Vegetable transplant producers supply approximately $250 million worth of transplants to vegetable growers throughout the United States. Proper nutrient application for transplant production is important for crop establishment and minimizing excessive fertilizer waste which can negatively affect surface and groundwater. However, nutrient application guidelines for vegetable transplants grown in soilless substrate are limited. Therefore, researchers undertook a study to determine optimal nitrogen (N) concentrations for the top five transplanted vegetable crops produced in California. Greenhouse trials were conducted on leaf lettuce, romaine lettuce, processing tomato, broccoli, and celery transplants. Three treatments were applied in each trial (One 200-cell plug tray per treatment, replicated five times each): (1) 400 ppm N; (2) 200 ppm N; (3) 50 ppm N. Fertigation was applied to trays placed on weighing-lysimeters and total daily transpiration was recorded. Once transplants were fully developed, they were harvested and analyzed for shoot fresh weight and shoot dry weight. Fresh plant tissue was sent to an agricultural laboratory for nutrient content testing. Total nutrient uptake (mg) was calculated by multiplying nutrient tissue content (%) by shoot dry weight (mg). N fertilizer concentration (mg*L-1) was calculated by dividing the total N uptake value (mg) by transpiration (L). Transplants in the 400 ppm treatment had significantly higher N tissue content, compared to the 200 and 50 ppm treatments, in all crop trials except for leaf lettuce. In the leaf lettuce trial, the 400 and 200 ppm treatments had similar N tissue content. Average shoot dry weight was similar between the 200 and 400 ppm N treatments in all five crops, indicating that both treatments provided sufficient N. Based off these results, we recommend applying 246 ppm N to leaf lettuce, 232 ppm N to romaine lettuce, 304 ppm N to processing tomato, 437 ppm N to broccoli, and 262 ppm N to celery transplants. These values are based off the calculated N fertilizer concentrations which produced the highest shoot dry weights.
