Chapter 15

Plant Nutrition of Greenhouse Crops

Greenhouse Fertilization Programs

Maintaining adequate nutrition is among the most critical aspects of producing greenhouse crops. With so many factors affecting plant nutrition and an ever increasing number of fertilizers and fertilizer systems to choose from, growers need hands-on advice on how to put it all together. The following sections are divided into pre-plant fertilization, post-plant fertilization, corrective procedures for nutrient deficiencies, and fertilizer formulating.

Pre-Plant Fertilization

There are four categories of pre-plant fertilization: 1) pH adjustment; 2) addition of nitrogen and potassium; 3) addition of phosphorous and sulfur; 4) addition of micronutrients; and 4) addition of slow-release fertilizers. Rarely do growers try to incorporate sufficient fertilizer in the potting media for the entire duration of the crop. The advantage of this approach is that the grower has the option of adding or withholding liquid fertilization to speed up or slow down crop growth as needed.

pH Adjustment

Since most greenhouse media components are acidic (mainly peat moss and pine bark), agricultural limestone is added to raise the pH suitable for plant growth. Dolomitic limestone is usually used, since it supplies both calcium and magnesium. Dolomitic limestone comes in different particle size distributions and grades that can have a big impact on how quickly the media pH rises and how long the pH stays at a desired level.

Nitrogen and Potassium

Some growers add a “starter charge” of soluble nitrogen and potassium to the potting mix so young seedlings or rooted cuttings have sufficient fertility to begin growth quickly.

Phosphorus and Sulfur

Plants also need anions for good growth. Nitrates (NO3-), chlorides (Cl-) and sulfates (SO4-) are examples of anions. Negatively charged anions are not attracted by the negative charge of the clay particles. Thus, they are not held like cations.

Micronutrients

The micronutrients iron, manganese, zinc, copper, boron, and molybdenum are often added to the potting medium using a commercial micronutrient product designed for this purpose. There are several commercial pre-plant micronutrient mixtures available for greenhouse use.

Controlled-Release Fertilizers

Controlled-release fertilizers are products manufactured from fertilizer salts designed to release fertility at a controlled rate over time. This is accomplished by coating the fertilizer with materials like sulfur, waxes, polyethylene, polyurethane, or acrylic resins. Many growers find that combining controlled-release fertilizer with water-soluble fertilizer in an overall fertility program provides better results than water-soluble fertilizer or controlled-release fertilizer alone.

Post-Plant Fertilization

The most common type of post-plant fertility for greenhouse crops is liquid fertilization using a water-soluble fertilizer. It is important to provide a continuous supply of nitrogen and potassium because these nutrients leach from the media quickly. Small amounts of phosphorus and sulfur are also often included despite being mixed in the media. These nutrients may leach from media quickly, and it is difficult to predict when their levels will fall short of crop needs. Most small to medium-sized commercial greenhouses use commercially blended fertilizers for convenience and dependability; however, for some growers it is economical to buy individual fertilizers and mix them together.  Following are some important notes about each of the essential plant nutrients:

Fertilizers

Plants also need anions for good growth. Nitrates (NO3-), chlorides (Cl-) and sulfates (SO4-) are examples of anions. Negatively charged anions are not attracted by the negative charge of the clay particles. Thus, they are not held like cations.

Nitrogen. Nitrate (NO3-) is the predominant form of fertilizer nitrogen used by greenhouses. Nitrogen may also be applied as urea or ammonia (NH4+). Urea is rapidly converted to ammonia in the media. Although ammonia is readily taken up by plants, it accounts for only a small percentage of any crop’s nitrogen uptake.

Phosphorus. Ammonium phosphate and urea phosphate are good sources of phosphorous. A nitrogen-to-phosphate (P2O5) ratio of 2:1 is acceptable for most crops.

Potassium. Potassium nitrate and potassium sulfate are good sources of potassium. A nitrogen-to-potash (K2O) ratio of 1:1 is acceptable for most crops.

Calcium and Magnesium. Dolomitic limestone, irrigation water, calcium nitrate, magnesium sulfate, and magnesium nitrate are good sources of calcium and magnesium. Calcium and magnesium provided by dolomitic limestone are released slowly over several months.

Micronutrients. Micronutrients are sold in different formulations.

Fertilization Rates

Rates of fertilization are often given in parts per million (ppm) of nitrogen (N), which is a way of expressing fertilizer concentration. One ppm is equivalent to 1 milligram/liter (mg/L). Fertilization rates for water-soluble fertilizers in the greenhouses vary based on the types of crops grown, pre-plant fertilization rates, the weather, and stage of growth, the desired rate of growth, and the leaching percentage. The decision whether or not to use pre-plant nutrients will impact the post-plant fertilization rates. When pre-plant nutrients are not applied the concentration of post-plant liquid fertilizer is typically at a higher rate and sometimes begins earlier. Growers often reduce the rate (lower end of recommended range) in the winter, during cloudy periods, or when other factors slow growth and raise the rate (higher end of range) in the summer, during sunny periods, or when other factors speed growth.

Fertilization Frequency

Two methods are commonly used by growers today: 1) constant liquid fertilization, which involves applying a dilute solution of fertilizer at each irrigation; and 2) scheduled fertilization, which calls for a more concentrated solution of fertilizer that is applied at periodic intervals, usually once per week.

Methods of Fertilizer Application

The best method of fertilizer application depends on the crop, available equipment, fertilizer-pesticide combination, labor, irrigation and tillage practices, and type of fertilizer. A major goal of many producers is to improve fertilizer efficiency (that is, greater crop yield per unit of fertilizer applied). The choice of method, however, also must meet future agronomic and environmental requirements. There are three methods of fertilizer application: 1) fertilizers can either be added during the pre-plant phase as a granular fertilizer or as a slow- or controlled release fertilizer to the potting media; 2) fertilizers can be applied to the plant’s growing media via the irrigation system using a water-soluble fertilizer known as fertigation; or 3) applied as foliar sprays directly to the plant foliage.

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