To ensure that narrow-leafed lupin (Lupinus angustifolius L.) meets feed quality standards, the concentration of alkaloids must be kept under the maximum acceptable limit of 200 mg kg−1 DM. One of the factors that may affect seed alkaloid concentration is soil nutrient deficiency. In this paper, we report the results of glasshouse and field experiments that tested the effect of potassium (K) deficiency on seed alkaloid concentrations. In the glasshouse, seed alkaloid concentrations increased by 385, 400 and 205% under severe K deficiency in sweet varieties (Danja, Gungurru and Yorrel, respectively) of L. angustifolius. The concentration of alkaloids in Fest, the bitter variety, was always high regardless of soil K status. At all levels of applied K (0–240 mg kg−1 soil), lupanine was the predominant alkaloid in sweet varieties, whereas 13-hydroxylupanine prevailed in the bitter variety. Seed yield of all varieties increased exponentially with increasing amounts of applied K, reaching a maximum at 60 mg K kg−1 soil. In the field, application of K to deficient soils decreased seed alkaloid concentration at Badgingarra, Western Australia (WA) but not at Nyabing, WA, in 1996. In both field trials, seed yield and mineral content were not affected by the amounts of K fertiliser applied. These findings highlighted the need for adequate K fertilisation of deficient soils in WA to avoid the risk of producing low quality lupin seed with high alkaloid concentrations. K deficiency is involved in stimulating alkaloid production in sweet varieties of L. angustifolius.