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 have adopted the following pre- and post-plant fertilization programs.
Pre- vs. Post-Plant 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 have adopted the following pre- and post-plant fertilization programs.
Pre-Plant Fertilization
As the name implies, pre-plant fertilization programs are initiated prior to planting crops. The biggest benefit of utilizing a pre-plant approach to fertilization is the potential to simplify managing mineral nutrition for your crops. 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. It is rare that a downward adjustment in growing media pH is required. Media most likely to require such correction are those that are alkalinemedia based (e.g., hardwood bark).
Post-Plant Fertilization
Post-plant fertilization programs are initiated after plants are transplanted. The most common type of post-plant fertility for greenhouse crops is liquid fertilization using a water-soluble fertilizer (WSF). 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.
Fertilization Frequency
The frequency of fertilizer applications also influences plant growth. Two methods are commonly used by growers today: (1) constant liquid fertilization, which involves applying a dilute solution of fertilizer at each irrigation known as fertigation; and (2) scheduled fertilization, which calls for a more concentrated solution of fertilizer that is applied at periodic intervals, usually once per week, and irrigate with clear water between feedings. In some cases it is important to supply nutrients at peak periods of vegetative or reproductive growth.
Fertilization Rates
Once the frequency of fertilization has been decided the next decision is the 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 one 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 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.
Substrate Influence
The substrate is an important parameter to consider in a fertility management program. Inert substrates, such as perlite and rockwool, have little or no Cation Exchange Capacity (CEC); therefore, all the nutrients provided are only found in the substrate solution. On the other hand, organic substrate components like peat moss have a CEC = 200 meg/100g; meaning it will retain some cations (NH4⁺, Ca2+, K⁺, Mg2+, and Na⁺) and release them back into the substrate solution in order to maintain their balance with anions.
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