Foliar and soil application of potassium nitrate improves tomato yield and fruit quality under salinity conditions.

A pot experiment was carried out with tomato plants(Solanum lycopersicum L.) to find out how to improve its yield and quality under salinity conditions. The experiment was conducted in a greenhouse in Pakistan to study the effects of different potassium nitrate doses and modes of application in three salinity treatments (0, 7.5 and 15 dS/m, induced by increasing the NaCl dose). Nitrate was applied to the soil (0, 3.3 and 6.6 mmol/kg) or as foliar spray (4.5 and 9 mM). All treatment combinations were conducted on four cultivars: 2 salt-tolerant (Indent-1 and Nagina) and 2 salt-sensitive (Peto-86 and Red Ball) field tomato genotypes. Tomato plants, upon reaching the 5-leaf stage, were transplanted into pots containing 12 kg of sandy-loam soil collected from the field. The recommended doses of N (210 kg/ha) and P (125 kg/ha) fertilizers were applied, and the amount of N applied in the form of KNO ₃ was also taken into account. No additional Ca, Mg, S or B fertilizers were applied, since the basic content of these nutrients contained in the soil was sufficient to support the growth of the tomato plants. The experiment was designed in a completely randomized block with 3 replications, using a factorial distribution.

The application of _KNO3 at increasing doses resulted in increased yield, both in terms of fruit weight and fruit number, regardless of the mode of application, in the absence of salinity stress. Potassium nitrate also resulted in increased fruit size. Generally, increasing salt stress (7.5 and 15 dS/m) results in significantly lower yields. This is mainly due to fruit size, which decreases as salinity increases. Conversely, fruit size increased as potassium nitrate concentration increased.

For those plants exposed to salinity, _KNO3 mitigated the negative effects of salinity and, depending on the dose, resulted in increased size and yield of both salt-tolerant and salt-sensitive plants compared to the respective control treatment groups. However, in soils with different salinity levels, salt-tolerant genotypes maintained better growth and higher yields than salt-sensitive genotypes and showed a better response to _KNO3 treatments. Total soluble solids content, titratable acidity, pH and fruit dry matter content improved significantly with both increased salinity and potassium nitrate application.

It can be concluded that the use of salt tolerant tomato genotypes and an increase in potassium nitrate application, either by foliar or soil application, can be used as an effective approach to utilize soils with some degree of salinity in tomato production.