Under the right conditions deeper crop roots can increase crop yield by providing valuable extra water during grain filling. But should we breed for deeper roots, try to manage crops to achieve deeper rooting – or both?
Research from CSIRO Agriculture has shown that when deep roots extract more water they leave the soil in a drier state – a legacy that influences the value of deep roots on different soils and seasons.
Using the Agricultural Production Systems sIMulator (APSIM) CSIRO Agriculture’s Dr Julianne Lilley has predicted the yield benefit and legacy effects of wheat varieties with deeper roots, early sowing systems and their interaction.
The results show that both early sowing and vigorous roots although beneficial to the current crop, can leave the soil drier for the subsequent crop. Extra water can only been taken up when it is there – either because a previous crop left it behind, or because the season has been wet enough to re-wet the soil to depth. In Australia many seasons are too dry for the water to penetrate to depth, especially at locations with low rainfall.
“Under these conditions deep roots are of little or no additional value,” explains Julianne.
“Much of the Australian cropping zone has shallow soils where root growth is significantly restricted to less than 1m due to high levels of salinity, sodicity, boron toxicity, acidity, alkalinity or gravel layers. Deep roots in these regions are of little to no value as the accessible soil water is fully exhausted in most years already.”
However, if you’ve got the right soil conditions and the seasons are wet enough, deep roots can provide a significant yield advantage if the crop is managed appropriately. Effective management can take a number of routes depending on soil, season and cultivar.
Maximising root depth by sowing early, extends the period of root growth allowing the crop to access more and deeper water, leading to increased yield. Early sowing of longer season cultivars also provides a benefit in shallow soils as more rainfall is captured during the longer period of crop growth and a lower proportion is lost to soil evaporation. But it is important to realise the extended crop duration means a longer period of water extraction which leaves the soil in a drier state.
“Overall we found that early sowing was beneficial in most circumstances because it combines deeper roots with other advantages for shoot growth that lead to increased yield. Cultivars with deeper roots were beneficial only on deep soils in specific seasons, but in those cases could add to the benefits of earlier sowing.”
Growers should consider their crop sequence and seasonal conditions to take advantage of deep water when it is there. But in drier seasons, or following crops which exploit soil water to a greater degree, growers should consider appropriate cultivar choices or select species which perhaps extract less water to give the profile a chance to recharge. For example, late-sown cultivars or grain legumes may be a less risky choice.
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