Fertilizers play a vital role in feeding the earth’s growing population, currently standing at 7.5 billion. But it is equally important that fertilizers are manufactured and used responsibly. From removal of dangerous heavy metals to preventing eutrophication, regulators, farmers and consumers must pay close attention to fertilizer quality and use.

What is Fertilizer?

boost-in-wheat-growth-with-fertilizers.jpg
Figure 1: Boost in wheat growth with fertilizers [1].

Soil that is fertile and supports good crop yields is a key element of food security, playing an important role in enabling agricultural producers to meet growing global demand for food. High-intensity agriculture, which produces the yields necessary to feed the world’s growing population, usually requires more nutrients than can be supplied by the soil. Fertilizers that contain one or more nutrients may be used to replenish the soil and support further plant growth. Fertilizers may be applied to the soil or, in some cases, directly to leaves. They not only replenish lost nutrients, but also provide additional stimulus to increase the volume and quality of crops. Fertilizers may be either organic (compost, manure, sludge) or mined from naturally-occurring inorganic resources like phosphate ore, in the case of phosphate-based fertilizers. The three main nutrients required by plants are nitrogen (N), phosphorus (P), and potassium (K), which are followed by secondary elements (sulphur, calcium and magnesium) as well as certain microelements such as boron, zinc, manganese, and others. Plants primarily require nitrogen, phosphorous and potassium. Therefore, most fertilizers will consist of one, if not all three.

Table 1: Primary nutrient groups [2].
  Nitrogen (N) Phosphorus (P) Potassium (K)
Effects on plant growth Found in every living organism, supporting protein & amino-acid development, boosting yields & quality. Plant’s source of energy, supporting photosynthesis, plant respiration & steady growth. Also increases/accelerates germination, root & seed development. Supports energy transfer, boosting disease & weather resistance. A deficiency can lead to fruit deformation, lower quality & poor germination.
Production Ammonia based, which itself is extracted from natural gas and/or coal. Key products include urea and ammonium nitrate. Phosphate containing ore is processed and usually reacted sulphur and ammonia to produce fertilizers. Key products include di-/mono-ammonium phosphate. Potassium-based fertilizers are made of potash containing salts. Key fertilizers include MOP, SOP and NOP.
Complex fertilizers Complex fertilizers contain a combination of macro, secondary and micro-nutrients needed by plants in different proportions. Their use supports balanced nutrition.

Mineral fertilizers

While humans have been using simple forms of fertilizer since the Neolithic Age, mineral or chemical fertilizers were first developed in the 1850s with an aim to replenish and to supplement nutrients already available in the soil. The first chemical manufacturing of phosphate-based fertilizers (made by treating bones with sulfuric acid) began in the 19th century. Potash fertilizer production started in Germany in 1861, and synthetic nitrogen fertilizers were first manufactured in 1903. Table 2 provides an overview of different types of mineral fertilizers used around the world.

Types of mineral fertilizers

Method of production

  • natural (as found in nature or only slightly processed).
  • synthetic (manufactured by industrial processes).

Number of nutrients

  • single-nutrient or straight fertilizers (whether for major, secondary or micro nutrients).
  • multi-nutrient (multiple nutrient) or compound fertilizers, with 2, 3 or more nutrients.

Type of combination

  • mixed fertilizers, i.e. a physical mixture of two or more single-nutrient or multi- nutrient fertilizers (for granular products this may comprise a blend of separate granules of the individual ingredients, or granules each containing these ingredients).
  • complex fertilizers, in which two or more of the nutrients are chemically combined (e.g. nitrophosphate, ammonium phosphates).

Physical condition

  • solid (crystalline, powdered, prilled or granular) of various size ranges.
  • liquid (solutions and suspensions).
  • gaseous (liquid under pressure, e.g. ammonia).

Mode of action

  • quick-acting (water-soluble and immediately available).
  • slow-acting (transformation into soluble form required).
Figure 2: Mineral Fertilizers [3].

What are Phosphates?

In agriculture, plants mostly absorb phosphorous through the soil. As a nutrient, it plays an important role during a plant’s growth phase, improving its metabolism, cell division, reproduction, and leading to the development of a strong and healthy root system. This, in turn, supports the transfer of moisture and nutrients, strengthens winter crops’ resistance to frost, and reduces the amount of water intake.

For animals and humans, phosphorous is accessible through plants and feed or food additives. Its role is just as important a here, since it is a part of bone tissue and is an indispensable element in the processes of ossification, muscle contraction, separation of exchange products and energy production. In the case of ruminants, it supports digestion and assimilation of nutrients.

