Chapter 16

Fertilizers for Greenhouse Crops

Organic Fertilizers

Organic fertilizers and soil amendments come from natural sources–plants, animals, and rocks. Examples of organic fertilizers include animal manure, sewage sludge, compost, fish emulsion, and other animal wastes. An organic fertilizer is a natural soil amendment that adds plant nutrients to the soil, most often nitrogen, phosphorus, or potassium. Organic fertilizers are often not well balanced and the release of nutrients can be unpredictable—either too fast or too slow. Most organic fertilizers have relatively low nutrient analyses, although actual concentrations may vary considerably depending on the type of material and stage of decomposition. On the one hand, an advantage of the low-nutrient concentrations of organic fertilizers, such as compost, is that it is more difficult to apply excessive amounts of fertilizer, to “over fertilize,” but other organic fertilizers, such as fresh chicken manure, could damage plants. On the other hand, the low levels of nutrients provided by organic materials may be insufficient to achieve the rapid plant growth expected of greenhouse stock.

Advantages and Disadvantages

Organic fertilizers come from plants, animals or their waste products are distinguished from organic fertilizers by the fact that they derive nutritional content delivered to plants from naturally occurring organic materials rather than by synthetic compounds. 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 organic fertilizers.

U.S. Department of Agriculture’s National Organic Program Rule

Most synthetic fertilizers are prohibited by the U.S. Department of Agriculture’s National Organic Program (NOP) rule [7 CFR 205.105(a)], with a few specific exceptions found on the National List [7 CFR 205.601(j)]. Note that the NOP forbids the use of human sewage sludge (biosolids) from a municipal wastewater treatment facility on an organic operation (§ 205.105 (g)).

Fertilizers Labeled as “Organic”

Fertilizer labeling laws are enacted state-by-state in the United States. The regulators of fertilizer labeling laws are organized through the Association of American Plant Food Control Officials (AAPFCO). Most states follow the AAPFCO model bill, which defines “organic fertilizer” as “[a] material containing carbon and one or more elements other than hydrogen and oxygen essential for plant growth” (AAPFCO, 2008).

Animal-Derived Fertilizers and Amendments

They include bi-products of the cattle industry (blood meal), poultry industry (feather meal), and fisheries (fish meal, crab meal, shrimp meal).

Livestock Manure

Livestock manure is a key fertilizer in organic and sustainable soil management. Manure provides plant nutrients and can be an excellent soil conditioner. Properly managed manure applications recycle nutrients to crops, improve soil quality, and protect water quality. Manures are most effectively used in combination with crop rotation, cover cropping, green manuring, liming, and the addition of other natural or biologically-friendly fertilizers and amendments.

Bat Guano

Guano is most commonly used as a source of nitrogen for plants, but some guano materials are also relatively enriched in phosphorus. Typically mined from the floors of caves and other areas that make up the habitat of the bat, this guano is rich in organic matter and is a ready, water soluble source of nitrogen, as well as some of the trace minerals.

Blood Meal

Blood meal is dried slaughterhouse waste containing about 12 percent nitrogen. Generally, blood meal products are relatively high in ammonia and must be used carefully to avoid damaging plant roots.

Bone Meal

Bone meal, prepared by grinding animal bones, is one of the earliest phosphorus sources used in agriculture. As with rock phosphate, bone meal solubility decreases as soil pH increases, and is most reactive in acid soils. Bone meal phosphorus availability also improves with increasing fineness of particle size. Commercial bone meal products are heat-treated to a temperature that ashes soft tissues and eliminates the potential to transmit diseases.


Compost is defined as the product resulting from the controlled biological decomposition of organic material (e.g., manures). Compost is typically derived from animal manure but can also be derived from crop residues. Mature compost has little resemblance in physical form to the original biodegradable from which it is made. Compost is valued for its organic matter content, and it typically used as a soil amendment to supply a variety of macro and micronutrients.

