Deep roots and soil structure

There is convincing evidence for the benefits of deep rooting, especially in relation to drought resistance (Lopes & Reynolds, 2010; Uga et al., 2013). Modelling has shown that greater root depth allows increased water uptake and higher yields (Lilley & Kirkegaard, 2011). Deep rooting is thought to be improved by combinations of traits that confer steeper growth and an ability to penetrate strong layers (Lynch, 2013). There is a view that natural variability in root depth between species and within the same species (e.g. Canadell et al., 1996), for example, for wheat (Triticum aestivum), provides a basis for developing breeding programmes to develop deep-rooted crops (e.g. Kell, 2011).However, an alternative explanation is thewidely reported effect of soil structure on rooting depth (White & Kirkegaard, 2010; Valentine et al., 2012). The primary purpose of this article is to alert plant scientists to the restrictions to deep rooting that are imposed by soil conditions simply by virtue of depth in the profile which has the effect of increasing soil strength because of the combined effects of hydrostatic pressure and internal soil friction (Richards & Greacen, 1986); in doing so we emphasize the role of soil structure. In some respects these are well reported: for example Valentine et al. (2012) demonstrated the importance of macropores, while White & Kirkegaard (2010) showed that at depth all roots were found in pre-existing pores. However, we will argue that in the field the increase in soil strength at depth that occurs irrespective of soil management, must inevitability restrict root growth to existing pore networks. The findings of White & Kirkegaard (2010) showing that deep roots are only found in pores should be considered to be the norm.