Towards optimization of growth via nutrient supply phasing: nitrogen supply phasing increases broccoli (Brassica oleracea var. italica) growth and yield

A greenhouse experiment on broccoli (Brassica oleracea var. italica, cvs Wi ndsor and Arcadia) was carried out in order to demonstrate that supplying n itrogen (N) to meet the nitrogen demands of plant growth stages, through N phasing, improves plant growth and yield, as compared to fertilizing at the conventional, optimal, constant N rate. Two broccoli cultivars and two rat es of starter nitrogen fertilizer (optimum, 250 mg l(-1) and sub-optimum, 1 50 mg l(-1)), were combined with three timings of fertigation change. Shift ing N rate, at 60% and 75% of the market plant growth cycle significantly i ncreased shoot dry weight and head fresh weight, compared to the constant-N rates treatments (controls). The highest yield and shoot dry weight were o btained when the N-rate was switched from the optimum level(250 mg l(-1))to the sub-optimum level (150 mg l(-1)) at inflorescence initiation. The nitr ogen-to-growth-stage-fitness effect was determined and partitioned into rat e effect and phasing effect. The phasing effect was greatest, on both shoot dry weight and head fresh weight, at inflorescence initiation, and subsequ ently decreased until harvest time. None of the interactions was significan t. The results demonstrated the superiority of nitrogen supply phasing over the conventional fixed-rate-supply method.