Although a story about complex and contrasting behaviours might sound like the latest television series, it is instead a story about nutrient management.

The latest deep-banding fertility research from the University of Queensland’s Professor Mike Bell and his team has found how differently phosphorus can behave in soils with apparently similar characteristics.

Professor Bell says there has recently been a strong research focus on understanding phosphorus availability in certain soils, because of how applied phosphorus is recovered.

“The key to crops’ successful phosphorus and potassium acquisition is having enough roots in the soil with available nutrients for long enough to meet crop demand. Ensuring nutrients are available for plant uptake is a critical success factor,” he says.

“The availability of potassium applied into the soil is generally good. But the recovery of applied phosphorus is more problematic. It is dependent on the interaction between soil properties and fertiliser type.”

This is where the work gets tricky, with a balance needed between contrasting chemical reactions and root access.

Professor Bell says crop roots forage most effectively in larger soil volumes, but mixing phosphorus through large soil volumes increases the risk of sorption. Sorption ‘binds’ the phosphorus to soil surfaces and reduces its availability to the crop.

Applying phosphorus in bands limits the sorption risk by reducing the exposure of phosphorus to those binding surfaces. However, it introduces a different risk – that of precipitation, where the phosphorus becomes insoluble, “a little bit like too much sugar in your tea”.

“Effective phosphorus management means knowing your soil characteristics and working in the ‘sweet spot’ that maximises crop phosphorus recovery.”

The difference the deep-banded phosphorus can make in chickpeas is illustrated here. The crop on the left was planted on deep-banded country, whereas the one on the right has had no added nutrition. Photo: Doug Sands

Without knowing the sweet spot between these competing chemical reactions from paddock to paddock and region to region, the less-risky option is to stick with banding phosphorus fertiliser, Professor Bell says.

“What we are trying to balance is the impact on phosphorus availability to crops. If we band-apply phosphorus, we treat small soil volumes that limit root access and run the risk of precipitating phosphorus in forms that the plants cannot get at.

“If we try instead to treat a larger volume of soil to improve root access and reduce phosphorus precipitation risk, we instead expose the phosphorus to more sorption sites.

“So, we are between a rock and a hard place – we need the ‘protection’ of bands to reduce sorption, but we also need to keep the in-band concentrations low to avoid precipitation risks.

“There is a sweet spot in there somewhere that will optimise the volume of soil exposed to phosphorus fertiliser and maximise crop recovery. Given we don’t yet know how to determine that sweet spot for each soil, the lower-risk option is still banding, but maximising the number of bands and their spatial distribution.

“That way, you increase the volume of phosphorus-enriched soil, maximise the chances of enough roots accessing these smaller bands and use the residual value to slowly re-enrich the depleted layers.”

Soil chemistry 101

To better understand the juggling act between crop roots having sufficient access to nutrients while reducing sorption risks and avoiding precipitation hazards, Professor Bell explains some key terms. Adsorption refers to phosphorus accumulation on the surface of soil minerals or organic matter.

“This process reduces the concentration of phosphorus in the soil solution and is the reason why we rarely see phosphorus leaching in anything other than coarse sands. The clay and organic matter ‘sorb’ or ‘bind’ the phosphorus to the surfaces, so there is little left in the soil solution to leach.”

Adsorption is typically a two-way process and the way that process occurs determines how readily phosphorus is released – or ‘desorbed’ – back into the soil solution ready to supply plants. There are two sorption mechanisms – a non-specific electrostatic attraction and a specific chemical bond formation.

Researchers are finding that if the specific mechanism dominates, phosphorus tends to become rapidly unavailable and stays that way – not the outcome wanted when the aim is to improve soil fertility.

Professor Bell says the Phosphorus Buffer Index used in commercial soil tests is a measure of the phosphorus sorption capacity. “Soils rated ‘high’ typically have a lot of this specific adsorption. It means fertilisers need to be banded to limit contact and interaction with the soil as much as possible.”

Vertosols tend to have both moderate sorption capacities and mostly non-specific sorption mechanisms. This means they are capable of both storing and releasing phosphorus.

However, vertosols can have a wide range of parent materials and mineralogy, and some do have quite specific sorption capacities. That means they grab the phosphorus and hold it tightly.

The team is also finding that across all vertosols, the more phosphorus-depleted they are, the greater proportion of these ‘clingy’ specific sorption sites. It means more fertiliser phosphorus is held back from plant access.

“So, when we apply phosphorus mixed through these depleted soils in an effort to enrich larger soil volumes, nothing much seems to happen. That is unless we apply enough phosphorus to saturate the specific sorption sites and get into filling the less-specific ones too, which is a costly exercise.”

Deep banding is a way to reduce this sorption risk but, on the flip side, it increases the risk of precipitation within and around the phosphorus band.

In a nutshell, Professor Bell says precipitation occurs when the concentrations of ions in a soil solution are too high to remain dissolved. “(It is) a little bit like too much sugar in your tea, except in this case the product that settles out is often not readily resoluble.”

Precipitation reactions determine how available phosphorus is when applied in bands. “The more concentrated the bands are, the greater the risk of these insoluble precipitates forming and reducing the benefits of the phosphorus fertiliser applied.

“Deep bands offer protection from strong phosphorus sorption reactions typically found in phosphorus-depleted subsoils, but the band concentration needs to be kept low to avoid some of the adverse chemical reactions that cause phosphorus to precipitate and become unavailable to plants.”