Most sandy soils used for cropping in south-western Australia (SWA) have now become potassium (K) deficient due to removal of K in hay and grain, so it is now profitable to apply K fertiliser to most barley (Hordeum vulgare L.) crops in the region. Leaf diseases of barley crops in the region have increased in recent years particularly in the in medium to high (350–600 mm annual average rainfall) areas of SWA. Seventeen field experiments were undertaken to determine the effect of applications of K fertiliser, either the chloride (KCl) or sulfate source (K₂SO₄), on grain yield increases and on the percentage leaf area diseased (%LAD) when diseases were controlled or not controlled by fungicide sprays.

Maximum grain yield of barley was achieved where adequate K fertiliser (~8–22 kg K/ha) was applied and leaf diseases were controlled by fungicide. Applying increasing amounts of applied K fertiliser (0–120 kg K/ha) to barley decreased the %LAD by powdery mildew (Blumeria graminis f. sp. hordei Syn.) and spot-type net blotch (Pyrenophora teres f. maculata (Sacc.) Shoem.) and increased grain yield. By contrast, when leaf rust (Puccinia hordei G. Otth) was present the %LAD was unaffected by K application. When powdery mildew was the major disease, larger increases in grain yields and larger reductions in %LAD were obtained when KCl was used instead of K₂SO₄. About twice as much K fertiliser as K₂SO₄ was required for 90% maximum grain yield compared with KCl where powdery mildew was present. Applying larger amounts (>40 kg K/ha) of K fertiliser than required to achieve maximum grain yields did not further reduce %LAD by powdery mildew. There were no significant differences between the 2 sources of K fertiliser on the %LAD by spot-type net blotch.

Generally, the percentage protein content and hectolitre weight of grain were unaffected by K fertiliser. Potassium fertiliser decreased the percentage grain < 2.5 mm (known locally as screenings) and control of the foliar leaf diseases by applications of fungicide resulted in a decrease in protein content and screenings and increased hectolitre weight of barley grain.

The concentration of K in dried shoots that was related to 90% of the maximum shoot yield (critical diagnostic K) decreased as the plant matured, and was ~41 g/kg at Z22, ~30 g/kg at Z32, ~20 g/kg at Z40, and ~15 g/kg at Z59. The concentration of K in dried shoots which was related to 90% of the grain yield (critical prognostic K) decreased as plant matured, and was similar to critical diagnostic K values. Leaf disease had little effect on critical concentrations of K at early growth stages (Z22 and Z32).