Today all commercial phosphate products are produced using mined phosphate ores. Due to their low levels of solubility and plant uptake, most ores are processed into concentrates, before being reacted with sulphuric or nitric acids and processed into fertilizers, feed and food additives. By far the most common and cost effective products are those based on wet phosphoric acid (WPA). In 2016, between 80-90% of all phosphates consumed, stemmed from products produced using WPA.

phosphorus-deficiency.jpgphosphorus-deficiency-on-corn.jpg
Figure 3: Comparison of phosphorus deficiencies [4].

Phosphate production begins by mining phosphate containing ore. The richest concentrations are found in phosphorite and apatite ore bodies. The former, phosphorite, are usually of a sedimentary nature. These are easily the most common, accounting for 90-95% of rock concentrate produced today [5].

The largest sedimentary deposits can be found in North Africa, the Middle East and the U.S. These are formed over millennia. In most cases, their ores are composed of softer rocks, requiring different mining techniques and less processing in comparison to igneous rock. Although this limits extraction/ production costs, it also means that concentrate from sedimentary ores is susceptible to a larger numbers of impurities. These can increase downstream production costs for final products like fertilizers in order to remove unnecessary and potentially harmful elements like heavy metals (see the Heavy Metals section for more information).

Igneous deposits, by contrast, form as a result of magma cooling over a period of tens or hundreds of million years. The largest known deposits are located in Brazil, Canada, Finland, Russia and South Africa. Mining of deposits of this type is associated with higher operational and capital expenditures, but the benefit thereof is associated with a higher grade concentrate, containing fewer impurities. Table 1 gives an overview of known sedimentary and igneous phosphate rock reserves around the world.

Table 2: Comparison of Key Phosphate Resources [6].
Location Russia Morocco Tunisia USA Jordan China South Africa
World phosphate rock reserves. (Bln tonnes) 2.05 50 0.1 1.4 1.5 3.7 3.7
Operational Capacity (Mln T/y) 16.6 34.8 9.8 34.6 12.1 82.1 3.9
Ore type Igneous Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Igenous & Sedimentary
Al2O3 content Moderate Very low Very low Very low Very low Very low Low to Moderate
Minor element ratio (MER) Low Very Low Very Low Low to Moderate Very Low Moderate to High Moderate
Cadmium content <0.1 15-40 40 9-38 5-6 2 2
Other Heavy Metals Content Very low Moderate Low to Moderate Moderate to High Low Low to Moderate Very Low

The final stage of the process is the production of phosphate fertilizers, feed of industrial products. The most common phosphate fertilizers are outlined below:

Superphosphates: Single (SSP), double (DSP) and triple-superphosphates (TSP) were once the most common types of fertilizer, with P2O5 concentrations ranging from around 12% through to 46%. However, their popularity has diminished in recent decades.

Ammoniated phosphates: Di-ammonium (DAP) and mono-ammonium (MAP) phosphates involve the reaction of WPA with ammonia. Due to the benefits associated with offering nitrogen and phosphate together, these fertilizers have evolved to become the most common, accounting for more than 50% of global fertilizer consumption [7].

Complex & compound fertilizers: NPK, NPS and other complex fertilizers containing a mix of primary and secondary nutrients as well as microelements. Demand for such fertilizers continues to grow at a steady rate, especially for those containing sulphur and various microelements (zinc, bromine, molybdenum, copper, manganese and iron).

Nutrient breakdown of common phosphate fertilizer products

Table 3: Nutrient breakdown of common phosphate fertilizer products [8].
* some with S and/or Mg and/or micronutrients, the above numbers represent the average range of nutrient content in commercially available fertilizers
Common names N P2O5 K2O S MgO
Single superphosphate 0 16-20 0 12 0
Triple superphosphate 0 46 0 0 0
Diammonium phosphate 18 46 0 0 0
Monoammonium phosphate 11 52 0 0 0
Ground rock phosphate 0 20-40 0 0 0
NPK fertilizers 5-25 5-25 5-25 Mod* *
NP fertilizers 15-25 15-25 0 * 0
NK fertilizers 13-25 0 15-46 Mode* 0
PK fertilizers 0 7-30 10-30 Mode* *
  1. Source: IGNI [return]
  2. Sources: Fertecon, CRU, USGS, ML2R Consultancy, Kemworks, Phosagro Estimates. [return]
  3. Source: IFA, "Fertilizers and their efficient use" (http://www.fertilizer.org/en/images/Library_Downloads/2016_ifa_reetz.pdf). [return]
  4. Source: IGNI [return]
  5. Humphrey Knight – Consultant Fertilizers. Mineral processing of phosphate ore and associated gangue minerals [return]
  6. Sources: Fertecon, CRU, USGS 2011, ML2R Consultancy, Kemworks, Phosagro Estimates. [return]
  7. International Fertilizer Industry Association (2015) [return]
  8. Sources: IFA [return]

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