Key points

• Soil potassium levels are usually low in sandy soils where water repellence and compaction occur
• Inversion tillage can improve soil potassium availability for plants – but testing is recommended after amelioration to check the levels of this nutrient
• Despite the scope for inversion tillage to improve soil potassium availability, a plant nutrient survey of crops on ameliorated soils showed potassium was still the nutrient most frequently below critical levels  Soil inversion decreases water repellence at the soil surface, allowing better water penetration and increasing the potential for cycling organic residues and nutrients

Potassium levels are typically low in sandy cropping soils in Western Australia but research is showing that it can become more readily accessed by crops after soil amelioration.

This has been one of the early findings by researchers studying nutrient cycling in ameliorated soils. It may be due to the way in which these sandy soils are formed, their low clay content, high plant nutrient removal rate as a percentage of the total available or the greater susceptibility of sandy soils to leaching.

“Amelioration is commonly used to reduce compaction and overcome water repellence, but we are finding it also has an effect on nutrient cycling,” says research team leader Dr Craig Scanlan from the Department of Primary Industries and Regional Development (DPIRD).

“In particular, there appears to be better access to soil potassium by crops after soil amelioration and this may be due to more even wetting of soils and better root growth following amelioration.”

The research is part of a larger co-investment in crop nutrition by GRDC involving DPIRD, The University of Western Australia, Murdoch University, Curtin University and CSIRO through a SoilsWest initiative.

We are finding that ameliorating soil constraints has a consistent benefit for improving crop access to inherent soil potassium in particular.

“The greatest change in availability of soil potassium after amelioration has been on grey sands, which have low levels of clay and organic matter and limited capacity to store potassium.”

The researchers have found ongoing benefits from ameliorating soil constraints and applying potassium fertiliser.

“At a site where we applied deep ripping and potassium treatments in 2019, deep ripping to 50 centimetres changed the yield response to residual potassium for lupins grown in 2020. No residual effect from the applied potassium in treatments without deep ripping was evident, but where deep ripping had been done there was a 0.9 tonne per hectare yield increase in lupin grain from potassium applied in the previous year,” Dr Scanlan says.

Sandy soils, prone to potassium deficiency, are often water repellent and commonly found in WA’s West Midlands region, central wheatbelt and also on the south coast. Water repellence leads to uneven wetting and inefficient use of rainfall, resulting in lower yields.

Ground-truthing

Last year the team surveyed 20 grower paddocks with the South East Premium Wheat Growers Association (SEPWA) and the West Midlands Group (WMG). the paddocks had been ameliorated using a variety of practices ranging  from ripping and mouldboard ploughing to modified one-way ploughing.

We used plant analysis to give us some insights into which nutrients are constraining growth in growers' paddocks where amelioration had been done.

Whole plants and flag leaves were sampled and analysed seperately to assess macrop and micronutrient concentrations.

Overall, we found that potassium was more frequently below critical levels than other naturient. forty-two per cent of flag leaf samples from WMG and 19 per cent from SEPWA were below critial levels for potassium,"  Dr Scanlan says (See Figure 1).

Figure 1: Potassium was found to be more frequently below critical levels than the other nutrients from a survey of 20 grower paddocks with SEPWA and WMG.

Source: DAFWA

"Interestingly there were no samples below critical levels for phosphorus, sulfur, copper or manganese."

Just six percent of samples were below critical levels for nitrogen in the SEPWA paddocks.

At first, the increase in soil potassium availability after amelioration seems to contradict the finding that potassium was the nutrient most frequently below critical levels in flag leaf samples in the survey. However, Dr Scanlan says that in some cases, while amelioration increases potassium uptake by plants compared to untreated soil, the crops grown on ameliorated soil may still have potassium levels in their flag leaves below that needed for maximum growth.

The outcomes from the survey align with findings from the field experiments.

“In one of our long-term experiments on soil inversion and potassium and phosphorus management, grain yield was closely related to shoot potassium concentration and we didn’t see big responses to changes in phosphorus management at that site.

We’ve observed a huge response to potassium at one of our sites, but potassium levels of the flag leaves were still below critical levels.

“It seems there’s still more yield to be gained with better potassium nutrition on sandy soils after amelioration. However, there is a need for care as other nutrients will be removed as crop yields increase and fertiliser programs will need to be reviewed. Our challenge is to find a way to do that profitably.”

Next steps

The field trials that have been established will be continued to quantify the longer-term effects of removing soil constraints on yield response to nutrient application, particularly from an economic perspective.

“We’re starting some new work in 2021 that focuses on potassium use efficiency. Here, we will be investigating whether potassium placement – split application or combinations of these – have an effect on the efficiency with which crops access and take up potassium,” Dr Scanlan says.

“We’re interested in potassium use efficiency, because better efficiency could possibly lead to lower levels of inputs and/or potentially higher profits resulting from the same level of potassium fertiliser where other soil properties are not constraining yield.

In this specific example, we may be able to determine if split applications can help reduce the risk of leaching through the season on these sandy soils, thus providing the plant greater opportunity to access potassium fertiliser.

While there is still work to do, Dr Scanlan says tissue testing is a valuable tool for monitoring crop nutrient status on ameliorated soils. The critical levels in crop plants for the different nutrients are well-established and are not expected to differ on ameliorated soils.

More information: Dr Craig Scanlan, craig.scanlan@dpird.wa.gov.au, 08 9690 2174, Soils West.