Although soil salinity and potassium (K) deficiency are widespread in agricultural lands, there is a paucity of knowledge about the interactive effects of sodium (Na) and K on the growth and yield of major grain crops. In pot experiments, we examined salt tolerance of canola (Brassica napus L.), narrow-leafed lupin (Lupinus angustifolius L.) and wheat (Triticum aestivum L.), and crop K requirement under Na supply ranging from low to high. Plant growth and seed yield of all three crops were lower at 40 mg K/kg than at 100 mg K/kg soil. Although 100 mg Na/kg (4 dS/m in soil solution) had little effect on canola cv. Boomer and wheat cv. Wyalkatchem, the salt-treated narrow-leafed lupin cv. Mandelup died at 47 days after sowing, regardless of amount of soil K. In low-K soils, canola with 100 mg Na/kg and wheat with 50 mg Na/kg did not show K-deficiency symptoms and produced greater seed yield than plants with nil Na addition. At 100 mg K/kg, Na-induced reduction in growth and yield occurred only to plants with 200 mg Na/kg. However, at 160 mg K/kg, 200 mg Na/kg did not have an adverse effect. In canola and wheat, shoot K concentration increased and shoot Na concentration decreased with increasing amount of soil K; however, high soil K did not reduce shoot Na concentration in narrow-leafed lupin. The study showed that narrow-leafed lupin was very susceptible to salinity, whereas canola and wheat plants were relatively salt-tolerant. The stimulation of growth and yield in canola and wheat by low–moderate Na in low-K soils suggests partial K substitution by Na, and that adaptation of canola and wheat to salt-affected soils can be enhanced by high K supply.