Factors of soil formation in the 21st century.

• Factors of Soil Formation is experiencing an exponential increase in scientific citations. • The book, patterned on thermodynamics, reformulated ideas introduced by Dokuchaev. • The State Factor approach is an organizing principle for the Critical Zone research program. • The State Factor theory reveals important remaining research questions. • The least examined State Factor is humans, a glaring need for societally important issues. The year 2021 marks the 80th anniversary of the publication of Factors of Soil Formation. A System of Quantitative Pedology by Hans Jenny. Given the exponential surge in annual citations of the book over the past 30 years, and this important milestone in its history, it is an opportune time to examine what the book presents, and how it is relevant to transdisciplinary research in the 21st Century. The book did not originate the concept of soil forming factors, which were by then already widely accepted in soil science, but instead translated it into a quantitative scientific framework through rigorous definition of the soil system, and the identification and separation of dependent and independent variables. The initial formulation of the theoretical framework was inspired by thermodynamics, but with pedologically-relevant state variables. Subsequent formulations of the model by Jenny articulated the connection and utility to examining energy and mass fluxes through the soil system, a point of departure for recent efforts to bring open system, non-equilibrium thermodynamic methods to bear on soil research. The State Factor theory is a powerful lens to examine a number of academic and societally relevant issues in the 21st century, such as the role of the biotic factor and its interplay with the physical soil environment. However, the most critical, and understudied, state factor is the effect of humans on soil systems. Earth science must embrace the challenge, and obligation to society, to explore the human footprint on the planet, and devise ways to remediate its impact. [ABSTRACT FROM AUTHOR]