Nutrient requirements for lettuce transplants using a floatation irrigation system. I. Phosphorus

Several levels of P were supplied via floatation irrigation to 'South Bay' lettuce (Lactuca sativa L.) transplants to determine the optimum P concentr ation necessary. Plants were propagated by floating flats (ebb and flow sys tem) in a nutrient solution containing P at either 0, 15, 30, 45, or 60 mg .L-1 in summer and fall experiments, and either 0, 15, 30, 60, or 90 mg .L- 1 P in a factorial combination with 60 or 100 mg .L-1 N in a winter experim ent. Adding more than 15 mg .L-1 P had minimal effect on growth. Transplant s produced with 0 P grew poorly, regardless of the level of N applied. Nitr ogen at 100 mg .L-1 improved the response of shoot growth to any level of P , but adversely affected root growth compared with N applied at 60 mg-L-1. In general, relative growth rate was improved, while net assimilation rate was reduced at all levels of P. High-quality transplants had a root to shoo t ratio of about 0.25, total root lengths between 276 and 306 cm, and total root area between 26 and 30 cm(3) in a 10.9-cm(3) cell volume. Only 30 % o f the plants produced without P could be pulled from the transplant flats, whereas 90 % could be pulled when P was added. Pretransplant P hastened mat urity and increased lettuce head weight at harvest in the field. This work suggested that at least 15 mg .L-1 P, supplied via floatation irrigation to a peat + vermiculite mix, was required to produce a transplant with suffic ient roots for ease of pulling, rapid field establishment, and earlier harv est.