The role of crop calcium nutrition in fresh produce quality and the control of damaging physiological disorders such as tipburn in lettuce is widely recognized within the California vegetable industry.  Application of calcium fertilizers with irrigation water is a common practice for lettuce (Lactuca sativa L.) production, despite the fact that most California soils have high soil test Ca levels.  Research was conducted in 2005-06 to evaluate the efficacy of this practice, and to identify factors other than soil Ca status that impact the occurrence and severity of lettuce tipburn.  Soil samples were collected from 15 representative mineral soils in the lettuce producing region and subjected to ammonium acetate extraction, saturated paste extraction and extraction of soil solution by centrifugation to characterize soil Ca status.  In a greenhouse experiment lettuce was grown in these soils and the plant Ca uptake determined.  Expressed as a percent of cation uptake, plant Ca was most closely correlated with soil solution Ca (r = 0.87, p < 0.01).  Saturated paste Ca was strongly correlated with soil solution Ca (r = 0.93, p < 0.01), although even after adjustment for dilution effects saturated paste Ca concentration was less than 50% of soil solution Ca.  In 2 field trials calcium nitrate, calcium thiosulfate and calcium chloride fertilizers applied at 15 meq L^-1 in drip irrigation water during the final 2 weeks of growth had minimal effects on tissue Ca concentration and tipburn severity.  A survey of commercial lettuce fields at harvest stage showed that tipburn severity was not correlated with either soil Ca status or lettuce Ca concentration.  Severe tipburn occurred only under conditions of low reference evapotranspiration (ET_o) and moderate temperature; under these conditions a transient Ca deficiency was apparently induced in rapidly growing leaves by the restricted volume of transpirational flow.