Study on nitrate accumulation in water spinach (Ipomoea aquatica) and leaf mustard (Brassica juncea L.) fertilised by the biogas effluent

Abstract

The objective of the study was to compare the content of nitrate in water spinach (Ipomoea aquatica) and leaf mustard (Brassica juncea L.) and their productivity between being irrigated with inorganic fertilizers and with biogas effluent at different dosages. Three crops for each plant with six treatments were arranged randomly, including irrigating by 100% biogas effluent (1), irrigating by 75% biogas effluent + 25% inorganic fertilizer (2), irrigating by 50% biogas effluent + 50% inorganic fertilizer (3), irrigating by 25% biogas effluent + 75% inorganic fertilizer (4), irrigating by 100% inorganic fertilizer (5), and irrigating by 100% pond water (6). The results showed that the yield of water spinach and leaf mustard in treatments using biogas effluent increased gradually through the crops. In the third crop, the yield of water spinach was highest when irrigated by 100% biogas effluent (at 2.9 kg/m2 ), and that of leaf mustard was highest when irrigated by 100% biogas effluent as well as 75% biogas effluent + 25% inorganic fertilizer (at 1.9-2.1 kg/m2 ) and was not different in comparison with the treatment by 100% inorganic fertilizers. There was no significant difference between the accumulation of nitrates in both water spinach and leaf mustard in each treatment, while the nitrate contents in the treatments using combined biogas effluent and inorganic fertilizers were always lower than that of the treatment using 100% inorganic fertilizers. The nitrate content of water spinach was lowest when irrigated by 100% biogas effluent (39.4-52.6 mg/kg fresh vegetable). The leaf mustard’s nitrate content was lowest when irrigated by 75% biogas effluent + 25% inorganic fertilizer (32.1-37 mg/kg fresh vegetable). And they had no significant difference compared with the nitrate content of those irrigated by 100% pond water treatment.