Industrial Fertilizers in Agriculture
Summary and Keywords
Agriculture has been said to be the key to civilization development. The longevity of the production of the soils which sustained the population development influenced, in fact caused, the rise and often the collapse of those ancient cultures. Furthermore, the fertilization of those soils, if by new sediment or by other means, enabled some civilizations to survive longer than others. It was only with the development of more consistent fertilization and newer, higher-analysis materials that crop production entered an era where it could reliably feed beyond the family unit but feed the city, and then the whole country. This modern industrial fertilization required fewer people to be devoted to food production so that their efforts could be directed to more secondary and tertiary careers. The growth of the use of fertilizer by over 200% in 40 years has led to an increased scrutiny of its environmental aspect in the early 21st century, and this has led to a revaluation of application procedures and to an increase in research and development of new forms of fertilizer and into ways to change modern fertilizers’ environmental footprints to better steward food production and remedy systems that are off target environmentally. These technologies are sometimes very basic, such as including combinations of elements which help stabilize each other (e.g. sulfur and nitrogen or phosphorus and sulfur). Other technologies include polymer-coating (e.g. slow-release coatings) and impregnatable coatings (e.g. nitrapyrin, NBPT). In other cases, new materials have been developed (e.g. methylated urea) and in yet others progress has come from a mixing of other compounds with the fertilizer (e.g. gypsum to phosphorus fertilizer, or humic acids to nitrogen formulations). Lastly, there has been a rise in the importance of micronutrients as production has increased (e.g. zinc, manganese, and boron) especially as yield levels have increased.
The First Fertilizers
If one were to be able to observe the first human realization of soil fertility and plant growth it might well have been of a plant performing better because of the proximity of excrement of an animal or from the burning of dead grasses or wood. This would have been noticed by early man as a curiosity. Fast forward thousands of years and we see the application of wood ash, compost, manure, limestone and other ground rocks, all of these providing “earth” to help plants grow and become sustainable (Russel & Williams, 1977). Nitrogen-15 content of archeologically excavated cereal and pulse seeds has revealed that manure use in crop production was taking place in Europe as far back as 7900 years ago (Balter, 2013). Perhaps the best example in the Americas is that of the Native Americans placing fish carcasses in with their corn plantings, as reported in the early 1600s (Ceci, 1975).
Modern fertilizer (primary macro nutrients), as one might define it in the early 21st century, started with Justus von Liebig in the middle of the 19th century recommending nutrients to promote growth, such as phosphorus (ground bones treated with acid) and potassium (wood ash). By this time modern chemistry had matured and elements were being identified and purified as technology allowed. This “recommendation,” together with the knowledge of limiting elements, may be regarded as the start of the practice (Liebig, 1840), although potassium was discovered in Germany in the mid-1800s in brine wells, and nitrogen compounds were being used in the mid-1850s (Packard, 1952). Nitrogen fertilizer production actually started in the 1700s, Priestley and Cavendish using electric sparks to oxidize nitrogen. The history of secondary macro nutrients (calcium, magnesium, sulfur) is not as clear. Ruffin in the United States has been credited by some as bringing the importance of controlling pH into the mainstream in 1832, using marl to increase corn yields (Roberts & Dibb, 2009). The use of alkaline materials like marl is often referred to as liming.
The growth of the fertilizer industry in the United States began its rapid expansion in the 1960s. Increasing by 215% between 1960 and 2004. Usage rates on a per acre basis changed from 46.2 nutrient pounds (nitrogen, phosphorus & potassium) to 146 pounds per acre. Nitrogen is the major contributor, making up 59% of the tonnage being used, while phosphorus and potassium are nearly balanced at 20% each (Figure 1). Because of this growth, current attention to environmental impact, and new detection methods, fertilizer has come under increasing examination, which has led to changes in management and the development of new fertilizer technologies for each element.
