ABSTRACT
Some of the vertical soil-structure nonuniformities are natural products of the soil formation process and are formally classified as soil horizons of a soil series. The root is an integrated system consisting of distributed meristems and zones of cell elongation and maturation. Soil structural features of importance are evidenced by pedality, penetrometer resistance, macroporosity, consistence, dry bulk density, air and water porosity, and micromorphology. Soil environmental factors of water, heat, gaseous exchange, mechanical impedance, and available nutrients influence root development and function. Respiration and growth of roots and aerobic microorganisms require that oxygen diffuse from the soil surface through the soil pores to the microsites of root and microbial activity. Carbon dioxide must move in the reverse direction away from these sites where it is being produced during respiration. The root zone is a heterogenous soil environment created by both natural causes and cultivation. Traffic and tillage influence both vertical and horizontal soil variations.
