The project reported here aims to help grain growers diagnose where K deficiency is limiting their productivity and profitability. It also aims to identify the most cost-effective applications of K fertiliser, where they are needed the most, and to avoid wasteful applications where K deficiency is unlikely.

• Two K response trials (0 and 150 kg K/ha) were established in 2022. 
  • At Maitland (Colwell K 98 mg/kg) wheat was responsive to K, increasing in grain yield from 5.4 t/ha to 7.4 t/ha with added K. Residual effects from the K application were also measured the following season in lentil.  
  • At Arthurton (Colwell K 200 mg/kg) lentil (2022) and wheat (2023) were not responsive to K fertiliser application.  
• Potassium application strategy trials showed wheat grain yields were highest where K (MoP) was banded below the seed at rates of 50 and 75 kg K/ha. These two treatments consistently showed increased biomass, shoot K and K uptake early in the season. 
• The response from broadcasting K was variable (no response or moderately responsive) across two seasons. 
• Foliar application of K was not an effective application strategy. 
• These results suggest the soil test K critical limit for wheat on soils of the Yorke Peninsula is much higher than the 45 mg/kg currently used by the grains industry.