Exploring potassium uptake in Balllidu

By Kate Parker, WMG Project Officer

Plant tissue testing at peak biomass this season has shed light on how soil type, fertiliser rates and rainfall interact to influence potassium uptake in crops at two GRDC funded K Extension Project Ballidu trial sites. Both sites were located in neighbouring paddocks, with Site 1 on a yellow sand and Site 2 on a sand over gravel.

Five potassium (K) fertiliser rates were applied across treatments — 0, 25, 37.5, 50 and 75 kg/ha — to assess crop response. Tissue samples were taken at GS30 and again at peak biomass to help understand how applied potassium and seasonal conditions affected plant nutrition.

Early growth (GS30) results

The early-season tissue tests taken at GS30 showed little separation between treatments, suggesting that differences in applied potassium were not yet reflected in the plant tissue. This may indicate that K availability early in the season was not limiting, or that the plants had not yet developed sufficient root mass to capture treatment differences. In many sandy soils, K uptake often becomes more distinct later in the season once soil moisture and root depth increase.

Peak biomass results

By peak biomass, clearer differences began to emerge — particularly at Site 2.

• At Site 1 (yellow sand), potassium concentrations ranged from 1.28 % to 2.61 %, with Treatment 3 (50 kg K/ha) showing the highest value and Treatment 5 (37.5 kg K/ha) the lowest.
• At Site 2 (sand over gravel), levels ranged from 1.16 % to 2.27 %, with a gradual increase in tissue K up to the higher fertiliser rates, aligning more closely with applied potassium.

This alignment at Site 2 may reflect the stronger nutrient-holding capacity of the subsoil clay and the impact of additional nitrogen (N) applied by the grower during the season. Following agronomic advice, extra N was applied at Site 2 to support crop growth and to balance the interaction between nitrogen and potassium — an important consideration in high-rainfall years when leaching and nutrient dilution can occur.

Conclusions

High rainfall through winter likely contributed to nutrient movement and potential K losses on the lighter-textured yellow sand, helping to explain why Site 1 did not show the same clear response pattern. In contrast, the sand over gravel profile at Site 2 may have helped retain more potassium in the root zone, allowing the applied rates to translate more directly into plant uptake.

While early tissue tests didn’t reflect fertiliser rates, by peak biomass the sand-over-gravel site began to show a clearer trend of increasing K concentration with higher applied rates. This highlights the role of soil type, rainfall, and nutrient balance in determining how effectively potassium fertiliser is taken up by the crop.

Ongoing monitoring, including yield and soil K changes post-harvest, will help determine whether these tissue test trends translate into productivity gains, and how future fertiliser decisions might be fine-tuned for Ballidu’s variable soils.