Effects of fertilisers on vegetable production 2. Effects of nitrogen fertilisers on nitrogen content and nitrate accumulation of spinach and beetroot

The effects of 5 different nitrogenous fertilisers (ammonium sulphate, ammonium sulphate + N‐serve, potassium nitrate, poultry manure, and urea) each at 5 application rates (0, 150, 300, 450, or 600 kg N/ha) on the total nitrogen (N) and nitrate content of 2 varieties of spinach ( Spinacea oleracea L.) and 1 variety of beetroot ( Beta vulgaris L.) were evaluated in a pot experiment using Wakanui silt loam soil. The nitrification inhibitor, N‐serve, was used to retard the microbial conversion of ammonium to nitrate. Total N and nitrate N generally increased with increasing rates of N applied. Potassium nitrate application resulted in the highest N uptake levels, and poultry manure application gave the lowest. N‐serve depressed N uptake at 600 kg N/ha applied. Plants accumulated nitrate to a level potentially hazardous to health at only the highest rate (600 kg N/ha) of potassium nitrate, ammonium sulphate, and urea applied. Differences between the 2 varieties of spinach tested were not significant.