Fertilizers can be applied to greenhouse crops in dry or liquid form. If applied in dry form, the fertilizer may be incorporated as the soil is mixed and before it is pasteurized unless a slow-release fertilizer is used. In that case, the fertilizer can be incorporated after pasteurization. Nutrients can be replenished later by supplemental applications of dry fertilizers to the surface of the benches.

Dry fertilizers require time to apply, especially if slow-release forms are not used. For potted crops, repeated application of dry fertilizer adds to labor costs. Slow-release dry fertilizer is believed to be used more efficiently by the plant with less loss through leaching than fertilizer applied in quickly available dry form.

The popular alternative to dry fertilizer is liquid fertilizer dissolved in water and applied as the crop is irrigated. Such methods save on labor but use more fertilizer overall.

The greenhouse watering system is the most common means of applying liquid fertilizer to crops. Three types of systems are used: the venturi proportioner, the positive displacement pump, and the eductor. In each case, concentrated liquid fertilizer is held in a container and injected into the greenhouse irrigation system when water is used or when fertilization is desired.

1. The venturi proportioner draws the fertilizer into the water system through a small tube that connects the water hose to the fertilizer concentrate. As the water pressure and flow rate vary, so does the rate at which the concentrate mixes with the water. The venturi proportioner is not precise but is serviceable for many crops.

2. The positive displacement pump may be water-driven or use an electric motor to power a fertilizer concentrate pump. It maintains a uniform proportion of fertilizer to water, regardless of the water pressure or rate of flow. It is usually not adjustable to permit a change of the injection ratio.

3. The eductor places the fertilizer concentrate in a pliable bag sur­rounded by water under pressure. As the pressure increases, it forc­es the fertilizer through a metering system and into the water line. The eductor system is a flexible one that does permit the injection ratio to be varied. Under normal operating conditions, it maintains a constant proportion of concentrate to water.

These fertilization systems must be checked for accuracy at least once each year and should be one of the first things checked if a crop begins to show signs of trouble. The choice of a system is usually based on the size of the operation, whether the injection ratios will be stan­dard or must be varied, whether the system will be mobile or perma­nently installed, and how easily a particular system can be serviced when necessary.

Fertilizers for the injectors can be mixed by the grower or purchased premixed from suppliers. Either way, the fertilizers must be easily sol­uble in water and contain a dye that will be visible in the water supply even when highly diluted to provide a visual check that the system is working.

The amount of fertilizer applied per application depends on the strength of the concentrate, the dilution ratio of the injector, and the amount of water applied. Most commonly, a ratio of 1:100 (one part concentrate to ninety-nine parts water) is used, but 1:200 is not uncom­mon. If fertilizer is only applied every few weeks, the concentrate must be stronger. If applied with each watering, the concentrate must be weaker.

Measuring Greenhouse Fertilizers

Greenhouse fertilizers are measured by the number of parts of nutrients to a million parts of water, or ppm (parts per million). Most soil test reports are expressed in parts per million. So, too, are most fertilizer recommendations for greenhouse crops.

To determine ppm, multiply the percent of any elements in a fertil­izer by 75. The answer will be the ppm of 1 ounce of the fertilizer in 100 gallons of water. For example, a 20-5-30 fertilizer is 20 percent nitrogen. In a solution of 1 ounce of 20-5-30 fertilizer in 100 gallons of water, the ppm of nitrogen is.20 X 75, or 15.

To make a 100 ppm solution, the number of ounces of fertilizer required per 100 gallons of water must be calculated. To do so, divide 100 by the ppm in 1 ounce. Continuing the example above, 100 ^ 15 = 6% ounces.