Fertilizers for Greenhouse Crops
Inorganic Fertilizers
The information on the label (Figure 22.1) is the guaranteed amount of the primary nutrients given in a series of three numbers, such as 10-6-4, and is referred to as the fertilizer grade. A fertilizer grade gives the minimum guaranteed primary nutrient content as percentage of total nitrogen (N), percentage of available phosphate (P2O5), and percentage of potassium oxide (K2O). Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus and potassium (N, P, K). It should be remembered that it is not N-P-K but N-P2O5-K2O. For example, if we have a 100-pound bag of fertilizer labeled 15-30-15, there are 15 pounds of N, 30 pounds of phosphate and 15 pounds of potassium oxide.
Fertilizer Labeling
The information on the label (See Figure 19.1) is the guaranteed amount of the primary nutrients given in a series of three numbers, such as 10-6-4, and is referred to as the “fertilizer grade.” A fertilizer grade gives the minimum guaranteed primary nutrient content as percentage of total nitrogen (N), percentage of available phosphate (P2O5), and percentage of potassium oxide (K2O). Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus and potassium (N, P, K). It should be remembered that actually it is not N-P-K but N-P2O5-K2O.
Other Nutrients
In addition to primary macronutrients, fertilizers may contain other nutrients, such as sulfur (S), iron (Fe), boron (B), zinc (Zn), and molybdenum (Mo). These nutrients may be added as additional nutrients or may be constituents (impurities) remaining in the fertilizer material following mining and manufacturing processes. If present as additional nutrients (as in Figure 19.1), nutrients will be listed on the fertilizer label on an elemental basis, similar to nitrogen (N).
Regulatory Standards
The law establishes minimum allowable levels of nutrients and provides specific labeling requirements. The law only requires that the manufacturer guarantee what is claimed on the label. So in some cases, a fertilizer will contain secondary nutrients or micronutrients not listed on the label, because the manufacturer does not want to guarantee their exact amounts. For this reason, some fertilizers (especially organic fertilizers) may have a higher total nutrient content than what is listed on the label.
Fertilizer Formulations
The type or form the fertilizer comes in is called the formulation. The formulation is presented on a bag of fertilizer as percent N to percent phosphorus (P2O5) to percent potassium (K2O). Many different physical and chemical forms of inorganic fertilizers are available. Appendix C, Common Macronutrient Fertilizers lists some of the more common macronutrient fertilizers and Appendix D, Common Micronutrient Fertilizers lists some of the more common micronutrient fertilizers for growing greenhouse crops.
Single- or Multiple-Nutrient Fertilizers
Based on their primary nutrient content (N-P-K), fertilizers are referred to as being single-nutrient or multiple-nutrient. Single-nutrient fertilizers, such as urea (46-0-0), contain only one primary macronutrient and are also called “simple” or “straight” fertilizers. In contrast, multiple nutrient fertilizers contain more than one nutrient, such as diammonium phosphate (DAP, 18-46-0) and may be referred to as “mixed” or “complex” fertilizers.
Fertilizer Forms
The most commonly used fertilizer forms used by greenhouses are solids and liquids. Each physical form has its own uses and limitations, which provide the basis for selecting the best material for meeting the plant’s needs.
Solid Fertilizers
Depending on the nutrient content and manufacturing processes, solid fertilizers may differ in size, shape, color, and bulk density. Solid fertilizer forms are classified by size and shape and include granules, prills, pellets, and powder. Granular fertilizer particles range in size from approximately 1 to 4 mm in diameter and are roughly round in shape. Granular fertilizers are the most common form of solid fertilizer used. Prills are made by solidifying free-falling droplets in the air to form nearly spherical particles.
Blended Fertilizers
Blended fertilizers are mixtures of dry fertilizer materials when two or more chemical compounds may be mixed together in a suitable proportion to supply nutrients at the same time from the same material. In blended fertilizers, the individual particles remain separate in the mixture, and there is a potential for segregation of the nutrients. The result is nonuniform distribution of the fertilizer nutrients in the field. Thus, a major precaution in bulk bending is that materials be of similar particle size; otherwise, segregation will occur during handling and application. Properly made blends are generally equal in effectiveness to other compound fertilizers.
Solution Fertilizers
Solution fertilizers can be diluted or concentrated for precise, even application. In solution fertilizers, nutrients are completely dissolved in water. Solution fertilizers are commonly applied in greenhouses with a fertilizer injector, which is more convenient than broadcasting or top-dressing with dry fertilizers. Solution fertilizers generally have a lower nutrient analysis than solid fertilizers due to the solubility limitations of nutrients in a solution.
Suspension Fertilizers
Suspension fertilizers are fluids in which solubility of the components has been exceeded and clay has been added to keep the very fine, undissolved fertilizer particles from settling out. The major advantage is that they can be handled as a fluid. Suspensions have some additional advantages over solutions. Production costs are lower because less materials are used to produce suspensions.
