Fertilizer

A fertilizer (American English) or fertiliser (British English) 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 liming materials or other non-nutrient soil amendments. Many sources of fertilizer exist, both natural and industrially produced.^[1] For most modern agricultural practices, fertilization focuses on three main macro nutrients: nitrogen (N), phosphorus (P), and potassium (K) with occasional addition of supplements like rock flour for micronutrients. Farmers apply these fertilizers in a variety of ways: through dry or pelletized or liquid application processes, using large agricultural equipment or hand-tool methods.

Historically fertilization came from natural or organic sources: compost, animal manure, human manure, harvested minerals, crop rotations and byproducts of human-nature industries (i.e. fish processing waste, or bloodmeal from animal slaughter). However, starting in the 19th century, after innovations in plant nutrition, an agricultural industry developed around synthetically created fertilizers. This transition was important in transforming the global food system, allowing for larger-scale industrial agriculture with large crop yields.

Nitrogen-fixing chemical processes, such as the Haber process invented at the beginning of the 20th century, and amplified by production capacity created during World War II, led to a boom in using nitrogen fertilizers.^[2] In the latter half of the 20th century, increased use of nitrogen fertilizers (800% increase between 1961 and 2019) has been a crucial component of the increased productivity of conventional food systems (more than 30% per capita) as part of the so-called "Green Revolution".^[3]

The use of artificial and industrially-applied fertilizers has caused environmental consequences such as water pollution and eutrophication due to nutritional runoff; carbon and other emissions from fertilizer production and mining; and contamination and pollution of soil. Various sustainable-agriculture practices can be implemented to reduce the adverse environmental effects of fertilizer and pesticide use as well as other environmental damage caused by industrial agriculture.

History[edit]

Management of soil fertility has preoccupied farmers for thousands of years. Egyptians, Romans, Babylonians, and early Germans are all recorded as using minerals or manure to enhance the productivity of their farms.^[1] The science of plant nutrition started well before the work of German chemist Justus von Liebig although his name is most mentioned. Nicolas Théodore de Saussure and scientific colleagues at the time were quick to disprove the simplifications of von Liebig. There was a complex scientific understanding of plant nutrition, where the role of humus and organo-mineral interactions were central, and which was in line with more recent discoveries from 1990 onwards.^{[citation needed]} Prominent scientists on whom von Liebig drew were Carl Ludwig Sprenger and Hermann Hellriegel. In this field, a 'knowledge erosion'^[6] took place, partly driven by an intermingling of economics and research.^[7] John Bennet Lawes, an English entrepreneur, began to experiment on the effects of various manures on plants growing in pots in 1837, and a year or two later the experiments were extended to crops in the field. One immediate consequence was that in 1842 he patented a manure formed by treating phosphates with sulfuric acid, and thus was the first to create the artificial manure industry. In the succeeding year he enlisted the services of Joseph Henry Gilbert; together they performed crop experiments at the Institute of Arable Crops Research.^[8]

The Birkeland–Eyde process was one of the competing industrial processes in the beginning of nitrogen-based fertilizer production.^[9] This process was used to fix atmospheric nitrogen (N₂) into nitric acid (HNO₃), one of several chemical processes generally referred to as nitrogen fixation. The resultant nitric acid was then used as a source of nitrate (NO₃⁻). A factory based on the process was built in Rjukan and Notodden in Norway, combined with the building of large hydroelectric power facilities.^[10]

The 1910s and 1920s witnessed the rise of the Haber process and the Ostwald process. The Haber process produces ammonia (NH₃) from methane (CH₄) (natural gas) gas and molecular nitrogen (N₂) from the air. The ammonia from the Haber process is then partially converted into nitric acid (HNO₃) in the Ostwald process.^[11] After World War II, nitrogen production plants that had ramped up for wartime bomb manufacturing were pivoted towards agriculture uses.^[12] The use of synthetic nitrogen fertilizers has increased steadily over the last 50 years, rising almost 20-fold to the current rate of 100 million tonnes of nitrogen per year.^[13]

The development of synthetic nitrogen fertilizer has significantly supported global population growth. It has been estimated that almost half the people on the Earth are currently fed as a result of synthetic nitrogen fertilizer use.^[14] The use of phosphate fertilizers has also increased from 9 million tonnes per year in 1960 to 40 million tonnes per year in 2000.

Agricultural use of inorganic fertilizers in 2021 was 195 million tonnes of nutrients, of which 56% was nitrogen.^[15] Asia represented 53% of world total agricultural use of inorganic fertilizers in 2021, followed by the Americas (29%), Europe (12%), Africa (4%) and Oceania (2%). This ranking of the regions is the same for all nutrients. The main users of inorganic fertilizers are, in descending order, China, India, Brazil and the United States of America (see Table 15), with China the largest user of each nutrient.^[15]

NPK Compound Fertilizer Granular High Tower Granulation Tumbling Granulation

A maize crop yielding 6–9 tonnes of grain per hectare (2.5 acres) requires 31–50 kilograms (68–110 lb) of phosphate fertilizer to be applied; soybean crops require about half, 20–25 kg per hectare.^[16] Yara International is the world's largest producer of nitrogen-based fertilizers.^[17]

Mechanism[edit]

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 many on fertilizers, emphasises the nutritional aspect. Fertilizers typically provide, in varying proportions:^[19]

• three main macronutrients (NPK):
  • Nitrogen (N): leaf growth
  • Phosphorus (P): development of roots, flowers, seeds, fruit;
  • Potassium (K): strong stem growth, movement of water in plants, promotion of flowering and fruiting;
• three secondary macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S);
• micronutrients: copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), boron (B). Of occasional significance are silicon (Si), cobalt (Co), and vanadium (V).

