Nutrient accumulation in vegetable crops is declining. New varieties, selected for high yield, may be subject to a dilution effect of nutrient concentration. Alternatively, soil fertility may be to blame. Here, we investigate how nitrogen fertilization can enhance or suppress calcium and potassium content in two lettuce varieties already known to accumulate high or low amounts of these nutrients. Effects of varying the ammonium:nitrate ratio and effects of calcium carbonate buffering on plant growth by mass and on uptake and accumulation of potassium and calcium in two lettuce (Lactuca sativa L.) cultivars, Two Star and Red Deer Tongue, were investigated in three greenhouse hydroponic experiments in which ammonium supplied none, 6%, 12%, 25%, 50%, 75% or all of the nitrogen. Ammonium, supplied as the sole nitrogen source (15 mM), was toxic under buffered or unbuffered conditions. It limited growth and concentrations of potassium and calcium in lettuce leaves. Proportions of ammonium-N greater than 50% of total N nutrition severely curtailed growth and nutrient accumulation for both cultivars. For both cultivars, optima for all three variables occurred in treatments that contained less than 50% NH4+-N in the total N supply. Application of calcium carbonate buffer did not result in improved maxima for growth and shoot potassium or shoot calcium concentrations compared to the best responses in unbuffered solutions. However, supplying calcium carbonate buffer did raise the minima for growth and shoot potassium and shoot calcium concentration. Both cultivars in buffered solutions compared to unbuffered solutions had significantly greater values for growth and for shoot potassium or shoot calcium concentration in treatments that contained 50% ammonium-N or greater in the total N supply. Although buffering relieved symptoms of ammonium toxicity, it did not eliminate symptoms, confirming the work of other researchers that ammonium toxicity is not due solely to acidification of the root-zone and that buffering has an effect on the capacity of plants to tolerate ammonium nutrition. Supplying nitrogen with ammonium:nitrate ratios in which nitrate predominates enhances yield and accumulation of calcium and potassium in lettuce. Two Star, the modern variety, is more ammonium-sensitive than Red Deer Tongue, the heirloom variety, if calcium carbonate buffering is not provided.