A continual model of soil organic matter transformations based on a scale of transformation rate

•A continual model of soil organic matter transformation has been proposed.•The rate of humification was used as a continual scale of humification degree.•The model is simple due to choice of the scale.•The model equation has analytical solution in elementary functions. The study proposes a model of transformation and decomposition of soil organic matter, based on using the rate of matter transformation as a continual scale of its transformation degree. The model is represented by one partial derivative differential equation that has an analytical solution. This model has only two adjustable parameters, while the number of experimentally determined parameters depends on how detailed the description of the plant litter is supposed to be. The model has been tested, and model parameters have been evaluated by comparing model predictions with the literature data on the stock of soil organic matter in different types of ecosystems.