We tested the impact of P deficiency, K deficiency, and their interaction on seed alkaloid concentrations and profile, yield and mineral content in sweet (low-alkaloid) and bitter (high-alkaloid) varieties of narrow-leafed lupin (Lupinus angustifolius L.). P deficiency reduced seed alkaloid concentrations in sweet, but not in bitter, varieties. Under P deficiency, the alkaloid profile in harvested seed of sweet varieties mimicked that of the bitter variety Fest, with 13-hydroxylupanine dominating over lupanine. With adequate or abundant P, lupanine was the predominant alkaloid in sweet varieties. K deficiency was associated with an 8-fold increase of seed alkaloid concentrations in the sweet variety Danja (from 1000 to 8000 mg kg(-1) DM), mostly due to the stimulation of lupanine production. There was a significant interaction between P and K that affected seed alkaloid concentrations in two ways: (i) the inhibitory effect of P deficiency was only apparent under K deficiency and (ii) the lowest seed alkaloid concentrations occurred with abundant K (240 mg K kg(-1)) and P (60 mg P kg(-1)). Seed yield of all varieties increased asymptotically with increasing P and reached a maximum at adequate P (30 mg P kg(-1)). There was no impact of K deficiency on seed yield. In sweet and bitter varieties P supply increased seed N, P and Zn concentrations, but not K. In contrast, seed K concentrations increased and P concentrations decreased with increasing K supply. These findings suggest that P fertiliser should be supplemented with K, to avoid high seed alkaloid concentrations stimulated by asymptomatic K deficiency at high P levels.