The optimal soil pH and sufficiency concentrations of N in plants and P, K and Ca + Mg in plants and soils were determined for alfalfa (Medicago sativa L.) and onion (Allium cepa L.) grown on three acid organic soils in a growth chamber experiment. The heaviest (14 t/ha) liming rate reduced soil acidity from pH 3.4 to 5.1 (1 N KCl), pH 3.9–6.2, and pH 2.6–4.9 on soils 1, 2 and 3, respectively. Liming increased alfalfa yields on all soils, but onion yields on only two soils out of three. Yields were, however, not increased by liming soils above pH 4.5 for alfalfa and 4.0 for onion. At maximum yields, the concentration of Ca + Mg in alfalfa at pH 4.5 was about 145 meq/100 g tissue while the concentration in onion at pH 4.0 was about 86 meq, concentrations that are typical for these crops grown successfully on other soils. Alfalfa and onion yields were not increased by fertilizing these soils at higher levels than 200 kg N, 100 kg P and 400 kg K/ha. The average sufficiency concentrations of plant nutrients for obtaining maximum yields were: tissue, 2.0 and 2.1% N, 0.2 and 0.24% P, 1.2 and 1.9% K; soil, 90 and 100 ppm P (13 and 15 μg P/cm3), and 0.8 and 0.9 meq K/100 g (44 and 50 μg K/cm3) for alfalfa and onion, respectively.