CSBP - What’s the risk of potassium leaching losses on loamy sands in the medium rainfall area - with and without lupins in the cropping rotation?

By James Easton, CSBP Senior Agronomist

• A CSBP trial at Regans Ford has indicated that there may be low risk of potassium (K) leaching losses on ameliorated sandplain in medium rainfall areas.
• Potassium recovery from soil and fertiliser by wheat was greater than by lupins.
• Applying K before or at seeding can simplify fertiliser requirements after seeding.

What’s the risk of potassium (K) being lost to leaching on deep loamy sands in medium rainfall areas? To help answer this and other questions, CSBP established a long-term K trial last year at Regans Ford. The paddock had a good history of lime applications, spading, and deep ripping down to 500 mm.

Muriate of potash (MoP) was topdressed in March to supply 0, 40 and 120 kg K/ha. Replicated blocks of lupins and wheat were sown in early May.

Growing season rainfall was about 500 mm, with 120 mm in June, 110 mm in July, and 130 mm in August—enough to create leaching potential.

To assess K uptake from soil and fertiliser, crop biomass cuts were taken at flowering from treatments receiving 0 and 120 kg K/ha.  

Potassium uptake and yield

Results showed that K uptake was about three times higher in wheat and K recovery from MoP was double that of the lupins (Figure 1).

Despite differences in K uptake, there was no yield response to MoP in either crop. Wheat yielded 4 t/ha and lupins yielded 3 t/ha.

Soil potassium after harvest

To investigate the effects of K applications and crop rotation on soil K levels, soil samples were collected after harvest from five depths: 0-10, 10-25, 25-50, 50-75 and 75-100 cm.

Unsurprisingly, with lower plant K uptake by lupins, soil levels were generally higher after lupins than after wheat (Figure 2).

There was some indication that K from MoP may have moved deeper down the profile after lupins, but not beyond 75-100 cm.

Practical takeaway

These results suggest that on ameliorated loamy sands in medium rainfall areas, there is little risk of K loss through leaching in cropping paddocks.

This should give growers greater confidence to consider capital applications of K before seeding or to maintain robust fertiliser rates at seeding. Doing so should improve the efficiency of K supply to crops and simplify fertiliser logistics after seeding.

This season, the trial will be planted to canola to assess the residual value of last year’s K applications and to compare responses to K applied in 2025 after lupins and wheat. For more guidance on K fertiliser strategies, please contact your CSBP Account Manager.