Fertilizers

Fertilizers increase food, feed, and fiber production by providing the minerals needed by plants. Organic refuse, gases, rock phosphate, and salts from ancient seas are the chief sources of materials for fertilizers. While they have greatly improved agricultural yields, fertilizers also contribute significantly to nutrient pollution.

Principal Terms

compound: a chemical combination of elements with distinct properties that may differ from the elements from which it formed

element: a pure substance that cannot be broken down into anything simpler by ordinary chemical means

grade: the percentage by weight of a nutrient present in a fertilizer, expressed as a standard element or compound

leach: to dissolve from the soil

macronutrient: a substance that is needed by plants or animals in large quantities; nitrogen, phosphorus, and potassium are macronutrients

micronutrient: a substance that is needed by plants or animals in minute quantities

mineral: a naturally occurring substance with definite chemical and physical properties

nutrient: a substance required for the optimal functioning of a plant or animal; foods, vitamins, and minerals essential for life processes

pH: a term used to describe the hydrogen ion activity of a system; a solution of pH 0 to 7 is acid, pH 7 is neutral, pH 7 to 14 is alkaline

sedimentary: formed from material that has been deposited from solution or eroded from previously existing rocks

Types and Grades

The major nutrients needed by plants are carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. They form 95 percent of a plant’s dry weight. Carbon and oxygen can be taken in directly from the air. Hydrogen comes from water absorbed by plant roots. Nitrogen, although present in the atmosphere, is not taken in by plants in the gaseous form. It must come from soluble nitrogen-containing compounds in the soil or be produced by bacteria that live in the roots of some plants. The remaining 5 percent of plant dry weight is composed of phosphorus, potassium, calcium, magnesium, silicon, sodium, sulfur, and chloride. Selenium, iron, boron, molybdenum, zinc, and manganese are necessary to most plants in very small amounts. Silicon has little biological use, but is present in water occupied by plants and typically is sequestered in silica grains within the plant. Because plants cannot take in nitrogen directly from the air, and because nitrates are easily leached from soils, nitrogen fertilizers are the most commonly used fertilizers. Phosphorus and potassium are commonly added to soil by fertilizers as well.

Two types of fertilizers may be used. Organic fertilizers, which are produced by plants and animals, include bone meal, blood meal, humus, peat, domestic sewage sludge, and manure. Guano, the accumulated wastes of birds or bats, is also an organic fertilizer. Inorganic fertilizers begin with atmospheric gases, natural gas, or deposits of sedimentary rock. The rocks precipitate from mineral-rich coastal waters or form when shallow lakes and seas evaporate. Inorganic materials are chemically processed to yield commonly used fertilizers such as liquefied ammonia, urea, and superphosphates.

The price and recommended use of commercial fertilizers depend on their grade. Both organic and inorganic fertilizers are labeled with a grade. Fertilizer grade is usually written as three numbers, such as 10-16-10 or 10-0-10. Each number tells the percentage by weight of one of the three macronutrients. They are always given alphabetically: nitrogen, phosphorus, potassium. The first number represents total nitrogen (in the form of elemental nitrogen) in percent by weight. Plants use the second macronutrient, phosphorus, when it is soluble in ammonium citrate. Usable phosphate is calculated as phosphorus pentoxide (P₂O₅). Potassium, the third major nutrient, is calculated as water-soluble potassium oxide. A fertilizer with a grade of 10-5-10 would contain 10 percent by weight of nitrogen, 5 percent by weight of available phosphorus, and 10 percent by weight of water-soluble potassium. A fertilizer may contain more of a nutrient than is stated on the label. If chemical tests show that the nutrient is in a form not usable by plants, the amount is not used in calculating the grade.

Nitrogenous Fertilizers

Ammonia is the principal nitrogenous fertilizer. Pure ammonia is a colorless, pungent, and irritating gas. As inorganic fertilizer, it becomes usable to plants when liquefied and injected into the soil. Most fertilizers sold are ammonia or a compound derived from ammonia. Anhydrous (without water) ammonia is made by combining atmospheric nitrogen and natural gas.

