The difficulty in establishing simple relationships between soil structure and crop growth does not mean that soil has little influence upon root growth; rather it points to the complexity of the interactions and the internal homeostasis which plants maintain. Roots both elongate and proliferate and spread laterally as they grow and age. Concomitantly some roots die and others become suberized and function as conduits, but not as absorbers, of water and nutrients.

Roots can elongate downwards as fast as 8 cm/d, as for example, soybean growing in a silt loam in a rhizotron (Kaspar et al. 1978). Deep-rootedness and maximum rooting depth reflect soil properties (for example, roots will not grow through pores that they cannot deform to a larger diameter than the root). However, relationships are not often reported. Maximum rooting depth varies with species and soil type.

For example, wheat roots penetrated to 0.8 m in heavy-textured soils and to 1.2 m in a loamy sand (Rickert et al. 1987) but it is often found that a variety will have a consistent rooting depth across similar soil types in a particular year (Hamblin and Hamblin 1985) or in one soil across several years (Pearson et al. 1991). Angus et al. (1983) found that rice and six dryland crops (mung bean, cowpea, soybean, groundnut, maize and sorghum) extracted different amounts of stored soil water (ranging from 100 mm for rice to 250 mm for groundnut) and that extraction was, in part, related to rooting depth.