Proper nutrition is one of the most critical factors in the production of greenhouse crops. Generally speaking, greenhouse crops may be classified as "heavy feeders", requiring relatively large quantities of fertilizers. However, the ratio and sources of elements supplied are as important as their amounts.

Research has shown that the balance between nitrate (NO₃), nitrogen (N) and ammonium (NH₄) can effect plant growth. In Texas it is recommended that no more than 50% of the N supplied should be in the NH₄ form. Increased amounts of NH₄ in the growing media may result in severe ammonium toxicity of foliage burn.

Most Texas growers currently supply phosphorus (P) in the form of phosphoric acid (H₃PO₄). This is done both to supplement P nutrition as well as to help acidify alkaline irrigation water.

Some growers also incorporate superphosphate into their growing media as a source of P. However, because superphosphate is relatively insoluble, the amount of P released during the growing season is not sufficient for potted poinsettias.

Potassium (K) is a key element in maintaining poinsettia nutrition. At present most growers supply K in the form of potassium nitrate (KNO₃). This material which contains both K and NO₃ is an excellent fertilizer for use on potted crops. Research has also shown that a 1:1 balance between N and K₂O is important for optimum bract size and color.

Another important aspect of greenhouse crop nutrition involves the secondary elements calcium (Ca) and magnesium (Mg). Due to continuous leaching during irrigation, these nutrients can run in short supply late in the growing season.

Most fertility programs designed for greenhouse crop production supply Ca in the form of calcium nitrate. However, growers using premixed fertilizers are generally recommended to make some supplemental applications of calcium nitrate near the end of the growing season.

Unlike Ca, most custom mixed fertilizer solutions do not contain Mg as a principal component. For this reason dolomitic lime is frequently used, not only to adjust pH, but also as a source of Mg. Once again, due to continuous leaching during irrigation, Mg levels in the medium can become low. Under these circumstances supplemental Mg may be applied in the form of magnesium sulfate (epsom salts) Table 1.

Table 1. Approximate amount (oz.) of epsom salts to add per 100 gallons of water to make 208 ppm or 182 ppm of supplemental magnesium fertilizer.

Perhaps the most commonly used method to apply fertilizer is through a nutritional regime referred to as constant fertilization. This system involves the application of soluble fertilizers at very irrigation. The most important factor in this fertilization program is to apply enough water at each irrigation to leach to pots thoroughly. This prevents the accumulation of soluble salts from previous irrigations.

Many growers also use a slow release fertilizer, such as Osmolote, in combination with a constant fertilization program. Generally speaking 4 - 6 lbs/cu. yd. of growing media (14-14-14) may be used to supplement a nutritional program. However, water quality must also be taken into consideration. Caution should be used in determining if slow release fertilizers are appropriate.

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 2 and Table 3 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 preincorporate 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.

Summary

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.

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