Chapter 16

Fertilizers for Greenhouse Crops

Inorganic Fertilizers

Inorganic fertilizers, also referred to as commercial and synthetic fertilizers, are chemically manufactured from petroleum products or from naturally occurring minerals containing one or more plant nutrients. For example, a common phosphorus fertilizer is monoammonium phosphate that contains 11 percent nitrogen in an ammonium form, and 22 percent phosphorus in a phosphate form. This compound is the result of treating mined and finely ground rock phosphate with sulfuric acid, to first produce phosphoric acid that is afterwards reacted with ammonia. Characteristics of inorganic synthetic fertilizers are that they dissolve in water and are immediately available to the plant for uptake. When used according to recommendations, these types of fertilizers are safe for the environment and can supply the required nutrients for plant growth. However, excessive rates of these fertilizers can injure the roots of plants causing death and potentially lead to environmental degradation.

Advantages and Disadvantages

Commercial, synthetic, or inorganic fertilizers are distinguished from organic fertilizers by the fact that they derive nutritional content delivered to plants from synthetic compounds rather than by naturally occurring organic materials like animal manure, fish emulsion, bone meal, and bat guano. Since both types of fertilizer promise to deliver the same nutrients to plants it is easy to conclude that it doesn't matter whether to use organic or inorganic fertilizers. However, there are advantages and disadvantages to using synthetic fertilizers.

Fertilizer Grade

The information on the label (See Figure 16.1) 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 water-soluble potassium (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.

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.

Complete versus Incomplete Fertilizers

A fertilizer is said to be a complete or mixed fertilizer when it contains nitrogen, phosphorus and potassium (the primary nutrients). Examples of commonly used complete fertilizers are 6-12-12, 10-10-10, 15-15-15 and 20-10-10. An incomplete fertilizer will be missing one or more of the major components. Examples of incomplete fertilizers are: 34-0-0 (ammonium nitrate), 46-0-0 (urea), 18-46-0 (diammonium phosphate), 0-46-0 (triple super phosphate), and 0-0-60 (muriate of potash).

Fertilizer Forms

Fertilizer materials can be solids, liquids, or gases. Each physical form has its own uses and limitations, which provide the basis for selecting the best material for the job.

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 fertilizers are the most common form of solid fertilizer used.

Granulated fertilizers. Granulated fertilizer materials are solid, homogenous mixtures of fertilizer materials generally produced by combining raw materials such as anhydrous ammonia, phosphoric acid, and potassium chloride. Granulated materials are N-P or N-P-K grades of fertilizer.

Blended fertilizers. Blended fertilizers are simple physical mixtures of dry fertilizer materials. The ingredients of a blended fertilizer can be straight materials, such as urea or potassium chloride; they can be granulated compound fertilizer materials mixed together; or they can be a combination of the two. An example of a blended fertilizer is a 12-10-8 + 4 percent magnesium + 8 percent sulfur + 2 percent calcium product, which was formed by adding triple superphosphate (0-46-0), potassium magnesium sulfate (0-0-22 plus Mg and S), and ammonium nitrate (34-0-0).

Particle size. Particle size of a fertilizer material can be important for both agronomic and handling reasons. In agronomic applications, particle size is most important for sparingly soluble materials such as rock phosphate. These materials must be very finely ground to ensure sufficient solubility.

Liquid Fertilizers

Fluids can be either straight materials, such as nitrogen solutions, or compound fertilizers of various grades. Fluid fertilizers are categorized into two groups: clear solutions and suspensions. Liquid fertilizers can be diluted or concentrated for precise, even application. Liquid fertilizers are commonly applied through irrigation water (fertigation), sprayed, knifed-in, broadcast, or banded. Unlike solid fertilizers, liquid fertilizers are applied on a volume rather than a weight basis and the liquid density (pounds nutrient per unit volume) is needed to calculate application rates.

Clear Solutions. In clear solutions, nutrients are completely dissolved in water. The major advantage is ease of handling. In addition, the phosphorus in these materials is highly water-soluble.

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. Another advantage is that they can be formulated at much higher analyses than clear solutions. These formulations may contain analyses as high as dry materials.

