Key points

• Research into root architecture has shown that adequate crop nutrition is critical for any genetic improvements to be reliably expressed
• Research into phosphorus and potassium subsoil fertility shows that deep banding is very important in systems that rely on stored soil water
• Professor Mike Bell says both research projects reinforce that deep banding, as part of a nutrient management strategy, is beneficial in addressing subsoil nutrient depletion in northern cropping systems

Adequate crop nutrition is critical for ‘designer’ roots to be reliably expressed, with combinations of shallow and deep bands proving to be the best application methods for this.

Working with the University of Queensland’s (UQ) Professor Mike Bell, research by former postdoctoral researchers
Dr Frederik van der Bom and Dr Nelly Raymond has reinforced that nutrient deficiencies need to be addressed.

For the past three years, Dr van der Bom has experimented with wheat and sorghum genotypes to test the interaction between root angles and nutrient acquisition – specifically phosphorus. He says inadequate phosphorus can eliminate any benefits from plant genetic selection for traits such as deeper root development.

“The importance of placement and timing of phosphorus applications to optimise the performance of ‘designer’ root systems cannot be overlooked. At the end of the day, phosphorus access is critical for ‘designer’ root architecture to be expressed.”

Narrow root angles are associated with a deeper exploration of the soil profile, while wider root angles are associated with scavenging for phosphorus in the topsoil, particularly early in the season. There might also be benefits from wide root angle genotypes because of their ability to exploit rewetting events. “But these would be highly dependent on in-season rainfall distribution.”

As access to surface-stratified phosphorus can quickly be reduced by the soil drying out, there is only a limited window of opportunity to take up phosphorus from this layer. The potential benefits of shallow or wider roots for improved phosphorus uptake are diminished.

He says this means crops need to have phosphorus access close to the seed. “The sooner the plant can access phosphorus the better, so starter fertilisers are important. But, unfortunately, they are not enough to meet crop phosphorus demand in most seasons.

“Likewise, deep phosphorus provides an important source of phosphorus in the subsoil but unless roots can access phosphorus during early growth, they can take too long to find and exploit the deeper P bands. So, we need both starter and deep phosphorus together to reliably improve crop phosphorus access.”

Reliable application

Earlier work by UQ’s Dr Gregor Meyer highlighted that concentrated fertiliser bands had some negative impacts on phosphorus availability to plants. Dr Raymond, a vertosol expert, investigated. She diluted phosphorus fertilisers by mixing them through large soil volumes, essentially what would occur with more conventional tillage.

Her research helped unveil the complexities that come with phosphorus application. It suggested that while the thorough mixing of fertiliser through large soil volumes may offer greater root exposure, this may not result in greater crop uptake.

This is because of strong sorption reactions that significantly reduce phosphorus availability. Wider, less-concentrated bands might be an effective compromise, enriching more of the soil volume and minimising precipitation reactions, but also limiting the exposure of phosphorus to sorption reactions.

Professor Bell says both research projects reinforce that all application strategies have limitations, but banding will continue to be an important part of nutrient management strategies that address subsoil nutrient depletion.

“Frederick’s work shows that if the root cannot access the phosphorus, it won’t grow. So, if your phosphorus levels are low, it is time to do something about it. Meanwhile, Nelly’s work shows that more-frequent, less-concentrated deep bands work best.

“Overall, if nutrient deficiencies are addressed, the crop can capitalise on whatever the season will be.”

Both Dr van der Bom and Dr Raymond have returned to the University of Copenhagen, winning prestigious Marie Curie research grants. They are continuing to study ways to sustainably manage soil phosphorus fertility.

More information: Nelly Sophie Raymond, nr@ign.ku.dk, Frederik van der Bom, fvdb@plen.ku.dk