Optimizing Nutrient Application for Vegetable Transplants

Producing high-quality vegetable transplants requires careful fertilizer management to ensure sufficient growth and uniform plant development while minimizing input costs and nutrient losses. With rising fertilizer costs and regulatory pressure to decrease nitrogen pollution in California, efficient fertilizer usage is increasingly important. 

Frequent irrigation and high fertilizer application rates can increase the risk of nutrient leaching, particularly when fertilizer applications exceed crop demand. But inadequate fertilization can result in reduced transplant growth, delayed production schedules, and lower-quality transplants. As a result, identifying fertilizer practices that maintain transplant quality while reducing nutrient loss is an important area of focus.

To investigate optimal fertilizer application rates for vegetable transplants, we selected the top 5 economically important transplanted vegetables in California: celery, broccoli, processing tomato, leaf lettuce, and romaine lettuce. For each vegetable transplant, we analyzed the effects that three different nitrogen application rates had on plant growth. We applied 50, 200, and 400 ppm N through fertigation using 20-10-20 NPK fertilizer. We ran the experiment twice for each vegetable, and we analyzed vegetable transplant growth rates, dry weight, and nutrient tissue content (Table 1). We also collected data on canopy cover through the course of each trial (Figure 1).

Aboveground Dry Mass

We recorded the dry mass of the shoots and leaves of vegetable transplants at the end of each trial to determine the impact of different fertilizer concentrations on aboveground plant growth. We expected to see increasing dry mass as N concentration increased. However, for every vegetable we studied, we found no difference in the aboveground dry mass between transplants treated with 200 or 400 ppm N. Fertigating with 50 ppm N resulted in significantly lower dry mass in at least one of the two trials we conducted for each vegetable transplant. These results indicate that 50 ppm N is not sufficient for plant development, while applying more than 200 ppm N will provide no benefit for aboveground plant growth.

Root Dry Mass

Dense, fibrous root balls support vegetable transplant establishment in the field. So we separated roots from their shoots, dried them, and recorded root dry mass to determine potential impacts of fertilizer concentration on root development. In general, nutrient stressed plants will put more energy into their belowground growth in comparison to their aboveground growth to find nutrients in the soil or substrate. Therefore, we expected to see higher root mass in transplants treated with low N concentrations. However, this only happened in our romaine lettuce trials. 

In our celery trials, we had conflicting results. In the first celery trial, celery fertigated with 50 ppm N had higher dry root mass in comparison to the 400 ppm N treatment. But in the second celery trial, celery fertigated with 50 ppm N had a significantly lower root mass in comparison to celery fertigated with 200 ppm N. The difference in root growth between the first and second trials could be due to the difference in the length of those individual trials. Celery trial 1 ran for 35 days, and celery trial 2 ran for 26 days. 

We saw no difference in root mass in our leaf lettuce, broccoli, and tomato transplant trials (Table 1). Overall, our results indicate that root growth is variable, depending on the crop, nutrient concentration, and development stage.

Canopy Cover

In addition to root development, achieving >90% canopy cover is a good visual indicator that vegetable transplants have achieved sufficient growth and will establish well in the field. So, we analyzed canopy cover to understand the growth rate of vegetable transplants treated with different concentrations of N fertilizer. We expected transplants fertigated with higher N concentrations to achieve 90% canopy cover faster.

For romaine lettuce, celery, and processing tomato, the 200 and 400 ppm N treatments achieved 90% canopy cover on the same dates. The 200 ppm N treatment achieved 90% canopy cover first in the broccoli trial, but in the leaf lettuce trial, the 200 ppm N treatment didn’t achieve this threshold until the last day of the trial (day 15). Across all 5 vegetable transplants, the 50 ppm N treatment had the lowest canopy cover percentage after the seedling stage. 

These results indicate that there is no negative impact on growth rates when applying 200 ppm N, rather than 400 ppm N. This was the case for our broccoli, processing tomato, celery, and romaine lettuce trial. For leaf lettuce, applying 400 ppm N provided a slight benefit. We only documented canopy cover consistently during the second trial run for each vegetable transplant, so we can only use this data for analysis.

Conclusion

Based off the data collected, fertigating vegetable transplants with 200 ppm N optimizes root, shoot, and leaf growth. Vegetable transplant growth rates do not benefit from additional fertilizer, with the possible exception of leaf lettuce. Applying N at a concentration of 400 ppm would decrease fertilizer use efficiency, create higher input costs, and encourage nutrient runoff for little to no benefit in transplant growth.