Fertilizers

For healthy growth, plants require many different nutrients. Of these, nitrogen is needed for leaf growth and protein production, phosphorus promotes root and early seedling growth, while potassium regulates the transport of other nutrients and water within the plant. Sulfur is essential for protein synthesis and magnesium is required for the formation of chlorophyll. Other necessary elements include Ca, Na, Fe, Mn, B, Cu, Zn and Co. Deficiencies in a particular soil can be treated by the addition of appropriate fertilizers.

Figure 1 In the 19th century, John Lawes and Henry Gilbert began a series of field experiments to examine the effects of inorganic fertilizers and organic manures on important crops such as wheat which have continued to this day. The first experimental crop of wheat was sown on Broadbalk, a field in Hertfordshire, England in 1843 and harvested in 1844. This same field experiment continues today and is used in current research programmes at Rothamsted Research. The field is divided into sections. On some wheat is grown continuously and on others it is rotated with other crops. Other experiments use different amounts of fertilizers, including triple phosphate, potassium chloride, sodium and magnesium compounds and farmyard manure.
By kind permission of Rothamsted Research Ltd.

Nutrients

Plant roots need to take up nutrient elements from the soil solution usually as positively or negatively charged ions, which must be present in the soil.

Nitrogen is taken up by most plants, other than legumes, in only two forms, as nitrate (NO₃^-) and ammonium (NH₄⁺) ions. Under conditions in the UK, ammonium ions are rapidly oxidized via microbes in the soil to nitrate ions. These are absorbed by the roots of the plant, eventually forming amino acids and hence, proteins.

In very wet conditions, where oxygen is in limited supply, as in rice paddy fields, most of the nitrogen is taken up as ammonium ions.

The most commonly used nitrogen-containing fertilizers in Western Europe are ammonium nitrate and calcium ammonium nitrate (a mixture of ca 80% ammonium nitrate and 20% calcium carbonate). However, worldwide, about one half of the nitrogen applied as fertilizer is as carbamide (urea) It is the most concentrated solid nitrogen fertilizer (46%N) and is converted rapidly into ammonium ions by an enzyme, urease, which occurs in many soils. All nitrogen-containing fertilizers are made via ammonia and over 150 million tonnes are produced annually.

Phosphorus is usually taken up by most plant roots as the dihydrogenphosphate ion, H₂PO₄^-. Phosphorus can be applied as triple superphosphate, which is water soluble calcium dihydrogenphosphate, Ca(H₂PO₄)₂, or as an ammonium phosphate. The manufacture of these water-soluble phosphates is from phosphate rock (apatite), which contains various forms of tricalcium phosphate Ca₃(PO₄)₂, which is itself insoluble in water. 180 million tonnes of the ore are mined annually, of which 73 comes from Asia and 43 from Africa (mainly Morocco).

Two ammonium phosphates are produced, ammonium dihydrogenphosphate and diammonium hydrogenphosphate. They are particularly useful as they contain both nitrogen and phosphorus. The one chosen for use depends on which ratio of nitrogen to phosphorus is most effective for growth of the plant. All are derived from phosphoric acid which is produced on a massive scale, some 38 million tonnes a year.

Potassium is taken up as the positively charged potassium ion K⁺ and is usually applied as potassium chloride. Very large amounts, over 50 million tonnes a year, of potash are mined, of which 19 million tonnes are mined in the FSU and 17 in North America.

Sulfur deposition from the atmosphere has decreased rapidly as emissions from industrial processes have declined in response to the requirement for 'clean' air. This is leading to a sulfur deficiency in the soils. Thus many companies now produce fertilizers containing sulfur, added usually in the form of ammonium sulfate or other sulfate salts. However, elemental sulfur is being increasingly used in some fertilizrs.

Solid fertilizers are available in various forms. Fertilizers such as ammonium sulfate and potassium chloride will be as crystals. Some, like urea, may be as small spheres (prills) produced when the molten material is sprayed through a perforated disc and the resulting droplets rapidly cooled and solidified as they fall down a tall tower (a prilling tower). The granules are usually 2-4 mm diameter, a form which is easy to handle and spread.

Multinutrient fertilizers

Multinutrient fertilizers contain two or more of the nutrients N, P and K and in some cases small amounts of sulfur, magnesium and trace elements like boron may be added. There are two types of multinutrient fertilizers. One type, known as a blended fertilizer, is simply a mixture of individual fertilizers. Because the individual fertilizers are used in their commercially produced form, they may be of different sizes and densities and after blending may separate out during transport and spreading. This results in an uneven distribution of the nutrient elements and uneven crop growth.

The other type of multinutrient fertilizer is a granular fertilizer where each granule contains the nutrients in the ratio required. They are made by mixing the individual nutrient salts, like ammonium nitrate, an ammonium phosphate and potassium chloride to create a slurry in a large vessel, known as a granulator. On drying, the granules have a moisture content of approximately 0.5% and may then be coated with a water-repelling material to prevent further moisture absorption. This improves storage life and enhances flow of the granules when they are being spread mechanically. These fertilizers are known as compound or sometimes as complex fertilizers.

Choosing the fertilizer to use

Farmers have a wide range of fertilizers to choose from, single nutrient fertilizers and blended or compound fertilizers. Both compound and blended are available with different compositions according to the ratio in which the individual nutrients are required for the crop being grown.

The choice of fertilizer is based on the amount of nutrient required. Nitrogen is applied each year and the amount needed will depend on the crop being grown and its expected yield. For phosphorus and potassium, available reserves of these nutrients accumulate in soil and the amount required as fertilizer will depend on the amount already in the soil. This can be determined by soil analysis.

Manufacture

A fertilizer plant contains several integrated processes:

ammonia is manufactured from nitrogen (air) and hydrogen (made from natural gas, naphtha or coal with steam)
potassium chloride is mined, and the ore crushed and purified
phosphoric acid is manufactured from phosphate rock (mainly calcium phosphate) and sulfuric acid, the latter having been manufactured from sulfur (obtained from oil and natural gas)
triple superphosphate (calcium dihydrogenphosphate) is manufactured from phosphate rock and phosphoric acid
ammonium nitrate is manufactured from ammonia and nitric acid (itself manufactured from ammonia)
the ammonium phosphates are produced from phosphoric acid (and ammonia.

Environmental Issues

A 'green' controversy exists between those who believe that no 'artificial' fertilizers should be used and those who believe that manufactured fertilizers have to be used in order to feed an ever growing world population. It is estimated that synthetic nitrogen fertilizers now provide about half the protein in crops worldwide.

Because manufactured fertilizers contain precisely the same plant nutrients in the chemical forms that are provided in nature, they are not in themselves environmentally harmful. However, it is important that the timing and rate of fertilizer or manure application are correct, calculated to optimize crop production and minimize adverse environmental pollution from nutrient losses.

The nitrate ion is particularly mobile in soil and problems occur if excessive amounts of nitrogen fertilizers or animal manures are used. This is because any unused nitrate remains in the soil and there is a risk that it will leach into the water courses.

Phosphate ions applied in fertilizer and manure become tightly bound to soil particles and are not normally leached. Some phosphate ions however are found in rivers and they come from soil which has been eroded into the water course.

Nevertheless, even very small amounts of phosphate transferred to water can lead to nutrient enrichment of the water - eutrophication - which stimulates plant growth. Decomposition of excessive plant growth requires large amounts of oxygen and this decreases the amount available for fish and other aquatic animals leading to their death and, in extreme cases, eventual conversion of a waterway into an anaerobic waterbody which is toxic to vertebrates and smells badly.