Effects of ammonium and inorganic carbon enrichment on growth and yield of a hydroponic tomato crop

Effects of varying the proportions of NO 3 — and NH 4 + in the growth medium on seedling growth and tomato fruit yield ( Lycopersicon esculentum L. cv. Trust F1) were investigated in greenhouse hydroponic experiments. The presence of NH 4 + as the sole N source (11 mM) was toxic: it curtailed growth and decreased chlorophyll content of the leaves. However, at low concentration (10 % of total N), the presence of NH 4 + , with or without added dissolved inorganic carbon (DIC), increased vegetative growth and fruit yield by ˜ 15 %, and enhanced taste/flavor of the fruits. In DIC‐enriched treatment, pH was maintained at 5.8 by addition of KHCO 3 or as CaCO 3 . The presence of NH 4 + , at 10 % of total N, inhibited NO 3 — uptake rates by ˜ 27 %. The rates of uptake of NO 3 — and NH 4 + were comparable (13.3 and 14.2 mmol plant —1 d —1 , respectively, in the presence of DIC, and 14.7 and 14.0 mmol plant —1 d —1 , respectively, in the absence of DIC), despite such a large difference in their concentrations in the nutrient feed solution. A higher proportion of NH 4 + (up to 50 % of total N) had no further significant effect upon early vegetative growth, but in a long‐term experiment resulted in a high incidence of blossom end‐rot (BER) disease, thereby severely curtailing fruit yield. The presence of even 1.1 mM NH 4 + reduced Ca 2+ and Mg 2+ accumulation in the leaves as well as in fruits.