Water-Soluble Fertilizers
Water-soluble fertilizers have long been the fertilizer mainstay for greenhouses. Water-soluble fertilizers come in either granules or water-soluble crystals. Soluble fertilizers are typically injected into the irrigation system, a process known as fertigation (See Chapter 20, Fertigation of Greenhouse Crops). Their popularity stems from the fact that the application rates can be easily calculated, distribution is as uniform as the irrigation system, and, if properly formulated and applied, the chance of fertilizer burn is very low. Because they are immediately soluble, the nutrients in these fertilizers are quickly available for plant uptake and it is easy to adjust nutrient levels and ratios, which gives the grower great control of plant growth rates.
Fertilizer Acidity/Basicity
Potting media pH can be somewhat controlled through fertilizer selection. Fertilizers with potential acidity will lower pH while fertilizers with potential basicity will raise pH. Most water-soluble fertilizers will change the potting media pH to some extent. Some are very acid, such as 20-20-20 (potential acidity 583), while others are mildly basic, such as 15-0-15 (potential basicity 420). Potential acidity, or basicity, is printed on the fertilizer label based on calcium carbonate equivalent (CCE) as a benchmark. This number provides some indication of acidity or basicity a particular fertilizer formulation. See Table 19.4 for some common acid and basic fertilizer formulations. Note that acid fertilizers tend to contain greater amounts of ammonium nitrogen while basic fertilizers contain much less ammonium nitrogen. Fertilizers with a considerable percentage of the nitrogen in the ammonium form tend to leave an acid residue in the media, indicated by the potential acidity
Fertilizer Salt Index
The salt index of a fertilizer is expressed as the ratio of the increase in osmotic pressure of the salt solution produced by a specific fertilizer to the osmotic pressure of the same weight of sodium nitrate, which is based on a relative value of 100. Fertilizers with a high salt index have a high potential to damage plants compared to fertilizers with a low salt index. In general, nitrogen and potassium fertilizers have much higher salt indices than phosphorus fertilizers.
Controlled- and Slow-Release Fertilizers
Slow- and controlled release fertilizers are described as materials that slowly release soluble nutrients over an extended period of time. While the terms “controlled-release” and “slow-release” fertilizer are often used interchangeably; however, the compounds they describe are quite different. Controlled-release fertilizers (CRF) are primarily water-soluble fertilizer salts or blended fertilizer substrates containing N-P-K or N-P-K plus micronutrients covered in a membrane that limits the solubility of the fertilizer. The membrane technology varies between companies and can be resin-, plastic- or polymer-based. Slow-release fertilizers (SRF) are another group of fertilizers with limited solubility. However, SRFs differ from controlled-release fertilizers in one important way.
Advantages and Disadvantages
Controlled- and slow-release fertilizers have demonstrated the following advantages: (1) nutrients are better utilized when slowly released throughout a season rather than applied in “bursts” or instantly soluble applications such as is the case in water-soluble fertilizers application, thus increasing nutrient use efficiency and perhaps more closely synchronizing release rates with plant demand; (2) the quantity of fertilizer used is also reduced, leading to less of a risk for plant injury through high soluble salt levels; (3) nutrient leaching is greatly reduced when using controlled- and slow-release fertilizers as compared to water-soluble fertilizers;
Controlled-Release Fertilizers
Controlled-release fertilizers are also called coated or encapsulated fertilizers because the release is controlled by a polymer coating that contains a water-soluble fertilizer. The thickness of the coating and air temperature dictate the release rate. The higher the temperature, the faster the fertilizer will be released. Most CRFs are sold with an expected release rate based on 75 degrees F (24°C). The coatings are made of acrylic resins, polyethylene, waxes, and sulfur.
Fertilizer Longevity. Controlled-release fertilizers are available with different longevities. Most manufacturers have products with 3, 6, 9, 12, or 16-month release periods. Products with different longevities are manufactured by blending fertilizer prills of different coating thicknesses: the thinner coated prills release nutrients first and then the thicker-coated prills release nutrients later. Short-term crops such as annuals would probably require 3-month release products and longer-term crops such as woody perennials would require fertilizer products with a 12-month release period. Always carefully consider crop type, feeding habit, length of growing season, timing of application, container size in relation to plant size, and irrigation.
Slow-Release Fertilizers
Slow-release fertilizers are released by hydrolysis (dissolved by water) or by microbial degradation. Therefore, the nutrient release from slow-release fertilizers is less predictable than from controlled-release fertilizers. In order for adequate release to occur, sufficient moisture and warm temperatures (generally above 68°F, 20°C) must be present in order to initiate and encourage microbial activity. Slow-release fertilizers are only slightly soluble and require additional time for mineralization, thereby giving them slow-release properties. Slow-release fertilizers may be organic or inorganic, and unlike controlled-release fertilizers are uncoated.
Chelates
The metal micronutrients iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) form salts when used in a non-chelated form. They can precipitate with phosphates, carbonates or hydroxides rendering them unavailable for plant uptake. Chelation is a process that can protect micronutrients from forming these precipitates; when chelated, the nutrients remain dissolved in solution and available to the plants. There are various forms of chelates such as EDTA, DPTA, and EDDHA, each with its own power to attract metal ions. These abbreviations refer to the chemical structure of the organic molecule.
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