Fertiliser nitrogen and potassium studies with Flora-Dade tomatoes grown with trickle irrigation and polyethylene mulch covered beds on krasnozem soils

Abstract

Field experiments were conducted with Flora-Dade tomatoes on krasnozem soils during 1985-86 (site I), 1986-87 (site 2), and 1987-88 (site 3) to examine the effect of nitrogen (N, 5420 kg/ha) and of potassium (K, 1120 kg/ha) on fruit yield and quality and leaf nutrient composition. Nitrogen and K were applied either pre-planting to first fruit set, or at increasing weekly increments from 1 week after transplanting to mid fruit harvest. At each site, one rate of N and one of K were based on a commercial soil chemical analysis. The yield and quality of fruit at all sites was not affected (P>0.05) by N or K fertiliser rate or by method and timing of application. Marketable yield was 83-1 18 t/ha and fruit firmness (compression) was 0.97-1.27 mm. At site 3, which had the lowest exchangeable K concentration [0.3 cmol(+)/kg], the addition of 90 kg K/ha increased the yield of large fruit. At all sites, and with the nil-N treatment (site 3), the youngest fully opened leaf (YFOL) petiole sap nitrate-N concentrations exceeded critical values (Coltman 1987, 1988; Huett and Rose 1988) at all sampling times. YFOL concentrations were highest at 2-6 weeks after transplanting, then declined over the growth period. The highest concentration recorded at site 1 was 5.6 g/L, and at site 2, 3.2 g/L. These concentrations were not affected (P>0.05) by N fertiliser rate, indicating greater mineralisation of organic N at sites 1 and 2 than at site 3, where the highest petiole sap nitrate-N concentration was 1.8 g/L. The pre-plant soil nitrate concentrations (0-15 cm depth) at sites 1 and 3 were similar (14 and 16 mg/kg), and when measured 6 weeks after transplanting at site 3, the concentrations in the nil and 120 kg N/ha treatments were 31 and 66 mgkg, respectively. Neither pre-planting soil test (N or K) accurately predicted fertiliser response by tomatoes. The application of supra-optimal rates of N and K to semi-determinate fresh market tomatoes of Flora-Dade type will not be detrimental to yield, composition, and firmness of fruit. For a 70 t/ha crop, 130 kg N/ha and 208 kg K/ha are equivalent to removal by fruit.