Organic Fertilizer Optimization for Enhanced Growth and Nutrient Uptake in Bell Pepper Transplants (Capsicum annuum L.)

Organic fertilization management for vegetable transplant production is challenging to growers due to the slow and unpredictable release nature of organic fertilizers. Nutrients in organic fertilizers, particularly nitrogen (N), often fail to meet the demands of rapidly growing transplants in soilless substrate. This study aimed to develop fertilization guidelines for organic bell pepper (Capsicum annuum L.) transplants by evaluating the performance of one conventional fertilizer, two organic fertilizers (Drammatic, Pre-Empt), and one naturally derived fertilizer (Bio-Matrix) at a range of N rates in supporting transplant growth. Bell pepper transplants were grown in an indoor growing chamber for 28 days with weekly fertilizer application. We found that the initial nitrate-N concentration in the fertilizer solution was the sole predictor of shoot dry weight (R2 = 0.62), confirming that N availability was the primary limiting factor for transplant growth. The conventional fertilizer produced the largest transplants (370.9 mg/plant in shoot dry weight) while Drammatic resulted in the lowest maximum shoot growth (196.6 mg/plant), likely due to its high salinity and the accumulation of ammonium in the substrate. Bell pepper transplants exhibited low nutrient uptake capability and resulted in low N recovery efficiency, especially with the two organic fertilizers, Drammatic and Pre-Empt (15.6% and 23.8%, respectively). Furthermore, we found no carryover effects of the fertilizer treatments during the transplant stage on bell pepper growth after being transplanted to the greenhouse for 18 days. The final shoot dry weight only correlated with transplant shoot dry weight at the time of transplanting (R2 = 0.87) but not with fertilizer type (p = 0.2849). Overall, Pre-Empt emerged as the most effective fertilizer for organic bell pepper transplant production. It is cost-effective, has low electrical conductivity, and is associated with low ammonium accumulation in the substrate. Therefore, it can be applied at high N rates to meet the N demand of bell pepper transplants. Based on our growing conditions, we recommend 23.1 g/L substrate of Pre-Empt for organic bell pepper transplant production.