Macronutrient Industrial Fertilizer
Nitrogen (N) was not isolated until 1772, by Daniel Rutherford and Carl Scheele (independently of one another) even though it had been known as a separate substance since 1674 (Shectman, 2003). Although one of the most important elements for plant growth, if not the most important, its development as a commercial fertilizer material lagged behind that of other elements, notably phosphorus and potassium. Higher-analysis nitrogen production actually started in the late 1700s with Priestley and Cavendish oxidizing nitrogen and producing nitric acid. Nitrogen really grew in use as an element in synthetic fertilizer in the early 1900s, thanks first to Adolph Frank and Nikodem Caro in Germany using a cyanimide process followed by Haber–Bosch process using an iron catalyst (Taylor, 1953) which enabled a higher-temperature and pressure process. This was further refined to lower temperatures using an iron base catalyst by Alwin Mittash working for the BASF corporation. This meant that large quantities could be produced at a higher analysis. The first factory in 1913 in Germany produced 27 tons of ammonia per day (Synder & Burnett, 1966); by 1945 new plants were producing 1,500 tons per day. Ammonia was not used directly but instead was converted to nitrate forms (calcium nitrate being first, ammonium nitrate later). This also became critical as war broke out, since ammonium nitrate was used for the manufacture of explosives. After World War II the war’s infrastructure was turned to agriculture; nitrogen becoming the dominant element of fertilizer production (Collings, 1949). Most postwar ammonia manufacture used natural gas or fuel oil instead of coal and coke.
Ammonium nitrate was the first mass-produced nitrogen fertilizer since only particle size need to be changed from ammunition to fertilizer. Urea was first produced on a small industrial scale in Germany in 1920 by I.G. Farbenindustrie and then by DuPont in the United States in 1935. Its use grew and by 1975 urea was the world leading nitrogen fertilizer source. In 1933 the Tennessee Valley Authority (TVA) was established to increase the manufacturing, efficiency, and use of fertilizer for agriculture. More than 75% of all fertilizers produced in the early 21st century come from processes developed by the TVA (Russel & Williams, 1977).
Hennig Brand discovered phosphorus (P) in 1669, in Hamburg, Germany, by preparing it from urine (Weeks, 1932). This process was refined when, in the late 1700s, Scheele found it could be made from bone. Early phosphorus use was fueled by guano imported from Peru, at first in the 1840s in the northern part of the eastern seaboard of the United States; its use moved southward over the next decade. This material was 12 to 14% N and 10–12% P2O5. As time progressed a higher-concentration source (20–25% P2O5) from more humid areas in Atlantic and Pacific islands and in the coastal Gulf of the United States was found, and this displaced the Peruvian guano. This material was then sometimes processed with other components to make mixed products or to increase solubility (Roberts & Dibb, 2009). A 1923 recipe from an anonymous Maryland manufacturer, quoted in Russel & Williams (1977), reads
. . . 1,000 pounds of ground bone was dumped in a wooden box holding about one and one-half tons that had been put on wheels to run outside of the little mixing plant on a track. To the bone was added 35 pounds of sulphuric acid to every 100 pounds of ground bone to acidulate the same. The acid used was 66°, which by adding a certain percentage of water, generated an intense heat and a cloud of sulphur fumes arose, which required this action to be done out in the open. Four . . . men with hoes would pull the bone back and forth in the diluted sulphuric acid until it was dissolved. Subsequently, to this 1,000 pounds of dissolved bone were added 300 pounds of 6% Peruvian guano, 500 pounds of Nova Scotia plaster, and 200 pounds of a filler of some sort, and this constituted the ton (p. 262).