The compound urea contains 45 to 46 percent nitrogen. The nitrogen in urea must be converted to the ammonium ion by soil enzymes and held in humus or clay to be useful to plants. Urea can be applied as pellets or in a water solution. Chemical manufacturing plants synthesize urea from ammonia and carbon dioxide. Another solid source of nitrogen is ammonium nitrate. It contains 33.5 percent nitrogen and includes both ammonium and nitrate forms of nitrogen. The nitrate can be used directly by plants, but it is easily leached. Ammonium fixed onto clays becomes available to plant roots. Ammonium nitrate is synthesized from ammonia and nitric acid. Natural deposits occur in Chile. Ammonium sulfate is made from coke-oven gases. It is 21 percent nitrogen as the ammonium ion, NH₃⁺. Ammonium sulfate supplies sulfur as well as nitrogen and is often used on wet soils.

Urea or other solid fertilizers are often coated with sulfur or compounded from materials of different solubilities. The sulfur coating slows the dissolution of the fertilizer granule. When materials of different solubilities are blended, some will dissolve immediately, while others take longer to dissolve. These fertilizers release nitrogen continuously and save on the costs of application when supplements are needed over a long period of time. Nurseries and turf farms prefer continuous-release fertilizers.

Phosphate Fertilizers

A second macronutrient supplied by fertilizers is phosphorus. Phosphate fertilizers are available in both inorganic and organic forms. Rocks rich in phosphorus are mined in the southeastern and western United States. Rock phosphate fertilizers contain about 4 to 5 percent equivalent of phosphorus pentoxide. The rock is ground into particles small enough to be applied to the soil. Bone meal, the phosphorus fertilizer that has been in use for the longest time, contains about the same amount of available phosphorus as rock phosphate. Bones from slaughterhouses provide the raw materials for bone meal.

Rock phosphate can be enriched in several ways. When mixed with sulfuric acid, it becomes superphosphate with 16 to 20 percent phosphorus pentoxide. Mixtures with phosphoric acid yield triple superphosphate, with a 0-45-0 grade. Other phosphate fertilizers are diammonium phosphate (46 to 53 percent phosphorus pentoxide and 18 to 21 percent nitrogen), monoammonium phosphate (48 percent phosphorus pentoxide and 11 percent nitrogen), and nitric phosphate (20 percent phosphorus pentoxide and 20 percent nitrogen). Nitric phosphates are made by treating phosphate rock with nitric acid and phosphoric acid. The addition of muriate of potash (potassium chloride) to nitric phosphates gives a 10-10-10 grade fertilizer containing all three basic nutrients. Polyphosphate fertilizers, another inorganic source of phosphorus, contain high amounts of phosphorus pentoxide. They form soluble combinations with iron and aluminum. These chemicals, called complexes, can be used by plants. Other phosphate fertilizers may form insoluble compounds in soils with high iron and aluminum content whereby the phosphate is not available to plants.

Phosphate rock can be mined in Florida, Tennessee, North Carolina, Idaho, Wyoming, and Montana. North Africa and the region west of the Ural Mountains also have large deposits. In the United States, geoclinal (West Coast) deposits form in deep areas at the margins of continents. The geoclinal deposits of the northern Rocky Mountains extend over hundreds of square kilometers. Platform (East Coast) deposits were formed closer to shore where phosphate-rich waters warmed on approaching land. Weathered or residual deposits form when calcium-rich rocks are leached and less soluble phosphate remains. The “brown-rock” deposits of Tennessee are of this type. The phosphate sometimes precipitates into solution and is deposited lower in the soil profile. The Pliocene Bone Valley formation of Florida contains phosphatic pebbles and nodules reworked from the underlying Miocene Hawthorn formation. Phosphatic guano occurs on some Pacific islands where old deposits of bird excreta have lost their nitrogen through decomposition. Guano that has decomposed and been leached by percolating waters may reach 32 percent phosphorus pentoxide.

Morocco had about 50 billion metric tons of phosphate in reserve in 2024, representing about 70 percent of the world's known phosphate rock deposits. Other nations with significant reserves include China, Egypt, Tunisia, Russia, Algeria, Brazil, and South Africa, as well as Australia, the United States, Finland, and Jordan. Phosphate rock deposits are likely to decline in availability toward the end of the twenty-first century. This event, dubbed “peak phosphorus,” will have profound effects on global food production. Fortunately, phosphorus supplies can be extended by better application of fertilizers to minimize waste, and by recycling of phosphorus that would otherwise be discarded as sewage.

