We combined published information and our own experimental results from the Gulf Coastal Plain to evaluate how soil aeration and strength interact with loblolly pine root growth. Our results demonstrate that soil aeration and strength differ by soil series and year and are subject to vertical and horizontal spatial variation. Comparison of loblolly pine root phenology and seasonal redox potential indicates that oxygen is frequently limiting to new root growth. The strength of these soils may also restrict loblolly pine root growth. Physiological process models that predict southern pine productivity should apply accurate calculations of plant-available soil in simulations of potential root zone water storage and plant-available water. We propose a conceptual root zone submodel that predicts the volume of plant-available soil with soil aeration, strength, and water retention by horizon. Model parameters are components of the Natural Resources Conservation Service (NRCS) Soil Interpretation Record (SIR) database collected across the United States.