Controlled-release fertilizer/ja

メチレン尿素（MDU）は、最も普及している緩効性肥料の成分である。

緩効性肥料（CRF）は、粒状の肥料であり、養分を土壌中に徐々に放出する（すなわち、制御された放出期間を持つ）。緩効性肥料は、制御放出性肥料、遅効性肥料、計量放出性肥料、または遅効性肥料としても知られている。通常、CRFは窒素ベースの肥料を指す。緩効性および制御放出性肥料は、肥料市場のわずか0.15%（56万2,000トン）（1995年）を占めるにすぎない。

歴史

尿素とホルムアルデヒドを組み合わせたポリマーに由来する制御型窒素放出技術は、1936年に初めて製造され、1955年に商品化された。初期の製品は、全窒素の60%が冷水不溶性であり、未反応（速効性）のものは15%未満であった。メチレン尿素、例えばメチレンジウレアは、1960年代から1970年代にかけて商品化され、窒素の25%から60%が冷水不溶性であり、未反応の尿素窒素は15%から30%の範囲であった。

1960年代に米国で、テネシー川流域開発公社国立肥料開発センターが硫黄被覆尿素の開発を開始した。硫黄はその低コストと二次養分としての価値から、主要な被覆材として使用された。通常、完全なカプセル化のためにワックスまたはポリマーが添加される。遅効性特性は、カプセル内の二次密封材の土壌微生物による分解、および機械的欠陥（ひび割れなど）に依存する。芝生への適用では、6〜16週間の遅延放出が一般的である。硬質ポリマーが二次被覆として使用される場合、その特性は拡散制御粒子と従来の硫黄被覆の間のものとなる。

利点

緩効性肥料（CRF）の使用を促す多くの要因があり、その一つは肥料のより効率的な使用である。問題を示すと、平均して従来の窒素系肥料の16%が蒸発（NH₃、N₂O、N₂として）またはアンモニアの流出によって失われると推定されている。CRTを支持するもう一つの要因は、作物に化学的損傷（肥料焼け）から保護することである。植物に栄養を供給することに加えて、過剰な肥料は同じ植物に有害となる可能性がある。最後に重要な利点は経済的なものであり、施肥回数が減り、全体的に使用する肥料の量が少なくなる。ほとんどの場合、結果（収量）は10%以上改善される。

Environmental considerations

CRF has the potential to decrease nitrogenous pollution, which leads to eutrophication. The efficient use of nitrogen-base fertilizers is also relevant to the emission of N
₂O into the atmosphere each year, of which 36% is due to human activity. The anthropogenic N
₂O is produced by microorganisms 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 functions as an inhibitor of the enzyme urease. Urease inhibitors, at levels of 0.05 weight percent, are added to urea-based fertilizers to control its conversion to ammonia.

Mechanisms of release

Crotonylidene diurea is also used as a CRF.

Isobutylidenediurea (IBDU) is yet another CRF.

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 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 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 inhibitor: A substance that inhibits the biological oxidation of ammoniacal-N to nitrate-N.
• Urease 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 ("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 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. Recently, biodegradable polymers as coatings for slow/controlled-release fertilizer have attracted interest for their potential to increase fertilizer/pesticide utilization efficiency and reduce negative environmental effects.

関連項目

• 粘土団子
• 被覆尿素