Horticulture Principles and Practices

A Venturi-
FIGURE 12–42
type proportioner.
Tap
Siphon mixer
Outlet hose for
irrigation
Container for
fertilizer concentrate
2. Preparing the stock solution.
pounds to add to stock tank:
= ounces/100 gallons second number of injector stock tank volume
(gallons)/100 16
= 18.3 200 50/100 16
= 183,000/1,600
= 114.4
Proportioners are calibrated in terms of the ratio of dilution of concentrated fertilizer with
irrigation water. Some have fixed ratios (e.g., 1:100 or 1:200), and others are variable. They
differ in mechanisms of operation, proportioning ratio, and cost. A relatively inexpensive
type is the Hozon proportioner, which operates on the Venturi principle (Figure 12–42).
Water enters the unit at very high velocity, generating a suction in a feeder line that is
dipped into the soluble fertilizer concentrate. The suction draws the fertilizer into the
watering hose, where it mixes with tap water according to a predetermined concentration.
Proportioners may lose their accuracy over time and require periodic callibration.
Problem of Accumulation of Soluble Salts Pot media are prone to accumulation
of soluble salts, which may result from factors such as poor drainage, excessive fertilization,
or insufficient irrigation (resulting in inefficient leaching). Such accumulation may
result in toxicity or deficiency of certain minerals. Monitoring the soluble-salt content of
growing media, which can be done with an instrument called a solubridge, is important.
Fertilizers differ in solubility and salt index (a measure of the effect of a fertilizer
on the soil solution. Potassium chloride has a salt index of 116, and urea, ammonia, and
regular superphosphate have salt indices of 75, 47, and 8, respectively. Fertilizers of low
salt indices are preferred to reduce salt accumulation. A greenhouse water supply may
contribute to the amount of soluble salts in the soil or growing medium.
12.3.5 CARBON DIOXIDE FERTILIZATION
Plants utilize carbon dioxide and water in the presence of light to manufacture food by
the process of photosynthesis. Carbon dioxide occurs in the atmosphere in a low concentration
of about 350 ppm. As long as adequate ventilation and freely circulating air are
present, greenhouses receive sufficient amounts of carbon dioxide for photosynthesis.
434 Chapter 12 Controlled-Environment Horticulture