An improved approach for estimating root elongation rate from penetrometer resistance and macropore porosity on a silty clay loam soil

The role of macropores is often ignored in classical models for predicting root elongation using soil penetrometer resistance (PR). In this study, we propose an empirical model that includes the effects of macropores and PR on maize (Zea mays L.) root elongation rate (RER) and compare its performance with three previous models. Undisturbed soil cores were collected from an 11-yr tillage experiment (including no-tillage and conventional tillage systems) in Northeast China. For each soil core, soil bulk density (BD), penetrometer resistance (PR), air-filled porosity (AFP), and pore size distribution from water release characteristics, and RER of maize seedlings at a matric potential of −20 kPa were determined. Results showed that RER negatively correlated with BD, PR, and the volume of ε<6 (the volume of pores less than 6 µm), but it was positively correlated with the AFP and ε>60 (the volume of pores greater than 60 µm) (P < 0.001). RER exhibited a 50 % reduction when PR was over 1.3 MPa or AFP was below 10 %. Additionally, RER became less sensitive to PR change at PR values greater than 1.3 MPa. The new RER model, which accounts for the influences of PR and macroporosity (> 60 µm), performed better in predicting RER than the previous models, with a root mean square error (RMSE) of 0.36. The new model is useful in simulating maize root distribution under field conditions.