Yield, nitrate levels and sensory properties of spinach as influenced by organic and mineral nitrogen fertiliser levels

Abstract

AbstractSpinach was fertilised at two levels of nitrogen, 140 kg/ha (x) and 420 kg/ha (3x), using split applications of ammonium sulphate and dried blood as nitrogen sources. A single application of ammonium sulphate (x) 19 days prior to harvest was also tested. A no‐nitrogen control was also employed. The x rate of mineral nitrogen produced statistically the same yield as the 3x organic source but at a lower tissue nitrate level. The 3x mineral fertiliser produced both the highest yield and nitrate concentration. Excluding late fertiliser application, nitrate levels were closely related to yield and dependent upon rate of nitrogen availability from each of the two sources. Late application of mineral nitrogen did not increase yield over a no‐nitrogen application control, but resulted in an eight‐fold increase in tissue nitrate. Triangle taste panels demonstrated significant differences in raw, cooked, and frozen samples when comparing the no‐nitrogen application against both the 3x organic and mineral nitrogen applications and the late mineral application. No significant sensory differences resulted between organic and mineral fertilisers at either application level. Colour measurements revealed that higher nitrogen application darkened colour. Organic sources gave darker colours than corresponding mineral fertiliser levels. Late application of mineral nitrogen resulted in the darkest colour. Gas–liquid chromatography headspace scans demonstrated that increasing nitrogen rates from x to 3x approximately doubled total peak area. No qualitative differences were noted. Late application of mineral fertiliser resulted in a headspace scan similar to those obtained at the 3x nitrogen rates.