Gaseous Fertilizers

Gaseous fertilizer requires some special considerations in handling and use. Anhydrous ammonia is a high-analysis nitrogen gaseous fertilizer used both in the manufacture of all other common nitrogen-containing fertilizers and in direct applications to the soil. Once applied, anhydrous ammonia behaves similarly to any other ammonium nitrogen source. But special handling methods and safety precautions are required, because anhydrous ammonia is stored as a compressed liquid.

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. Water-soluble fertilizers are typically injected into the irrigation system, a process known as fertigation (see the discussion in the following chapter). 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.

Solubility of Fertilizers

Fertilizer solubility is a measure of how much fertilizer material will dissolve in water and is a property that strongly influences the availability of nutrients to the crops and type of application methods—fertigation and direct media application.

Fertilizer Acidity/Basicity

The potential acidity or basicity indicates how the fertilizer will affect media solution pH. The potential acidity refers to the fertilizer’s tendency to cause the media pH to decrease, while the potential basicity refers to the fertilizer’s tendency to cause a media pH increase. Many water-soluble fertilizer labels state the potential acidity or basicity of the fertilizer in units of equivalent pounds of calcium carbonate (CaCO3, or agricultural lime) per ton of fertilizer referred to calcium carbonate equivalent (CCE). Potential acidity or basicity indicates the type of reaction produced, while calcium carbonate equivalency indicates the strength of that reaction. For example, 20-10-20 (40% NH4+) has a potential acidity of 422 pounds per ton of fertilizer. If one ton of 20-10-20 were applied to a field soil, we would estimate that 422 pounds of CaCO3 (lime) would be required to neutralize the long-term acidity produced from the fertilizer. See Table 16.1 for some common acid and basic fertilizer formulations.

Fertilizer Salt Index

Most fertilizer materials are readily soluble because they are salts. Once they are dissolved in the soil, they increase the salt concentration of the soil solution, which in turn increases the solution’s osmotic potential. The greater the osmotic potential, the more difficult it is for the plants to extract the soil water they need for growth. The selection of the proper fertilizer material can help to keep the soluble' salt concentration of a nutrient solution at a low level.

Slow- and Controlled-Release Fertilizers

Greenhouse production systems are intensively managed requiring high amounts of fertilizer for appropriate quality growth. Nutrients are generally applied as water soluble fertilizers in greenhouses. However, because water soluble fertilizers can be easily leached, there is the potential of high nutrient losses. Phosphates and nitrates are prone to leaching in greater quantities because they do not readily bind to negatively charged colloids.

Advantages of Slow- and Controlled-Release Fertilizers

Slow- and controlled-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) . . .

Disadvantages of Slow- and Controlled-Release Fertilizers

Applying sulfur-coated urea almost always lowers soil pH as aforementioned. However, this acidification may cause nutrient disorders such as calcium deficiency or magnesium deficiency if there is not a proper nutrient management program. Nutrient deficiencies may occur if nutrients are not released as predicted because of low temperatures or poor activity of soil microbes. 

Slow-Release Fertilizers

Slow-release fertilizers also slowly release nutrients, but the rate, pattern, and duration of release are not controlled because they depend on microbial organisms whose effectiveness is dependent on temperature and moisture conditions. 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.

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. Controlled-release fertilizers 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. All controlled-release fertilizer products are based on similar principles with soil and/or media temperature being the main driving force for nutrient release. Nutrients are released out of coated fertilizer “prills” (a small aggregate or granule of material) through osmosis at a rate that is positively correlated with increased temperature.

Combining Controlled-Release and Water-Soluble Fertilizers

Water-soluble fertilizers with controlled-release fertilizers can work hand-in-hand, providing greater benefits than either fertilizer type alone—especially when growing a variety of crops. Growers can use a low rate of controlled-release fertilizer as a steady base feed for all plants with a supplement of water-soluble fertilizer as needed adjusting rates to accommodate heavier feeders.

Application Methods of Slow- and Controlled-Release Fertilizers

Slow- and controlled-release fertilizers can be applied in a variety of ways. Greenhouse growers most commonly incorporate slow- and controlled-release fertilizers during pre-plant fertilization. The key objective of incorporation is to ensure consistent fertilizer distribution throughout the mix for the most uniform delivery to each pot. Depending on the chosen product and rate, growing media containing slow- and controlled-release fertilizers should be used up as soon as possible—typically within several weeks after the date of manufacture—to avoid soluble-salt buildup in the mix.

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