AGGREGATE: NEWS & EVENTS
The highs and lows of potassium application
WHEN is it OK to apply K?
Well, if a farmer wants to avoid leaching of the essential macronutrient, then application time should match plant uptake, says Dr Miaomiao Cheng, Research Scientist at the Department of Primary Industries and Regional Development (DPIRD), Agricultural Resource Management & Assessment of Fisheries & Sustainability.
With many seeding programs throughout Western Australia finished or nearing completion, farmers will be considering their potassium inputs through to harvest.
It’s a delicate equation. The majority of WA surface soils have adequate potassium levels for crop uptake, but relatively low buffering capacity – meaning potassium can be depleted under continued crop removal or lost through leaching during rainfall.
Over time it makes for a negative potassium balance, where output is greater than input – and possibly compounded by farmer preference for nitrogen and phosphorus.
“In lots of farming systems, farmers don’t apply enough potassium,” Dr Cheng tells soilswest.org.au.
“If you take potassium from the soil every year, low buffering capacity will mean that one day you will run out of your available potassium. Deficiency will happen, even in the loamy or clay soils.”
Determining a soil’s buffering capacity perhaps means understanding its cation exchange capacity (CEC) – the ability to hold positively charged ions.
Potassium is generally less mobile in soils with a high cation exchange capacity, such as those with high clay or organic matter content. Farmers can improve a soil's cation exchange capacity by adding lime to improve soil pH (in the case of an acidic soil), and manure, compost or clay.
While it isn’t necessarily new research, Dr Cheng, in her former position with Murdoch University, Emeritus Professor Richard Bell (Murdoch University) and Dr Craig Scanlan (The University of Western Australia, formerly of DPIRD) have progressed an important correlation between the two.
Their findings are detailed in their December 2025 paper ‘Labile potassium dependency and limited buffering capacity in highly weathered loamy and clay soils of Southwest Australia revealed by Q/I isotherms and K release’ – an additional outcome of the Grains Research and Development Corporation (GRDC) investment ‘Increasing profit from N, P, and K fertiliser inputs’.
“Buffering capacity is a little bit hard to predict or to detect on merit in the lab, but we found that it was well related to soil CEC or effective CEC,” Dr Cheng says.
“If you can determine CEC, you can then predict if your soil has higher buffering capacity or lower buffering capacity before you determine potassium application.
“You always need to think about potassium levels and soil’s holding capacity together. For example, if your soil has a high buffering capacity, you can apply a larger amount early in the season.
“If your soil has adequate available potassium and a strong capacity to hold it, the soil can effectively bank the potassium. The potassium stays in the soil until the crop needs it later.”
Dr Cheng says farmers need to be prepared for the likelihood of low buffering capacity in many WA agricultural soils, particularly sandy soils where potassium can be vulnerable to leaching.
In those soils, splitting potassium application can be an effective management approach.
“Rather than apply a very high amount that a soil can’t retain, you can split the potassium into smaller amounts to better match crop demand and reduce losses,” Dr Cheng says.
“You also can delay application as the surface soil may already have adequate available potassium for the germination and early seedling growth. You can then wait to apply at the tillering stage or a later stage. Just avoid the winter high rainfall seasons.
“Some duplex soils, however, may contain heavier subsoils with a greater ability to retain potassium than the sandy surface layer. In these situations, strategic placement or incorporation of K deeper in the profile may help improve longer-term potassium availability and reduce leaching risk.”
Dr Cheng says that while the GRDC project research focused on cropping in low rainfall areas between 2018 and 2023, its outcomes on potassium dynamics were applicable to all WA soils.
MORE STORIES
SoilsWest to talk constraints, courses at Open Day
SoilsWest and the School of Agricultural Science will have a shared presence for Murdoch University’s annual event.
Soil carbon focus for postdoctoral research role at Curtin
Curtin University is inviting applications for a four-year postdoctoral research role in its Soil and Landscape Science Laboratory Team.
Soil ecologist’s step into national soil innovation
Dr Ramesha H Jayaramaiah’s new project role will give him more opportunities to connect lab knowledge with practical agricultural systems.
Research spotlights soil fauna’s role in nutrient cycling
A review led by Dr Ramesha Jayaramaiah provides new insights into how soil fauna may help regulate nitrogen cycling and support more sustainable agricultural systems.