The nitrogen form and NH4+:NO3- ratio affect the calcium uptake and concentration in a number of fruit vegetables, such as tomato, sweet pepper, eggplant, melon and watermelon (Bar-Tal, 2001a; McLaurin, 1998). High levels of ammonium in the nutrient or soil solution will act antagonistic towards the uptake of calcium, which might result in calcium deficiency, shown as blossom end rot (BER) in the fruit of various fruit vegetable crops (Wilcox et al, 1973; Marti and Mills, 1991; Bar-Tal et al, 2001a, b; Langenhoven, 2004; Pill and Lambeth, 1980). In particular, fruit vegetable crops grown in hydroponics or in inert growing media, which are exposed to an imbalanced, high ammonium to calcium ratio in the root environment, will be more sensitive to BER incidence. Therefore, Dutch guide values for analytical data of the nutrient solution in the root environment, for crops grown in substrates or in water, recommend to keep the ammonium level below 0,5 mmol NH4+ per litre, while the ammonium level in the nutrient solution ranges from 1,0 to 1,5 mmol NH4+ per litre (Sonneveld and Straver, 1994).
Bar-Tal, A., B. Aloni, L. Karni and R. Rosenberg. 2001 a. Nitrogen nutrition of greenhouse pepper: I. Effects of nitrogen concentration and NO3:NH4 ratio on yield, fruit shape, and the incidence of blossom-end rot in relation to plant mineral composition. HortScience 36: 1244-1251.
Bar-Tal, A., B. Aloni, L. Karni, J. Oserovitz, A. Hazan, M. Itach, A. Avidan, I. Posalski and R. Rosenberg. 2001 b. Nitrogen nutrition of greenhouse pepper: II. Effects of nitrogen concentration and NO3:NH4 ratio on growth, transpiration, and nutrient uptake. HortScience 36: 1252-1259.
Langenhoven, P. 2004. Yield and quality response of hydroponically grown tomatoes (Lycopersicon esculentum Mill.). to nitrogen source and growth medium. PhD thesis University of Stellenbosch, South Africa. 211 pp.
Marti, H.R. and H.A. Mills. 1991. Calcium uptake and concentration in bell pepper plants as influenced by nitrogen form and stage of development. Journal of Plant Nutrition 14: 1165-1175.
McLaurin, W.J. 1998. Blossom-end rot. Horticulture Fact Sheet H-98-036. The University of Georgia College of Agricultural & Environmental Sciences Cooperative Extension Service.
Pill, W.G. and V.N. Lambeth. 1980. Effects of soil water regime and nitrogen form on blossom-end rot, yield, water relations and elemental composition of tomato. Journal of the American Society for Horticultural Science 105: 730-734.
Sonneveld, C. and N. Straver. 1994. Nutrient solutions for vegetables and flowers grown in water or substrates. No 8, series: Voedingsoplossingen glastuinbouw. 10th ed. Proefstation voor Tuinbouw onder Glas te Naaldwijk, Proefstation voor de Bloemisterij te Aalsmeer. 45 pp.
Wilcox, G.E., J.E. Hoff and C.M. Jones. 1973. Ammonium reduction of calcium and magnesium content of tomato and sweet corn leaf tissue and influence on incidence of blossom end rot of tomato fruit. Journal of the American Society for Horticultural Science 98: 86-89.