Greenhouse Quality Concerns

Water Quality

Water quality (bicarbonate) correction should be performed before the trays are floated. Add the required amount of acid after the beds are filled. A flow meter, available at plumbing supply houses, is helpful in determining the amount of water in the bed. The amount of acid to add is calculated from a recent water analysis report. First, calculate the total carbonate (TC) concentration by adding the concentrations of HCO3 and CO3 from the report. Then multiply TC by 1.11 to get the fluid ounces of battery acid (9.19 N sulfuric acid) per 100 gallons of float water. The amount of acid to use is now calculated at the Clemson laboratory and will be added to the water analysis report. Use only virgin acid, as recycled acid may contain harmful concentrations of heavy metals. Add the acid several days before the trays are floated to allow time for the acid to react. The final water pH should be in the 6.0 to 6.5 range before fertilizer is added. Do not add more acid than called for by the formula or an excessively low pH will result. Consult your county agent for advice. It is not beneficial to preheat the float water.


All the fertilizer should be added through the waterbed with the float system. One-hundred (100) ppm N early in the season is adequate. After four weeks, add another 150 ppm N. The initial application has been reduced to lessen the possibility of salt injury. Research at VPI has indicate that plant quality is improved when initial fertility application is delayed until two-three days after seeding. Since the second application has been increased, the total rate is the same as in past years. This change is based on several years of research conducted at both Clemson University and at North Carolina State University.

Split applications of fertilizer are recommended to reduce soluble salt problems sometimes experienced during germination. The addition of fertilizer at four weeks after seeding should coincide with the first addition of water to the beds. In severe situations (TC greater than 5), acid should be added along with the water. Adding water with the second fertilizer application should aid in mixing in the beds. It may be helpful to add the solution at several spots in the bed. Please refer to the following section for recommended fertilizer programs and rates.

Due to limitations in formulation of soluble fertilizers, no fertilizer can supply all nutrients needed for good seedling growth. Calcium, magnesium, and/or sulfur may not be supplied by a particular fertilizer. In order to supply a complete fertility program, gypsum (calcium sulfate) and Epsom salts (magnesium sulfate) may be needed (see Fertility Programs section for use and rates). Sulfuric acid, if needed, will supply plant available sulfate. Acid should be used only for water quality correction, not solely as a fertilizer.

Over-fertilization with nitrogen and phosphorus may result in succulent plants which are more prone to disease. In addition, over-fertilized plants will have to be clipped more frequently. Each clipping carries with it the possibility of introducing disease. Do not exceed 10% phosphorus in the fertilizer. To avoid excessive algae growth, float trays as soon as possible after adding fertilizer. Recent research and a grower waste solution survey have indicated that previously used fertility programs (20-20-20, 20-10-20) contained too much phosphorus. Potential negative effects of high phosphorus include higher cost, the tendency to produce "leggy seedlings, and increased clipping demand, "tender" transplants, and greater waste nutrient disposal problems. Seedlings grown with the low phosphorus programs will appear to grow slower, but will be ready at the normal time. Since the phosphorus status of the plants may be low at transplanting, a high phosphorus starter fertilizer in the transplant water may be advisable.

Calcium deficiency has been observed in several greenhouses. Some varieties are quicker to exhibit deficiency symptoms, but all should respond to calcium fertilization. Use of calcium in a complete fertility program will prevent potential problems. Calcium fertilization is necessary when the calcium level of the water is less than 20 ppm. If calcium is not in the regular fertilizer, it can be supplied by the addition of gypsum to the water bed.

Boron deficiency can occur when fertilizer without boron is used with water low in boron. To prevent deficiency, a fertilizer containing boron as part of its micro nutrient package should be used when water analysis indicates less than 0.5 ppm boron. Since boron can be toxic and only very small amounts are needed for good growth, use of Sol-u-bor or Borax to supply boron is not recommended.

Next Step - Tray Filling