The rate of C storage in the soil (C_s) depends on the balance between the inputs of C from organic residues (C_i) and the outputs due to microbial oxidation of organic matter. We hypothesize that simple mathematical models can be effective at representing soil carbon dynamics. We present empirical models of soil C dynamics of the general form C_sdt = h(C_s)C_i - k(C_s)C_s, where h and k are the residue humification and apparent soil C decomposition coefficients. The simplest solution to this equation results from assuming that h and k are constant and hence at steady state C_s = hC_i/k. This implies that the storage capacity of the soil is linearly dependent on C_i. There is evidence, however, that soils have a finite capacity to store C. In addition, the C_s constituents have different turnover rates i.e. k is a weighted average of the individual k's of each soil C fraction. Therefore, we developed models in which h and k are functions of C_s. Making h a function of C_s provides a simple way of representing a “C_s carrying capacity”. Making k a function of C_s provides a means for accelerating the turnover rate as C_s increases, with the underlying assumption that the higher the C_s content the lower the soil recalcitrant C fraction. If properly parameterized, a single mathematical function can be used for different soil layers provided that texture does not change dramatically and that C inputs from roots at different depths are known. The effect of residue composition on h such as variations in lignin content can also be accommodated. These simple models can be useful to predict C_s dynamics and provide feedback information to improve existing mechanistic models.