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 macro-nutrients 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 respectively in carbon dioxide and in water. 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 in proteins (amide bond between amino-acids), DNA (puric and pyrimidic bases) and other components (e.g., tetrapyrrolic heme in 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 (NH₃). Phosphate (PO3−4) is required for the production of DNA (genetic code) and ATP, the main energy carrier in cells, as well as certain lipids (phospholipids, the main components of the lipidic double layer of the cell membranes).

Microbiological considerations[edit]

Two sets of enzymatic reactions are highly relevant to the efficiency of nitrogen-based fertilizers.

Urease

The first is the hydrolysis (reaction with water) of urea (CO(NH₂)₂). Many soil bacteria possess the enzyme urease, which catalyzes the conversion of urea to ammonium ion (NH+4) and bicarbonate ion (HCO−3).

Ammonia oxidation

Ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas, oxidize ammonia (NH₃) to nitrite (NO−2), a process termed nitrification.^[20] Nitrite-oxidizing bacteria, especially Nitrobacter, oxidize nitrite (NO−2) to nitrate (NO−3), which is extremely soluble and mobile and is a major cause of eutrophication and algal bloom.

Classification[edit]

Fertilizers are classified in several ways. They are classified according to whether they provide a single nutrient (e.g., K, P, or N), in which case they are classified as "straight fertilizers". "Multinutrient fertilizers" (or "complex fertilizers") provide two or more nutrients, for example N and P. Fertilizers are also sometimes classified as inorganic (the topic of most of this article) versus organic. Inorganic fertilizers exclude carbon-containing materials except ureas. Organic fertilizers are usually (recycled) plant- or animal-derived matter. Inorganic are sometimes called synthetic fertilizers since various chemical treatments are required for their manufacture.^[21]

Single nutrient ("straight") fertilizers[edit]

The main nitrogen-based straight fertilizer is ammonia (NH₃) ammonium (NH₄⁺) or its solutions, including:

• Ammonium nitrate (NH₄NO₃) is also widely used.
• Urea (CO(NH₂)₂),another popular source of nitrogen, having the advantage that it is solid and non-explosive, unlike ammonia and ammonium nitrate.
• Calcium ammonium nitrate (Ca(NO₃)₂ · NH₄ · 10 H₂O), reportedly holding few% of the nitrogen fertilizer market (4% in 2007).^[22]

The main straight phosphate fertilizers are the superphosphates:

• "Single superphosphate" (SSP) consisting of 14–18% P₂O₅, again in the form of Ca(H₂PO₄)₂, but also phosphogypsum (CaSO₄ · 2 H₂O).
• Triple superphosphate (TSP) typically consists of 44–48% of P₂O₅ and no gypsum.

A mixture of single superphosphate and triple superphosphate is called double superphosphate. More than 90% of a typical superphosphate fertilizer is water-soluble.

The main potassium-based straight fertilizer is muriate of potash (MOP, 95–99% KCl). It's typically available as 0-0-60 or 0-0-62 fertilizer.

Multinutrient fertilizers[edit]

These fertilizers are common. They consist of two or more nutrient components.

Binary (NP, NK, PK) fertilizers

Major two-component fertilizers provide both nitrogen and phosphorus to the plants. These are called NP fertilizers. The main NP fertilizers are monoammonium phosphate (MAP) and diammonium phosphate (DAP). The active ingredient in MAP is NH₄H₂PO₄. The active ingredient in DAP is (NH₄)₂HPO₄. About 85% of MAP and DAP fertilizers are soluble in water.

NPK fertilizers

NPK fertilizers are three-component fertilizers providing nitrogen, phosphorus, and potassium. There exist two types of NPK fertilizers: compound and blends. Compound NPK fertilizers contain chemically bound ingredients, while blended NPK fertilizers are physical mixtures of single nutrient components.

NPK rating is a rating system describing the amount of nitrogen, phosphorus, and potassium in a fertilizer. NPK ratings consist of three numbers separated by dashes (e.g., 10-10-10 or 16-4-8) describing the chemical content of fertilizers.^[23][24] The first number represents the percentage of nitrogen in the product; the second number, P₂O₅; the third, K₂O. Fertilizers do not actually contain P₂O₅ or K₂O, but the system is a conventional shorthand for the amount of the phosphorus (P) or potassium (K) in a fertilizer. A 50-pound (23 kg) bag of fertilizer labeled 16-4-8 contains 8 lb (3.6 kg) of nitrogen (16% of the 50 pounds), an amount of phosphorus equivalent to that in 2 pounds of P₂O₅ (4% of 50 pounds), and 4 pounds of K₂O (8% of 50 pounds). Most fertilizers are labeled according to this N-P-K convention, although Australian convention, following an N-P-K-S system, adds a fourth number for sulfur, and uses elemental values for all values including P and K.^[25]

Micronutrients[edit]

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 or less than 0.04% dry matter.^[26][27] These elements are often required for enzymes essential to the plant's metabolism. Because these elements enable catalysts (enzymes), their impact far exceeds their weight%age. Typical micronutrients are boron, zinc, molybdenum, iron, and manganese.^[19] These elements are provided as water-soluble salts. Iron presents special problems because it converts to insoluble (bio-unavailable) compounds at moderate soil pH and phosphate concentrations. For this reason, iron is often administered as a chelate complex, e.g., the EDTA or EDDHA derivatives. The micronutrient needs depend on the plant and the environment. For example, sugar beets appear to require boron, and legumes require cobalt,^[1] while environmental conditions such as heat or drought make boron less available for plants.