Abstract:
Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai var.
Crimson Sweet) was seeded in a commercial peat mix in multicell
containers (72 cells-tray⁻¹, 40 cm³-cell⁻¹) in July 1989 and April 1990.
In 1989, the medium was amended with CaC0₃ at 10% w:v. In 1990, no
CaC0₃ amendment was made. Watermelon seedling growth and mineral
composition response to a factorial treatment combination of 5 nitrogen
form ratios and 5 levels of supplemental calcium applied within a
100-31-265 mg-liter⁻¹ NPK pretransplant nutrition regime were analyzed.
In 1990 seedlings were transplanted in the field to determine
pretransplant treatment effects on seedling establishment and yield.
In 1989 shoot growth decreased with increasing NH₄-N; in 1990, N
ratio had varying effects on shoot growth parameters. In 1989, CaC0₃
amendment ameliorated N ratio effects on plant mineral composition; in
1990, increasing NH₄-N depressed cation uptake. In 1989, plants given N
ratios with greater than 50% N03-N had greater N content and total
uptake. In 1990, increasing NH₄-N produced greater shoot % N.
Reduction in plant growth with increasing Ca was greater in 1990
than 1989, due to higher medium EC. Increasing Ca did not affect shoot
N in 1989, but in 1990 N accumulation was greatest with 4 to 8
mmol-liter⁻¹ Ca. Increasing supplemental Ca reduced K and Mg uptake in
1989, and Mg in 1990.
In the field, seedling establishment and early yield were greater
with 100% NH₄-N and supplemental Ca at 8 and 16 mmol-liter⁻¹. Late
yield was not affected by treatments. Total yield was not affected by N
ratio; however, total yield was greater with supplemental Ca than
without. Differences in yields were due to an increase in fruit number,
and not fruit size. Although yield and fruit number increased with
increasing shoot N concentration, shoot N was not the primary factor
affecting yield.
It was concluded that NH4-N at 100 ppm and Ca level of 8 and 16
mmol-liter⁻¹ in the pretransplant fertilization regime were optimal for
watermelon transplant production.
In an additional experiment, supplemental Ca was applied within an
NH4-N based NPK (100-31-265 mg-liter⁻¹) pretransplant fertilization
regime with and without calcium carbonate amendment to the medium (10%
w:v) .
Supplemental Ca had no effect on dry weight, leaf area, or shoot N
content 4 weeks after seeding. Calcium carbonate amendment decreased
shoot dry weight and plant height, while increasing shoot N
concentration. Shoot N accumulation was not affected by CaC0₃, medium
amendment. Medium pH increased with CaC0₃ addition.
No net nitrification was observed in response to any treatment
over the duration of the experiment.
