Nutrient requirements for lettuce transplants using a floatation irrigation system - II. Potassium

Although floatation irrigation has numerous advantages for vegetable transp lant production, including improved seedling health, lettuce (Lactuca sativ a L.) transplants grown with floatation (ebb and flow) irrigation can have poor root systems. Floatation fertigation of 'South Bay' transplants with K at 15, 30, 45, or 60 mg .L-1 K applied every 2 to 4 days, increased fresh and dry root weight at 28 days. Higher K (24 mg . kg(-1)) in the medium did not affect root weight. Fresh and dry shoot weight, leaf area, relative sh oot ratio (RSR), relative growth rate (RGR), leaf weight ratio (LMR), and r oot weight ratio (RMR) were unaffected by applied K, regardless of the init ial K concentration in the medium. Available K in a vermiculite-containing medium may have supplied all the K required. When 60 was compared with 100 mg .L-1 N at various levels of K, the applied K again did not influence dry root weight; however, at 100 mg .L-1 N, root weight was reduced as compare d with 60 mg .L-1 N, regardless of the level of applied K. In a field exper iment, pretransplant K had no effect on growth. Transplants grown with no a dded K in a peat + vermiculite mix with at least 24 mg .L-1 water-extractab le K produced yields equivalent to transplants supplied with 15, 30, 45, or 60 mg .L-1 K via floatation irrigation.