224px, Methylene diurea (MDU) is component of the most popular controlled-release fertilizers. A controlled-release fertiliser (CRF) is a

granulated Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation invo ...

fertiliser A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

that releases

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

s gradually into the

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

(i.e., with a

controlled release Modified-release dosage is a mechanism that (in contrast to immediate-release dosage) delivers a drug with a delay after its administration (delayed-release dosage) or for a prolonged period of time (extended-release R, XR, XLdosage) or to a sp ...

period). Controlled-release fertilizer is also known as controlled-availability fertilizer, delayed-release fertilizer, metered-release fertilizer, or slow-acting fertilizer. Usually CRF refers to nitrogen-based fertilizers. Slow- and controlled-release involve only 0.15% (562,000 tons) of the fertilizer market (1995).

History

Controlled-nitrogen-release technologies based on polymers derived from combining urea and formaldehyde were first produced in 1936 and commercialized in 1955. The early product had 60 percent of the total nitrogen cold-water-insoluble, and the unreacted (quick-release) less than 15%. Methylene

ureas 220 px, B vitamin, is a urea.">B_vitamins.html" ;"title="Biotin, a water-soluble B vitamins">B vitamin, is a urea. In chemistry, ureas are a class of organic compounds with the formula (R2N)2CO where R = H, alkyl, aryl, etc. Thus, in addition to ...

, e.g.

methylene diurea Methylene diurea (MDU) is the organic compound with the formula CH2(NHC(O)NH2)2. It is a white water-soluble solid. The compound is formed by the condensation of formaldehyde with urea. Methylene diurea is the substrate for the enzyme methylen ...

, were commercialized in the 1960s and 1970s, having 25% and 60% of the nitrogen as cold-water-insoluble, and unreacted urea nitrogen in the range of 15% to 30%. In the 1960s in the U.S., the

Tennessee Valley Authority The Tennessee Valley Authority (TVA) is a federally owned electric utility corporation in the United States. TVA's service area covers all of Tennessee, portions of Alabama, Mississippi, and Kentucky, and small areas of Georgia, North Carolina ...

National Fertilizer Development Center began developing sulfur-coated urea. Sulfur was used as the principal coating material because of its low cost and its value as a secondary nutrient. Usually wax or polymer is added to perfect the encapsulation. The slow-release properties depend on the degradation of the secondary sealant by soil microbes as well as mechanical imperfections (cracks, etc.) in the capsule. 6 to 16 weeks of delayed release in turf applications is typical. When a hard polymer is used as the secondary coating, the properties are a cross between diffusion-controlled particles and traditional sulfur-coated.

Advantages

Many factors motivate the use of CRF, including more efficient use of the fertilizer. Illustrating the problem, it is estimated that, on average, 16% of conventional nitrogen-based fertilizers is lost by evaporation (as NH₃, N₂O, N₂) or run-off ammonia. Another factor favoring CRT protecting crops from chemical damage ( fertiliser burn). In addition to their providing the nutrition to plants, excess fertilizers can be poisonous to the same plant. Finally important advantages are economic: fewer applications and the use of less fertiliser overall. The results (yield) is in most cases improved by >10%.

Environmental considerations

CRF has the potential to decrease nitrogenous pollution, which leads to

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

. The efficient use of nitrogen-base fertilizers is also relevant to the emission of into the atmosphere each year, of which 36% is due to human activity. The anthropogenic is produced by

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s acting on ammonia faster than the plant can uptake this nutrient.

Implementation

The fertiliser is administered either by topdressing the soil, or by mixing the fertiliser into the soil before sowing. Polymer coating of fertilizer ingredients gives tablets and spikes a 'true time-release' or 'staged nutrient release' (SNR) of fertilizer nutrients.

NBPT ''N''-(''n''-Butyl)thiophosphoric triamide (NBPT) is the organophosphorus compound with the formula SP(NH2)2(NHC4H9). A white solid, NBPT is an "enhanced efficiency fertilizer", intended to limit the release of nitrogen-containing gases followi ...

functions as an inhibitor of the enzyme

urease Ureases (), functionally, belong to the superfamily of amidohydrolases and phosphotriesterases. Ureases are found in numerous bacteria, fungi, algae, plants, and some invertebrates, as well as in soils, as a soil enzyme. They are nickel-contai ...

. Urease inhibitors, at levels of 0.05 weight percent, are added to urea-based fertilizers to control its conversion to ammonia.

Mechanisms of release

The rate of the release is determined by various main factors: (i) the low solubility of the compounds in the soil moisture, (ii) the breakdown of protective coating applied to fertilizer pellets, and (iii) the conversion of the chemicals into ammonia or similarly effective plant nutrient. Conventional fertilisers are

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

in water, the nutrients disperse. Because controlled-release fertilisers are not water-soluble, their nutrients disperse into the soil more slowly. The fertiliser granules may have an insoluble substrate or a

semi-permeable Semipermeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecule ...

jacket that prevents dissolution while allowing nutrients to flow outward.

Definitions

The Association of American Plant Food Control Officials (AAPFCO) has published the following general definitions (Official Publication 57): * Slow- or controlled-release fertilizer: A fertilizer containing a plant nutrient in a form which delays its availability for plant uptake and use after application, or which extends its availability to the plant significantly longer than a reference ‘rapidly available nutrient fertilizer’ such as ammonium nitrate or urea, ammonium phosphate or potassium chloride. Such delay of initial availability or extended time of continued availability may occur by a variety of mechanisms. These include controlled water solubility of the material by semi-permeable coatings, occlusion, protein materials, or other chemical forms, by slow hydrolysis of water-soluble low molecular weight compounds, or by other unknown means. * Stabilized nitrogen fertilizer: A fertilizer to which a nitrogen stabilizer has been added. A nitrogen stabilizer is a substance added to a fertilizer which extends the time the nitrogen component of the fertilizer remains in the soil in the urea-N or ammoniacal-N form. *

Nitrification ''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of am ...

inhibitor: A substance that inhibits the biological oxidation of ammoniacal-N to nitrate-N. *

Urease Ureases (), functionally, belong to the superfamily of amidohydrolases and phosphotriesterases. Ureases are found in numerous bacteria, fungi, algae, plants, and some invertebrates, as well as in soils, as a soil enzyme. They are nickel-contai ...

inhibitor: A substance that inhibits hydrolytic action on urea by the enzyme urease.

Examples

Most slow-release fertilizers are derivatives of urea, a straight fertilizer providing nitrogen.

Isobutylidenediurea Isobutylidenediurea (abbreviated IBDU) is an organic compound with the formula (CH3)2CHCH2. It is a derivative of urea (OC(NH2)2), which itself is highly soluble in water, but IBDU is not. It functions as a controlled-release fertiliser owing to ...

("IBDU") and urea-formaldehyde slowly convert in the soil to urea, which is rapidly uptaken by plants. IBDU is a single compound with the formula (CH₃)₂CHCH(NHC(O)NH₂)₂ whereas the urea-formaldehydes consist of mixtures of the approximate formula (HOCH₂NHC(O)NH)_nCH₂. Controlled release fertilizers are traditional fertilizers encapsulated in a shell that degrades at a specified rate. Sulfur is a typical encapsulation material. Other coated products use thermoplastics (and sometimes ethylene-vinyl acetate and surfactants, etc.) to produce diffusion-controlled release of

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

or other fertilizers. "Reactive Layer Coating" can produce thinner, hence cheaper, membrane coatings by applying reactive monomers simultaneously to the soluble particles. "Multicote" is a process applying layers of low-cost fatty acid salts with a paraffin topcoat.