Crop emergence, the impact of mechanical impedance

To emerge from a germinated seed, the shoot has to be capable of reaching the soil surface, while continued root growth is required to gain access to water in drying seedbeds. This is illustrated in Figure 1 where the seed must first germinate rapidly, then have rapid initial downward growth, and finally have high potential for upward shoot growth in soil of increasing impedance (Figure 1). Once a seed has germinated, seedling growth depends on temperature, water potential, and the mechanical strength of the seedbed (Collis-George and Yoganathan, 1985a, b; Finch-Savage et al., 1998; Townend et al., 1996; Whalley et al., 1999, 2001). Root and shoot elongation rate decrease with water potential in vermiculite (Sharp et al., 1988), but as soil dries it also tends to become stronger and mechanical impedance rather than water stress can become limiting (Weaich et al., 1992). Understanding the impact of mechanical impedance on seedling emergence can be difficult...