Feather Meal

Feather meal (14 to 16% N), a by- product of the poultry industry, contains as much as 70 to 90 percent protein. It is mostly present as non-soluble keratin stabilized by highly resistant disulfide bonds. When treated with pressurized steam and animal-derived enzymes, the feather-based protein becomes a good source of available nitrogen for crop nutrition.

Fish Meal and Emulsion

Fish meal and fish emulsion are, like most animal by-products, rich in nitrogen. Fish meal contains about 10 percent nitrogen, along with about six percent phosphate. It is most frequently used as a feed additive, but can be used as a fertilizer on organic farms. These high-nitrogen animal byproducts have relatively rapid nitrogen mineralization.

Plant-Derived Fertilizers and Amendments

One of the more popular families of fertilizers used by organic farmers is plant-based fertilizers, primarily plant meals. All of these materials are low in N-P-K content and are not be sufficient as a standalone fertilizer in either organic or conventional crops. These materials usually contain additional nutrients in slowly available organic forms. They are often applied by organic farmers as starter fertilizers. None of the meals can be used on certified organic crops, if they come from GMO crops.

Alfalfa Meal

Alfalfa meal (or pellets) contains around three to four percent nitrogen and is commonly used as an animal feed. They require microbial mineralization before the nitrogen is available for crop uptake. Mineralization of these nitrogen-rich materials is generally rapid. It is an excellent horticultural fertilizer and is said to contain “unknown growth factors” that make its mineral content more effective as a plant nutrient source.

Cottonseed Meal

After most of the oil is extracted from cottonseed for food-grade products, the hulls are finely ground to create this product. It is a rich source of nitrogen (7%). The meal is slightly acidic—a boon to those with alkaline soils and/or water—and it’s a good source of trace elements.

Fruit Pomace

Fruit pomace are what remain after the juice is extracted. They are heavy, wet products normally available only locally, and best composted before use.

Soybean Meal

Soybean meal is, like alfalfa, most commonly used as a protein supplement for animal feeds. With about seven percent plant-available nitrogen, it can be a useful, although somewhat expensive, fertilizer. Soybean meal has a similar nutritional composition to that of cottonseed meal, but has a neutral pH.

Seaweed Fertilizers and Amendments

Seaweed fertilizers, soil amendments, and growth promoters are usually derived from kelp (Ascophyllum spp.) and other species of seaweed harvested primarily in the North Atlantic. Dried seaweed contains about one percent nitrogen, a trace of diphosphorus pentoxide (P2O5), 2 percent potassium oxide (K2O), varying amounts of magnesium and sulfur, and numerous trace elements.

Kelp Meal

Ground kelp meal is most often used for production of high-value horticultural crops in situations when the high product cost is most likely recoverable.

Raw Seaweed Products

Raw seaweed products are prepared by various methods and sold under a variety of brand names. Check the OMRI list to ensure that a particular product is allowed.

Seaweed Extracts

More often, compounds from kelp and other seaweed are extracted by various methods to concentrate both micronutrients and naturally occurring plant hormones in a soluble, easily transportable form. Kelp extracts are usually foliar-applied by farmers seeking a natural, supplemental source of micronutrients. Generally, the micronutrient concentrations of kelp extracts are low and may not correct deficiencies of nutrients in the soil.

Mined Mineral Fertilizers

Another nutrient source used by organic farmers is the application of mined minerals. The mined minerals that are most commonly applied on organic farms are rock phosphate, gypsum, limestone, potassium sulfate, and magnesium sulfate. These are all significant sources of primary (N-P-K) and/or secondary plant nutrients such as calcium, magnesium, and sulfur. Less common are other rock powders that are limited sources for the major nutrients but are rich in micronutrients or have some other soil-improving characteristic. Among these are glauconite (greensand), glacial gravel dust, lava sand, granite meal, and others.