This recipe is an example of the adding of sulfuric acid to phosphate, which increased solubility. Rock phosphate (apatite, 20–30% oxide form) had been discovered in Europe but it was not until 1842 that John B. Lawes and James Murphy in England patented a process to make soluble “superphosphate.” In the early 2000s most phosphorus fertilizers were made using this process (Van Kauwenbergh, Stewart, & Mikkelsen, 2013). This acidification, followed by reaction with phosphoric acid, created triple superphosphate (46% oxide form). With the discovery of deposits in the United States, domestic production began in 1867 in South Carolina, and in Canada in 1869. Florida deposits, a richer ore source, were not found until the 1880s, but the largest deposits were not discovered until the late 1890s in the western Rocky Mountains of the United States (Wyoming, Montana, Idaho, and Utah) (Roberts & Dibb, 2009). These ore sources and the “wet process,” using acid, led to production of triple superphosphate fertilizer which by 1976 averaged 42.5% oxide, up from 14% in the 1920s, using just rock phosphate or simple sulfur additions (Bixby, 1980; Nielsen & Janke, 1980; Russel & Williams, 1977) (Figure 2).
The next leap in phosphorus fertilizer involved ammonification, still the products in use at the end of the 2010s. Initially, in 1873, superphosphate was used as an absorbent for free ammonia, but it was found to absorb toxic gases as well (Ross & Merz, 1932). As industrial ammonia came into being it was reacted with triple superphosphate to create ammoniated phosphate fertilizers. These materials were not easily handled, due to their fragility in granule form (Russel & Williams, 1977). In 1940, Hardesty and colleagues proposed to create a more stable granule by using the ammoniating solutions as the wetting agent and a source for heating the reaction in a rotating drum (Hardesty, Ross, & Jacob, 1940). This was successful, but it took more than another decade before this became a commercial standard in manufacturing and moved phosphate fertilizers another step forward. This procedure was further modernized to a point where ammonia, phosphoric acid and sulfuric acid are all three mixed simultaneously with the material still emptying into a drum granulation (Achorn & Balay, 1974). This gives rise to dry multinutrient-analysis fertilizers such as diammonium phosphate and monoammonium phosphate.
Approximately 150 years ago fluid fertilizer application was common, but with the advent of higher-analysis solids, use of it subsided (Palgrave, 1974, 1975). In the 1950s the Tennessee Valley Authority (TVA) developed superphosphoric acid, which enabled the production of higher-testing material. The TVA then began producing nitrogen and phosphorus liquid materials that had analyses given in the form %N-%P2O5-%K2O. These materials contained no potassium (K, from kalium in Neo-Latin) and initially had an analysis of 11-33-0, later 11-37-0. In 1964 this gave way to 10-34-0 which was easier to make, although it was a lower analysis. These materials became the basis for suspension fertilizer development in the late 1960s and into 1970s. Clay was used as a suspending agent for potassium fertilizer additives and additional nitrogen formulations, as well as micronutrients when needed (Meline, Lee, & Scott, 1972).
At the same time phosphorus was being developed so were potassium fertilizers. Ashes from the burning of wood had been used as a fertilizer for many years, in the knowledge that it enhanced plant growth without knowing why. This had developed to the point that 1790 Samuel Hopkins was granted a patent by President George Washington for an improvement “in the making of Pot ash and Pearl ash by a new Apparatus and Process” (Hopkins & Maxey, 1998). In spite of this widespread and ancient practice potassium was not discovered until 1807 by Sir Humphry Davy, who electrolyzed potash (Davy, 1836). Potash (ashes from burning) was recommended as a fertilizer by Liebig in 1840. Potassium salts had also been discovered in Germany in 1839 in a brine well (Liebig, 1840) but were not commercialized until 1861. The United States continued to import potash until 1931, when sylvite (potassium chloride) deposits were discovered in New Mexico, and then in Canada in 1958 (Russel & Williams, 1977). Commercial fertilizer production was not economic until 1963. In North America the world’s fourth-largest high-grade potash deposit was exploited in Saskatchewan, Canada. This deposit runs 725 kilometers long and 240 kilometers wide, under parts of Manitoba, North Dakota and Montana in the United States, ranging in depth from 1000 to 2500 meters (Tisdale, Nelson, & Beaton, 1985). Besides the Canadian deposit, the other major deposits in North America are in New Mexico and Utah (Foth & Ellis, 1997). These deposits, along with others all over the world, were created by evaporation of ancient seas. Potassium is mainly supplied as muriate of potash (KCl), potassium sulfate (K2SO4), potassium nitrate (KNO3), and potassium magnesium sulfate (K2SO4–MgSO4). Potassium sulfate is largely used wherever chlorine is a concern, such as in tobacco and potato production, although there is some interest in soil quality and the effects that chlorine may or may not have on the microflora. Potassium nitrate (discussed briefly at the end of the section “Nitrogen”) is used mainly in specialty crop production and became commercially available only in the 1960s; prior to then most was imported from Chile. Potassium magnesium sulfate (commonly referred to as SulPoMag) has a wide variety of uses when both elements are needed. Additionally it is pH neutral. Additional potassium sources, mainly used in the liquid or foliar fertilizer industry, include potassium phosphate, potassium carbonate, potassium hydroxide, and potassium thiosulfate (Tisdale et al., 1985).
