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FERTILIZATION GUIDE
 

•  CORRECT FERTILIZERS

•  EXAMPLES OF FERTILIZERS

•  FERTILIZATION GUIDE
 
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fertilizer (or fertiliser in British English) is any material of natural or synthetic origin (other than liming materials) that is applied to soils or to plant tissues (usually leaves) to supply one or more plant nutrients essential to the growth of plants.

The three main macronutrients are nitrogen (N), phosphorus (P), and potassium (K) - but not in their elemental form. Generally, their effects on plants are as follows.

Nitrogen: Leaf growth

Phosphorus: Development of roots, flowers, seeds, fruit

Potassium: Strong stem growth, movement of water in plants, promotion of flowering and fruiting.

Mechanism

Fertilizers enhance the growth of plants. This goal is met in two ways, the traditional one being additives that provide nutrients. The second mode by which some fertilizers act is to enhance the effectiveness of the soil by modifying its water retention and aeration. This article, like most on fertilizers, emphasizes the nutritional aspect. Fertilizers typically provide, in varying proportions:

  • three main macronutrients: nitrogen (N), phosphorus (P), potassium (K);
  • three secondary macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S);
  • micronutrients: copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), boron (B), and of occasional significance there are silicon (Si), cobalt (Co), andvanadium (V) plus rare mineral catalysts.

The nutrients required for healthy plant life are classified according to the elements, but the elements are not used as fertilizers. Instead compounds containing these elements are the basis of fertilizers. The macronutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.15% to 6.0% on a dry matter (DM) (0% moisture) basis. Plants are made up of four main elements: hydrogen, oxygen, carbon, and nitrogen. Carbon, hydrogen and oxygen are widely available as water and carbon dioxide. Although nitrogen makes up most of the atmosphere, it is in a form that is unavailable to plants. Nitrogen is the most important fertilizer since nitrogen is present inproteins, DNA and other components (e.g., chlorophyll). To be nutritious to plants, nitrogen must be made available in a "fixed" form. Only some bacteria and their host plants (notably legumes) can fix atmospheric nitrogen (N2) by converting it to ammonia. Phosphate is required for the production of DNA and ATP, the main energy carrier in cells, as well as certain lipids.

Micronutrients are consumed in smaller quantities and are present in plant tissue on the order of parts-per-million (ppm), ranging from 0.15 to 400 ppm DM, or less than 0.04% DM. These elements are often present at the active sites of enzymes that carry out the plant's metabolism. Because these elements enable catalysts (enzymes) their impact far exceeds their weight percentage.

Single nutrient ("straight") fertilizers

The main nitrogen-based straight fertilizer is ammonia or its solutions. Ammonium nitrate (NH4NO3) is also widely used. About 15M tons were produced in 1981, i.e., several kilograms per person. Urea is another popular source of nitrogen, having the advantage that it is a solid and non-explosive, unlike ammonia and ammonium nitrate, respectively. A few percent of the nitrogen fertilizer market (4% in 2007) is met by calcium ammonium nitrate (Ca(NO3)2•NH4NO3•10H2O).

Production

Nitrogen fertilizers

Top users of nitrogen-based fertilizer
Country Total N use

(Mt pa)

Amt. used for feed/pasture

(Mt pa)

China 18.7 3.0
U.S. 9.1 4.7
France 2.5 1.3
Germany 2.0 1.2
Brazil 1.7 0.7
Canada 1.6 0.9
Turkey 1.5 0.3
UK 1.3 0.9
Mexico 1.3 0.3
Spain 1.2 0.5
Argentina 0.4 0.1

All nitrogen fertilizers are made from ammonia (NH3), which is sometimes injected into the ground directly. The ammonia is produced by the Haber-Bosch process. In this energy-intensive process, natural gas (CH4) supplies the hydrogen and the nitrogen (N2) is derived from the air. This ammonia is used as a feedstock for all other nitrogen fertilizers, such as anhydrous ammonium nitrate (NH4NO3) and urea (CO(NH2)2). Deposits of sodium nitrate (NaNO3) (Chilean saltpeter) are also found in the Atacama desert in Chile and was one of the original (1830) nitrogen-rich fertilizers used. It is still mined for fertilizer.

Phosphate fertilizers

All phosphate fertilizers are obtained by extraction from minerals containing the anion PO43−. In rare cases, fields are treated with the crushed mineral, but most often more soluble salts are produced by chemical treatment of phosphate minerals. The most popular phosphate-containing minerals are referred to collectively as phosphate rock. The main minerals are fluorapatite Ca5(PO4)3F (CFA) and hydroxyapatite Ca5(PO4)3OH. These minerals are converted to water-soluble phosphate salts by treatment with sulfuric or phosphoric acids. The large production of sulfuric acid as an industrial chemical is primarily due to its use as cheap acid in processing phosphate rock into phosphate fertilizer. The global primary uses for both sulfur and phosphorus compounds relate to this basic process.

In the nitrophosphate process or Odda process (invented in 1927), phosphate rock with up to a 20% phosphorus (P) content is dissolved with nitric acid (HNO3) to produce a mixture of phosphoric acid (H3PO4) and calcium nitrate(Ca(NO3)2). This mixture can be combined with a potassium fertilizer to produce a compound fertilizer with the three macronutrients N, P and K in easily dissolved form.

Potassium fertilizers

Potash is a mixture of potassium minerals used to make potassium (chemical symbol: K) fertilizers. Potash is soluble in water, so the main effort in producing this nutrient from the ore involves some purification steps; e.g., to remove sodium chloride (NaCl), i.e. common salt. Sometimes potash is referred to as K2O, as a matter of convenience to those describing the potassium content. In fact potash fertilizers are usually potassium chloride, potassium sulfate, potassium carbonate, or potassium nitrate.

Compound fertilizers

Compound fertilizers, which contain N, P, and K, can often be produced by mixing straight fertilizers. In some cases, chemical reactions occur between the two or more components. For example monoammonium and diammonium phosphates, which provide plants with both N and P, are produced by neutralizing phosphoric acid (from phosphate rock) and ammonia (from a Haber facility):

NH3 + H3PO4 → (NH4)H2PO4
2 NH3 + H3PO4 → (NH4)2HPO4

Organic fertilizers

Main article: Organic fertilizer
 
 
 
 

The main "organic fertilizers" are, in ranked order, peat, animal wastes, plant wastes from agriculture, and sewage sludge. In terms of volume, peat is the most widely used organic fertilizer. This immature form of coal confers no nutritional value to the plants, but improves the soil by aeration and absorbing water. Animal sources include the products of the slaughter of animals. Bloodmeal, bone meal, hides, hoofs, and horns are typical components. Organic fertilizer usually contain less nutrients, but offer other advantages as well as appealing to environmentally friendly users.

Other elements: calcium, magnesium, and sulfur

Calcium is supplied as superphosphate or calcium ammonium nitrate solutions.