Sodium Nitrate

While listed on the National List as a prohibited non-synthetic, sodium nitrate (NaNO3, 16% N) mined from naturally occurring deposits in Chile and Peru can be used in organic production in accordance with its annotation. The annotation allows sodium nitrate to provide no more than 20 percent of the crop's total nitrogen requirement. The NOP Rule restricts its use by requiring documentation in the Organic System Plan and evidence that the restrictions placed on their use are met.

Colloidal Phosphate

Colloidal phosphate, also called soft rock phosphate, consists of clay particles surrounded by natural phosphate. Total phosphate is around 20 percent and available phosphate about two to three percent. It also contains about 25 percent lime and other trace minerals.

Rock Phosphate

Rock phosphate from apatite ore has not been acidulated or otherwise chemically treated. Rock phosphates are usually derived from ancient marine deposits. Synthetic fertilizers such as super- or triple superphosphate are made by reaction with sulfuric or phosphoric acid to increase the solubility of the phosphorus in rock phosphates.

Potassium Sulfate

There are two forms of potassium sulfate on the market. One is derived by reacting sulfuric acid with potassium chloride. It is a good fertilizer, but not acceptable in certified organic production. The other is derived from natural sources and it is allowed for organic crop production.


Langbeinite is listed by the Organic Materials Research Institute (OMRI) as allowable in certified organic production if it is used in the raw, crushed form without any further refinement or purification.

Granite Dust

Granite dust is often sold as a “slowly available” potash source for organic production. Total potash contents in granite dust typically vary from one to five percent, depending on overall mineral composition of the rock, but granite is mostly feldspar, a mineral with low solubility. Therefore, little potash fertility is derived from this material.


Glauconite is a clay-type mineral, commonly sold as greensand, which is listed by OMRI as allowed for organic production. Total potassium oxide (K2O) content of greensand is around seven percent, but most of the potash is unavailable.


Feldspar is one of the major potassium-bearing minerals of granite. Unfortunately, most feldspar potash is as tightly bound within its mineral structure as is the potash in greensand. Unless particular circumstances provide a clear indication that feldspar is the most appropriate source of potash, it is probably not cost-effective.

Wood Ash

Ash from hardwood trees served as one of the earliest sources of potassium for building soil fertility. This highly variable material is composed of the elements initially present in the wood, which were not volatilized when burned. Wood ash is an alkaline material, with a pH ranging from 9 to 13, and has a liming effect of between 8 and 90 percent of the total neutralizing value of commercial limestone.

Gypsum and Limestone

Gypsum and limestone are applied for their calcium content, and to help balance the pH of soil. In many alkaline or sodic soils, application of mined gypsum is a common practice to displace sodium from the soil. The sodium must be leached, usually by irrigation sufficient to wash the salts into the drainage system.


Organic farming focuses on the macronutrients of nitrogen, phosphorus, potassium, sulphur, calcium, and magnesium. However, the crops cannot reach full potential without the major micronutrients, which are boron, chloride, copper, iron, manganese, molybdenum, nickel, and zinc. Micronutrients are important for plant growth, as plants require a proper balance of all the essential nutrients for normal growth and optimum yield.


Cobalt, copper, iron, manganese, molybdenum, selenium, and zinc—can be applied to correct a deficiency provided that they are from sulfate, carbonate, oxide, or silicate sources. Nitrate and chloride forms of these micronutrients are explicitly prohibited. Synthetic soluble sources of boron can also be applied.

Basalt Dust

Basalt dust, if available at a reasonable cost, can provide a wide range of trace minerals to agricultural systems over a period of several years; as with most rock powders, transportation costs are a major factor in determining cost-effectiveness.

Chelating Agents

Chelating agents are compounds to which an element in its ionic form can be attached. Micronutrients can be made more available to plants by chelation with various compounds. Naturally occurring chelating agents such as citric acid may be used. Synthetic chelating agents on the National List such as lignosulfonic acid and its salts; and humic acids are more commonly used. Synthetic chelating agents not on the National List such as EDTA and DTPA are prohibited.

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