Effects of nitrogen, phosphorus and potassium on the acidity of tomato fruits

Abstract

These experiments were carried out to study the effects of mineral nutrients on the acidity of tomato fruits (var. Fukuju No. 2).1. In the solution culture, the titratable, total and combined acidities in the water extracts of whole fruits at the higher level of potassium (117ppm) were greater than those at the lower level (20ppm). Higher level of potassium induced the increases of the contents of potassium and inorganic phosphate. For the effects of phosphorus levels, the acidities and the potassium content at the higher level (31ppm) were lower than those at the lower level (5ppm). Inorganic phosphate content was increased at the higher level of phosphorus. To analyze such converse effects of both nutrients, partial correlation coefficients were calculated between the titratable acidity and the content of potassium or that of inorganic phosphate. It was shown that the titratable acidity was significantly correlated with the potassium content (r=0.96), but not with the inorganic phosphate content(r=-0.03). The same was true for the total acidity. Therefore the phosphorus effect seemed to be an indirect one on the acidities of fruits. The combined acidity and the potassium content at the higher level of nitrogen (140ppm) were lower than those at the lower level (40ppm). On the titratabie and total acidities, however, nitrogen levels had the different effects depending on the level of potassium supplied. At the lower level of potassium, the higher level of nitrogen decreased these acidities, whereas it increased them at the higher level of potassium.Gelatinous pulp weight expressed as a percentage of whole fruit was significantly decreased at the higher level of either nitrogen or phosphorus, but not affected by potassium levels.Examination of interaction between nutrient effect and fruit tissue indicated that effects of nitrogen or phoshorus on the acidities and the potassium content were not different with regard to the kind of tissues, while effect of potassium on the titratable and total acidities and the potassium content were greater in gelatinous pulp than in wall tissues.2. To study the effect of forms of nitrogen, the ratio of NO3-N to NH4-N was varied (10:0, 5:5 and 3:7), while the concentration of total nitrogen was held constant at 70ppm. Titratable and total acidities of fruits supplied with ammonium nitrogen (NO3: NH4=5:5 or 3:7) were lower than those of fruits supplied with nitrate nitrogen only (10: 0). Combined acidity and the potassium content were, however, decreased at the 5:5 solution only. The titratable acidity expressed as a percentage of total acidity was lower in the fruits supplied with ammonium nitrogen than those supplied with nitrate nitrogen only.3. The effects of various rates of phosphorus or potassium fertilizers were examined on fruits from plants grown in pots containing the volcanic ash soil. Potassium fertilizers had almost the same effect on the acidities and the potassium content with those found at the experiment in solution culture. Phosphorus showed no significant effects on the acidities.These results obtained above suggested that effect of potassium on the acidities was distinctly greater than those of nitrogen or phosphorus, and that the acidities of fruits were intimately related to the content of potassium. The apparent effect of potassium were thought to be based on the following two facts, namely (a) the almost proportional relations among the titratable, total and combined acidities were maintained in tomato fruits at the incipient colour stage, and (b) the combined acidity, i.e., the content of total cations, was linearly related to the potassium content.