desirable to investigate in a theoretical way the principal changes that could arise from these and other transformations. The results secured as well as the methods used offer the possibility of wider application than to this specific problem, being perhaps most applicable to the fields of plant physiology, soil chemistry, and bacteriological chemistry. For that reason, they have been prepared for publication. Certain changes in the nitrogen cycle produce acidity, other changes produce alkalinity, and still other changes in the nitrogen cycle have little or no effect on the pH of the medium. Many of these changes have been worked out quantitatively, others only qualitatively.2 Confusion exists in some plant and soil literature bearing on these changes. This paper is an attempt to develop by theoretical reasoning, from the fields of inorganic and biological chemistry, a quantitative expression of the and alkalinity produced by changes in the nitrogen cycle, culminating in a chart which shows these relations graphically. Some attention will be given also to production of and alkalinity resulting from transformations within the carbon and sulphur cycles. A subsequent paper will give experimental data bearing on a part of this cycle. It will be convenient in the following development to speak of the production of one equivalent of acidity as meaning also the disappearance of one equivalent of alkalinity, or a fraction of each, such that the sum of the fractional equivalents of acid produced and alkalinity disappearing will equal one; and conversely, with the production of one equivalent of alkalinity. 1 Contribution from the Division of Agronomy, University of California, Davis,, California.