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Fertilizer regimes were evaluated on edible amaranths to understand fertilizer management on the red soil (pH 5.1) of Okinawa. Effects of fertilizers 0 (Control), N, P, K, N+P (NP), N+K (NK), P+K (PK) and N+P+K (NPK) were evaluated on red leaf amaranth in two experiments. Each of the N, P and K fertilizers was applied at 50 g m-2. In addition, the effects of NPK (N:P:K=1:1:1) fertilizer at 0, 10, 20, 30 and 40 g m-2 were evaluated on red stem amaranth and red leaf amaranth. Growth and yield of amaranth cultivated under N, P, K, NK and PK treatments were very poor, but significantly higher with the NPK followed by NP. Growth parameters and yield greatly increased with the NPK fertilizer at 30-40 g m-2 for red stem amaranth and 20-30 g m-2 for red leaf amaranth. Mineral contents in the amaranths did not clearly differ with the different fertilizers. Mineral contents were higher or same in the amaranths cultivated with the fertilizer NPK at 30-40 g m-2, compared to those under control treatments. The results indicate that combined fertilizer NPK at 30-40 g m-2 is effective for higher yield and quality of amaranth in the red soil. [ABSTRACT FROM AUTHOR]

Amaranthus is a promising food crop with unique nutritional compositions. Fertilizer treatments including 0 (Control), N, P, K, N + P (NP), N + K (NK), P + K (PK) and N + P + K (NPK) were evaluated based on growth, yield and quality of edible red leaf-amaranth (Amaranthus spp.) cultivated on dark-red soil (pH 6.6) to understand fertilizer management. N, P and K were applied at 50 g m-2 according to the treatments in one of the experiments. Effects of the fertilizer NPK (N:P:K = 1:1:1) at 0, 10, 20, 30 and 40 g m-2 were evaluated on red stem-amaranth and red leaf-amaranth in the same soil during another experiment. P and K fertilizers applied alone did not promote growth parameters and yield of amaranth, whereas N alone did. The fertilizer K promoted the function of N slightly but P did the same significantly regarding amaranth growth. Growth parameters and yield were the highest with the combined fertilizer NPK followed by NP. Yield and L-ascorbic acid content of amaranth were the highest with fertilizer NPK at 40 g m-2. This study indicates that fertilizer N and P are more effective, and combined fertilizer NPK at 40 g m-2 is better for higher yield and quality of amaranth on dark-red soil in Okinawa. [ABSTRACT FROM AUTHOR]