For a number of years I’ve studied the use of organic fertilizers for growing commercial greenhouse crops. To start I chose to evaluate fertilizers that could be mixed and applied using methods familiar to growers using traditional water-soluble or granular slow-release chemical fertilizers.
Right now I recommend Nature’s Source 3-1-1 liquid fertilizer and Sustane 8-4-4 granular slow-release fertilizer. Both of these are readily available, cost effective, OMRI-certified, and have good label directions for greenhouses.
My work has led me to recommend using different organic fertilizers in combination rather than relying on one fertilizer. I suggest using Nature’s Source and Sustane together to take advantages of each fertilizer’s strengths. This would be done by incorporating Sustane in the growing medium at planting and then fertilizing on a regular basis with Nature’s Source starting about 4 weeks after planting. Combinations should be considered regardless of what brands or types of organic fertilizer are being used.
These are probably best for short-term crops of less than 6 weeks duration when environmental conditions are most favorable for plant growth (e.g., April-September). Bedding plants, herbs, and vegetable transplants are good candidates for trying organics. Assuming the plants are of good quality and color, reduce or stop using the fertilizer within a week or two of planned marketing. This practice will reduce the chance of ammonium toxicity symptoms.
For important specific recommendations on how to use organic fertilizers to grow greenhouse plants and a list of organic fertilizers that I tested, see:
Organic Fertilizers - Thoughts on Using Liquid Organic Fertilizers on Greenhouse Plants
Doug Cox, UMass Extension, Stockbridge School of Agriculture
