Fertilizers for Greenhouse Crops
Organic Fertilizers
Organic fertilizers comprise a variety of plant-derived materials that range from fresh or dried plant material to animal manures and litters to agricultural by-products. They are considered the more environmentally friendly option. The nutrient content of organic fertilizers varies greatly among source materials, and readily biodegradable materials make better nutrient sources. Nitrogen and phosphorus content are lower, often substantially lower, in organic fertilizers compared to inorganic fertilizers. There’s little to no risk of toxic buildups of chemicals and salts that can be deadly to plants. However, many of these nutrients have to be converted into inorganic forms by soil bacteria and fungi before plants can use them, so they typically are more slowly released, especially during cold weather when soil microbes are not as active. Moisture content is another factor that reduces or dilutes the nitrogen and phosphorus concentrations of organic fertilizers. Thus, it can be cost ineffective to transport high-moisture organic fertilizer long distances. However, use of locally available sources is perfectly reasonable if its use is consistent with the production strategy. Nutrient value of animal manures is more variable than that of agricultural by-products. The animal's diet, the use and type of bedding material, manure age, and how it was stored are factors that affect manure nutrient value; these factors can vary seasonally on and among farms, and regionally or on a larger geographic scale. In contrast, nutrient content of agricultural by-products is less variable but can be affected by the industrial process used to produce the by-product. Plant- or animal-derived organics like manure not only supply plant nutrients but also organic matter which improves soil physical condition and stimulates beneficial soil microorganisms. It always is advisable to analytically determine the nutrient content of the organic fertilizer.
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
Many of the most popular organic fertilizers and soil amendments contain animal ingredients such as manues, compost, bone meal, blood meal, feather meal, fish-based fertilizers, and guano. 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. 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.
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 (or pellets) contains around three to four percent nitrogen and is commonly used as an animal feed.
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. 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.
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 greenhouse operations 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. 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. Colloidal phosphate, also called soft rock phosphate, consists of clay particles surrounded by natural phosphate.
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