The effect of varying P concentrations on the growth, yield and development of leaf abnormalities, nutrient content and uptake of other nutrient elements was investigated for lettuce.Three cultivars (Chaparal, Wintercrisp and Robinvale) were studied in sand cultures and soil. The frequency of changing the nutrient solution had an effect on the P level that gave the highest yields or which caused reduced yields and leaf abnormalities. The leaf P content for optimum yields varied between 0.4 and 0.9, while higher P contents caused leaf abnormalities in Chaparal and Wintercrisp. With high P concentrations, more Ca needed to be taken up to overcome the detrimental effect of P. In soil, maximum yields were obtained with 540 mg kg#x2212;1P applied. Soil analysis at the end of the experiment on these treatments showed that a P content of 250 mg kg#x2212;1P (Bray II) coincided with the highest yields. Yield suppression due to high P concentrations was reduced when the Ca concentrations was increased. The leaf abnormalities which coincide with high P concentrations were very similar to those which have previously been described for auxin-type herbicides.
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