Fertilizing Greenhouse Crops

Maintaining adequate nutrition is among the most critical aspects of producing greenhouse crops. At present most growers utilize a liquid feed program as their primary means of supplying plant nutrients. This program may also be supplemented with granular or slow release fertilizers added to the growing medium.

The frequency of fertilizer applications also influences plant growth. In some cases it is important to supply nutrients at peak periods of vegetative or reproductive growth. However, it is generally accepted that a constant feed (soluble fertilizer at each irrigation) is the best system to optimize plant growth.

A constant feed program may also be modified so that nutrients are applied at every other irrigation. This approach may be necessary under conditions of high soluble salts.

The balance of plant nutrients is important in producing vigorous, efficient plants. In some cases when nutrients are out of balance severe deficiencies or toxicities may occur. Therefore it is important to consider both the source and amount of fertilizer used.

Selecting Fertilizers:

Several “complete” fertilizers are available from commercial sources for the production of greenhouse crops. These provide N, P and K in the balance desired (i.e. 15-16-17, 20-20-20 etc.). However, many growers “custom blend” fertilizers from several different sources to achieve the best balance for plant growth. Table 1 and Table 2 provide a quick guide to several of the most commonly used fertilizer materials as well as the amounts required to make up desired concentrations.

Selecting the type of fertilizer to be included in a nutritional regime is a key to optimum plant growth. The following is a brief description of the nutrients frequently used:

Nitrogen (N) – is often thought of as the most important element in a nutritional program. However it is only one of several essential elements to plant growth. The most common sources of N used in liquid feed programs include: ammonium nitrate, calcium nitrate and potassium nitrate. Generally speaking no more than 50% of the total N supplied to the plant should be in the ammonium form.

Phosphorus (P) – is another element required in relatively large quantities for plant growth. However, over supplies of P may render other nutrients insoluble and therefore unavailable for plant uptake. Phosphorus is generally supplied in nutrient solutions by phosphoric acid or in some cases superphosphate may be incorporated in the growing medium to supply P.

Potassium (K) or potash is used by the plant in a number of ways, but is primarily required in water relationships. Among the many greenhouse crops produced, poinsettias are notably heavy feeders of K. The most common source of K in liquid feed programs is potassium nitrate, however other sources may be used.

Secondary and Micronutrients a “complete” nutritional program must take into consideration the secondary and micronutrients as well as N, P and K (macronutrients). These two classes of elements generally include: calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), and chloride (Cl). Although many of these may be inherently supplied by the growing medium, others require supplemental application.

Both secondary and micronutrients may be included as a component of a liquid feed program. However, many growers pre-incorporate these into the growing medium. Dolomitic lime is perhaps the most common source of Mg used in this manner. There are also several commercial blends of micronutrients which may be incorporated into the growing medium.


The primary method of supplying nutrients to greenhouse crops is by means of liquid feed program. This can be supplemented with the addition of secondary and micronutrients to the growing medium.

Constant feed programs supply optimum quantities of nutrients for plant uptake. However the level of nutrition must be based on the presence of soluble salts. Complete fertilizers containing N, P and K in desired ratios may be used for greenhouse crops, or fertilizers can be custom blended to meet the needs of the plant.

Table 1. Parts per million (ppm) of nutrient element in solution when specified fertilizer compound is dissolved in water.
Fertilizer Fertilizer & Element Percentage Amount/Gal N P K Ca Mg S Cl
Potassium Nitrate 13.75-0-44.5 (36.9K) 1g 36 97
Potassium Sulfate 0-0-50 (41.5K, 17S) 1g 110 45
Muriate of Potash 0-0-60 (49.8K, 45Cl) 1g 131 119
Mono Potassium Phosphate 0-22.8-28.7 1g 53 75
K-Mag of Sul-Po-Mag 0-0-22 (18K, 11Mg, 22S) 1g 48 29 58
Mono Calcium Phosphate 0-46-0 (20P, 13Ca) 1g 34
Calcium Nitrate 15.5-0-0 (19Ca) 1g 50
Ammonium Nitrate 33.5-0-0 1g 41
Ammonium Sulfate 21-0-0 (24S) 1g 88
Urea 46-0-0 1g 55 63
Nitric Acid 7)% HNO3 (15.5N) 1g 121
Diammonium Phosphate 18-46-0 (20P) 1g 41 53
Phosphoric Acid 75%H3PO4 (0.363g.P/ml) 1 ml 47 96
Gypsum (Calcium Sulfate) (18.6Ca, 14.9S) 1g 49 39
Epsom Salts (9.9Mg, 13S) 1g 26 34
Note: For practical purposes, 3.5 ounces of fertilizer dissolved in 100 gallons of water yields the same ppm of element in solution as those given for 1gram/gallon. (28.35 grams = 1 ounce)

Table 2. Fertilizer sources for greenhouse crop production.
Name & Formula of Material N-P-K Analysis Others
Ammonium Chloride
25-0-0 0
Ammonium Nitrate
33.5-0-0 0
Ammonium Phosphate (di)
21-53-0 0
Ammonium Phosphate (mono)
11-48-0 1.4% Ca, 2.6% S
Ammonium Sulfate
20-0-0 24% S
Calcium Nitrate
15-0-0 37% S
Sodium Nitrate
16-0-0 0
45-0-0 0
Urea-Formaldehyde 38-0-0 0
0-20-0 18% Ca
Treble Phosphate
0-42-0 12% S
Phosphoric Acid
0-52-0 12% Ca
Potassium Chloride
0-0-62 0
Potassium Nitrate
13-0-44 0
Potassium Sulfate
0-0-53 0
MagAmp-Magnesium 7-40-6 18% S
Ammonium Phosphate 0 0
Magnesium Sulfate
0 10% Mg
Magnesium Nitrate
Mg(NO3)2 6H2O
0 9.5% Mg
0 23% Ca
Manganese Sulfate
0 28% Mn
Boric Acid
0 17% B
Copper Sulfate
0 25% Cu
Iron Sulfate
0 20% Fe
Iron Chelates 0 Various % Fe
Ammonium Molybdate
(NH4)6 Mo7O22H2O
0 53% Mo
Zinc Sulfate
0 36.4% Zn

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