Potassium Fertilizers

The third nutrient commonly supplied in fertilizers is potassium. The most widely used potassium supplements contain muriate of potash, sulfate of potash, or sulfates of potassium and magnesium. These are all inorganic salts. Muriate of potash, chemically potassium chloride, is the most commonly used potassium source, but due to its chlorine content muriate of potash can be toxic to some plants. Potatoes, tobacco, and avocados are sensitive to excess chlorine. Sulfate of potash contains 48 to 50 percent available potassium oxide. Sulfate of potash-magnesia is about 18 percent magnesium oxide and 22 percent potassium oxide. Other potassium fertilizers include potassium nitrate and potassium polyphosphate.

Potassium salts are mined in Canada, New Mexico, Utah, and elsewhere from deposits left behind by the withdrawal or evaporation of ancient shallow seas. Brines from the Great Salt Lake are evaporated to yield magnesium and potassium chlorides. Potash salts are the last salts to come out of solution when seawater evaporates. This fact, combined with their solubility in water, makes potash salts more rare than gypsum and rock salt. The largest mineral deposits of potash salts occur in the Devonian muskeg or Prairie formation of western Canada, North Dakota, and Montana. The Saskatchewan deposits occur nearest the surface and are, for that reason, the leading production sites. The Permian Zechstein evaporites of England, Germany, and Poland are equally known for deposits of potash salts. Guano derived from the wastes of birds or bats is an organic source of potassium nitrate fertilizer.

Specialty Fertilizers

Specialty fertilizers supply nutrients that may be depleted by fertilizer use, soil management practices, irrigation, or heavy crop production. Sulfur deficiencies can be alleviated by using ammonium or potassium sulfate fertilizers. The use of ammonia fertilizers, leaching, or excessive moisture may lead to acidic soils. Dolomitic limestone adds calcium and magnesium, and corrects soil pH. Some ammonia fertilizers include recommendations for sweetening soils that may become acidic because of the production of hydrogen ions. Boron is supplied by borax. Borates, from which borax is made, occur in bedded deposits beneath old playas, brines of saline lakes, and hot springs or fumaroles. Glass particles added to fertilizers provide boron.

Micronutrient metals, such as molybdenum, iron, zinc, manganese, and copper, can be added to inorganic fertilizer as needed. Soil tests, chemical analysis of crop samples, and crop performance help indicate such needs. Molybdenum provided by sodium or ammonium molybdates can be added in small quantities to other fertilizers or mixed with water and used to soak seed. Iron, zinc, and copper sulfates supply soluble metals.

The formation of water-insoluble chelates may lead to shortages of micronutrient metals. Chelates are organic chemicals that bond to a metal to keep it from forming a precipitated salt. Helpful chelates are soluble in water. Manure contains water-insoluble chelates that may tie up metals. Excessive use of manure as a fertilizer, therefore, can cause deficiency of metals.

Commercial solid fertilizers combine several types of ingredients. The “carrier” contains the nutrient element. Nitrates, ammonium sulfates, and ammonium carbonate carry nitrogen. Phosphates carry phosphorus. Potassium can be carried with nitrogen in potassium nitrate. The second ingredient of many fertilizers is a conditioner. Conditioners prevent caking and assure good flow. Vermiculite and organic wastes condition by absorbing water. Diatomaceous earth, oils, plastic, and waxes maintain flow. Often, inert substances are selected so that reactions will not affect the solubility of the carrier. Neutralizers, such as ground dolomitic limestone, may be added to counteract acidity. Fillers, such as sand, bring the product up to a standard weight. Special additives—including micronutrients, fungicides, herbicides, and insecticides—help save on labor costs or prevent unwanted plants from absorbing nutrients intended for the crop. Fertilizers are often blended for the needs of specific soils growing specific crops.

Agricultural and Environmental Concerns

Much research on fertilizers is related to more efficient agricultural use and assessing and reducing the effects of fertilizer on the environment. When the nutrients in fertilizers are not completely used up by plant growth, they can enter the surrounding environment as pollution. Both water and air quality can be harmed by nutrient pollution, particularly from nitrogen and phosphorus. In particular, eutrophication of waterways can kill many species while supporting algal blooms that have further negative effects on wildlife and humans. Nitrogen lost in gasification is a greenhouse gas that contributes to global warming.

