Effect of seaweed extract (Kappaphycus alvarezii) on the growth, yield and nutrient uptake of leafy vegetable Amaranthus polygamous

A greenhouse study was conducted during the dry (March to April) season of 2018 at the District Agriculture Training Center (DATC), Thirunelvely, Jaffna, Sri Lanka to study the effect of seaweed (Kappaphycus alvarezii) extracts (SWE) as foliar spray at the rates of 5.0% and 10.0% (v/v) on growth and yield of common leafy vegetable crop Amaranthus polygamous. The study was conducted as a three factor factorial with two levels of fertilizers (100% and 50% recommended chemical fertilizer dose (CF)) and two sources of irrigation water with different salinity levels collected from Thirunelvely (high salinity water, EC = 1 500 μS/cm) and Moolai (very high salinity water, EC = 12200 μS/cm). At the harvesting stage, highest plant height was observed with 100% chemical fertilizer and Thirunelvely water. However comparable plant growth was found with 50% chemical fertilizers when supplemented with 10% (v/v) SWE. In addition, number of leaves per plant, fresh weight of leaves, stem and whole plant (28.6 g) as well were highest in T2 (100% CF + Thirunelvely water). Replacement of 50% of fertilizer with SWE and Thirunelvely water yielded significantly comparable fresh plant weight (22.8 g), while addition of Moolai water resulted in poor yield (20.1 g) even with addition of 10% SWE. Leaf N, P, K and Na content further confirmed the effect of SWE on plant at harvesting stage. There was no significant difference in the percentage of N in leaves between chemical fertilizer alone and foliar application of SWE combined with 50% CF. In all treatments, which has received Moolai water contained significantly the highest Na concentration in Amaranthus leaves. The study indicates that foliar application of 10% (v/v) seaweed extract combined with 50% of recommended chemical fertilizer dose and irrigation with Thirunelvely water (at 1 500μS/cm) could be an effective alternative for sustainable cultivation of Amaranthus polygamous.