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Greenhouse Tomatoes: Fertilizing Plants Growing in Soilless Media

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The most popular method of growing greenhouse tomatoes is in bags or other containers filled with an artificial peat-based mix. With such a system, a fertilizer mixture is injected into a trickle irrigation system every time water is applied. This is called constant feed. A common fertilizer program is to use a 7-1 1-27 plus calcium nitrate. The 7-1 1-27 is mixed at a rate of 15 ounces per gallon to make a concentrate stock solution.

The calcium nitrate is mixed at 10 ounces per gallon of stock solution. These two stock solutions should be in separate containers. If they are mixed together as a concentrated stock solution, they will react, causing precipitates to settle out. This changes the character of the fertilizer and can clog the emitters. Every time the crop is watered, each of these two fertilizer solutions is injected into the irrigation water at a dilution ratio of 100 to 1. This provides a final concentration of 200 parts per million (ppm) of nitrogen which is appropriate for tomato plants that are bearing. (You should start newly set plants at half this concentration and gradually work up to 200 ppm.) To do this right requires two injectors; one for each of the fertilizers. They should both inject at the same time, so the two materials are mixed in the water line. This is safe because they are diluted when they are mixed and they are not in the line long before they pass through the emitter into the growing media.

Some growers have only one fertilizer injector and so they alternate fertilizers. This means that each fertilizer material is injected at every other watering and results in the nutrients being applied at half the recommended rate. There are a number of crops showing nutrient deficiencies as a result. It is best to use two injectors so you caii inject the materials at the same time. Another option is to move the injector suction tube from one stock solution to the other half way through the irrigation cycle, hut make each stock solution double strength. This should apply the correct amount of each material, but be sure to make the switch half way through the cycle or you can over apply certain nutrients.

John Howell
UMass Extension Vegetable Program

Commercial Horticulture
Commercial Horticulture topics: 
Plant Nutrition