Various methods to reduce this impact have been explored. Coated fertilizers and combination fertilizers with timed release of nutrients are being developed. Studies show that for some crops, applying fertilizer as a liquid and at certain stages of plant growth is preferable to solid application. Proper management of agricultural fields is also important, as establishing plant buffers, reducing tillage, and limiting livestock access to waterways all can help prevent nutrient pollution.

In the western United States, excessive use of fertilizers combined with irrigation from salty river waters has left some soils too salty to grow certain crops. Research continues on the use of low-salt fertilizers and salt-tolerant crop varieties. Fertilizer use is also an important issue in many developing nations due to the importance of agriculture on the economy. Using locally available materials, such as greensand, manure, chitin from crustacean shells, or phosphate rock, may boost agricultural production in the developing world without incurring foreign debt, and can also be a way to reduce environmental damage.

More broadly, environmental impact can be reduced and agricultural yields maximized by more efficient use of fertilizers. A soil test can reveal whether there is a need for fertilizers at all. Agricultural specialists may then suggest the best type of fertilizer for a particular soil and crop. As researchers learn more about plant metabolism and the role of specific nutrients at different stages of the plant life cycle, needed fertilizers can be used at the minimum necessary amount. Carefully planned fertilization avoids waste and minimizes hazards to the applicator and the environment. Finally, research on the use of organic or recycled fertilizers, such as domestic septage, sewage sludge or recycled livestock wastewater—rather than inorganic types that rely on mining and chemical treatment—may hold additional environmental benefits. The accumulation and uptake of heavy metals may be prevented by the addition of fungi, which concentrate metals and prevent them from entering crop plants. Though pollution risks must still be monitored, the overall environmental impact may be lowered.

Bibliography

"Agriculture Nutrient Management and Fertilizer." United States Environmental Protection Agency, 25 Jan. 2024, www.epa.gov/agriculture/agriculture-nutrient-management-and-fertilizer. Accessed 3 Dec. 2024.

Bates, Robert L. Geology of the Industrial Rocks and Minerals. Dover, 1960.

Birkeland, P. W. Soils and Geomorphology. 3rd ed., Oxford University Press, 1999.

Buol, S. W., R. J. Southard, R. C. Graham, and P. A. McDaniel. Soil Genesis and Classification. 6th ed., Wiley-Blackwell, 2011.

Colwell, J. D. Estimating Fertilizer Requirements: A Quantitative Approach. CAB International, 1994.

Donahue, Roy L., Raymond W. Miller, and John C. Shickluna. Soils: An Introduction to Soils and Plant Growth. 6th ed., Prentice-Hall, 1990.

Foth, Henry D., and Boyd G. Ellis. Soil Fertility. 2nd ed., CRC Lewis, 1997.

Gowariker, Vasant, et al. The Fertilizer Encyclopedia. John Wiley & Sons, 2009.

Havlin, John L., et al. Soil Fertility and Fertilizers. 8th ed., Pearson, 2013.

Jensen, Mead L., and Alan M. Bateman. Economic Mineral Deposits. 3rd ed., John Wiley & Sons, 1981.

Pettijohn, F. J. Sedimentary Rocks. 3rd ed., Harper & Row, 1975.

Philpott, Tom. "A Brief History of Our Deadly Addiction to Nitrogen Fertilizer." Mother Jones, 19 Apr. 2013, www.motherjones.com/food/2013/04/history-nitrogen-fertilizer-ammonium-nitrate/. Accessed 3 Dec. 2024.

"Phosphate Reserves by Country 2024." World Population Review, 2024, worldpopulationreview.com/country-rankings/phosphate-reserves-by-country. Accessed 3 Dec. 2024.

Schlossberg, Tatiana. "Fertilizers, a Boon to Agriculture, Pose Growing Threat to US Waterways." The New York Times, 27 July 2017, www.nytimes.com/2017/07/27/climate/nitrogen-fertilizers-climate-change-pollution-waterways-global-warming.html. Accessed 3 Dec. 2024.

Sopher, Charles D., and Jack V. Baird. Soils and Soil Management. 3rd ed., Reston Publishing, 1986.

"Sources and Solutions: Agriculture." United States Environmental Protection Agency, 18 Nov. 2024, www.epa.gov/nutrientpollution/sources-and-solutions-agriculture. Accessed 3 Dec. 2024.

Whalen, Joann K., ed. Soil Fertility Improvement and Integrated Nutrient Management: A Global Perspective. InTech, 2012.