J Sci Food Agric 1998, 76, 341È346 Nitrogen Nutrition, Yield and Qualit y of Spinach A Elia,1,* P Santamaria2 and F Serio1 1 Institute of Vegetable Crops for Processing, CNR, via Amendola, 165/A, 70126, Bari, Italy 2 Institute of Agronomy and Field Crops, University of Bari, via Amendola, 165/A, 70126, Italy (Received 21 April 1997 ; accepted 24 June 1997) Abstract : When grown in solution culture spinach plants conÐrmed the prefer- ence toward nutrition and showed heavy toxicity to In open Ðeld NO 3 ~ NH 4 ` . condition the highest yield was achieved with the ammonium sulphate in Bari (autumnÈwinter cycleÈ110 days) and with calcium nitrate in Policoro (winterÈ spring cycleÈ64 days). By increasing N level, yield, nitrates and oxalates leaf content increased. Oxalate content was not a†ected by nitrogen form. Remark- able di†erences were observed between leaf petiole and blade in nitrate (4062 vs 925 mg kg~1 of fresh mass) and oxalate (1051 vs 6999 mg kg~1 of fresh mass). ( 1998 SCI. J Sci Food Agric 76, 341È346 (1998) Key words : spinach ; N-form ratio ; N-fertilisers ; inorganic anions ; nitrate ; oxalate INTRODUCTION Plants only use about 50% of the applied N (Newbould 1989), which implies a large loss in money and energy. Farmers use at least double the quantity required by the plants, and unused is leached or denitriÐed NO 3 -N causing environmental pollution (Byrnes 1990 ; Smith et al 1990 ; Davies and Sylvester-Bradley 1995). N efficiency, the lowest the greater applied N (Brown 1978), can be improved by maintaining higher level of compared to in the soil. This can be achieved NH 4 NO 3 by nitriÐcation inhibitors (Prasad and Power 1995), or slow-release fertilisers (Shaviv and Mikkelsen 1993), or fertilisation with ammonium fertilisers (Lips et al 1990) or large quantity of ammonium fertilisers (Shaviv 1988). On account of the above remarks, we have been carrying out studies on nitrogen and ammonium nutri- tion in vegetables aimed to increasing nitrogen use effi- ciency. A large part of such a research work focuses on leaf vegetables which under high availability, NO 3 -N accumulate large amounts of nitrates (Santamaria et al 1997 ; Santamaria and Elia 1997), a compound believed to be potentially toxic to human health (Walker 1990 ; Gangolli et al 1994). On 31 January 1997, the European Commission issued a regulation stating the maximum acceptable * To whom correspondence should be addressed. nitrate concentrations in lettuce and spinach (European Community O J, L31 of 1 February 1997). For spinach the following limits are Ðxed : 3000 and 2500 ppm on fresh weight basis for crops harvested from 1 November to 31 March and from 1 April to 31 October, respec- tively. On 1 January 1999 only the concentration of 2500 ppm shall be accepted (for packaged, frozen or deep frozen spinach this concentration is 2000 ppm). Spinach (Spinacia oleracea L) is greatly responsive to nitrogen (Cantli†e 1972a,b ; MagniÐco et al 1992) and even if increased total amino acid concentration (Eppendorfer and Bille 1996) like other Chenopo- SÔren diaceae it not only accumulates nitrates, but is also rich in oxalates, which are hazardous to human health (Libert and Franceschi 1987). The goal of the present study is to evaluate yield response of spinach for the two nitrogen inorganic forms applied and both in solution culture (NH 4 ` NO 3 ~) and in open Ðeld conditions and to assess accumulation of nitrates, soluble oxalate and inorganic anions under di†erent N regimes and cultivation conditions. MATERIALS AND METHODS Three trials were carried out. One in growth chamber and in solution culture and two in open Ðeld conditions. In all trials the cultivar Gladiator Hybrid was used. 341 1998 SCI. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain (