Effect of nitrogen, phosphorus and potassium fertilisers on the non‐volatile organic acids of tomato fruit

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Abstract
The effect of two levels each of N, P and K in factorial combination on the acidity and the individual non‐volatile organic acids in ethanolic extracts of whole, ripe tomato fruit has been studied. The concentrations of the two principal acids, citric and malic, were both significantly increased by high levels of K in the soil. Although the main effect of N was not significant, significant interactions between N and K were found with respect to both acids, and a significant interaction between N and P with respect to citric acid.Glutamic and aspartic acids and pyrrolidonecarboxylic acid (PCA) were all increased by N applications and decreased by phosphate. There was no significant main effect of K on glutamic acid concentrations, but aspartic acid and PCA were significantly decreased by high K levels in the soil. Changes in glutamic and aspartic acids were reflected in the total α‐amino‐N content; the highest concentrations were found where high N was combined with low phosphate. Addition of K decreased the total α‐amino‐N at the low, but not at the high level of N.