Although sulfur (S) was known in Old Testament times it was not until 1789 that it was recognized as an element. Liebig (1843) again recognized its importance, not from an agronomic perspective but from an economic one. He stated “It is no exaggeration to say, we may fairly judge of the commercial prosperity of a country from the amount of sulfuric acid it consumes.” Sulfur was used in a great variety of manufacturing, including that of superphosphate from bone and guano. Its role as an agronomic element did not gain wide recognition until the early 2000s. Traditional atmospheric accumulation from the burning of fossil fuels and deposition in precipitation has traditionally provided the “fertilizer” crops needed, along with sulfur from organic matter decomposition. Only in the early 2000s has attention to the environment and movement to more environmentally friendly energy sources lessened the atmospheric contribution. During the mid-1980s over 70% of the sulfur in the atmosphere was from non-natural sources. Legislation started being introduced to “clean” the atmosphere of this and other contaminates. This, along with higher crop yields, has led to the detection of deficiencies in land areas that traditionally had not responded to sulfur fertilizer additions (Camberato & Casteel, 2017).
Naturally occurring sulfur in soils is found from mineralization of organically bound sulfur compounds to inorganic forms. This form is more than 90% of the soil sulfur, and includes sulfur mineralization from plant residues. Sulfur in the living soil biology accounts for up to 4% of total soil sulfur. This is typically mineralized into elemental sulfur, sulfides and thiosulfate forms. These sulfur forms are oxidized into the sulfate form by certain autotrophic (Thiobacillus) and heterotrophic microorganisms, and some fungi (Lawrence & Germida, 1988). Use of elemental sulfur in the soil (or as applied fertilizer) is directly related to the speed of oxidation. Cold, dry conditions often limit mineralization, and saturation conditions drive sulfur into iron compounds, reducing availability as well (Malhi, Schoenau, & Grant, 2005). Thiosulfate is a relatively rapid convertible form with 56–70% converted to sulfate in 25 days of incubation (Janzen & Bettany, 1986). Since elemental sulfur has very low water solubility, particle size is critical in affecting its oxidation; it can increase oxidation rates by up to 16 times (Wen Schoenau, Yamamoto, & Inoue, 2001). Regardless of the type of original sulfur compound it will convert over time to sulfate, which is subject to environmental loss mechanisms such as leaching.
Modern sulfur fertilizer sources are mainly found from the “cleaning” of atmospheric pollutants which generates gypsum (calcium sulfate) and ammonium sulfate as by-products. In addition, other manufactured sources are present in the market, including 85–90% elemental sulfur granules, magnesium sulfate (Epsom salts), potassium sulfate, and magnesium-potassium sulfate. Gypsum is also found as a mined product. Two main liquid sources are available as ammonium thiosulfate and potassium thiosulfate. These breakdown into plant-available sulfate and colloidal elemental sulfur, which then goes through the oxidation processes described above. Plants only use sulfate; hence, products already in sulfate form are easily useable. The remaining cations of the fertilizer compounds (calcium, magnesium, potassium, and ammonium) also become part of the fertilizing use of the materials (Tisdale et al., 1985).
Calcium (Ca) was isolated in 1808 by Sir Humphry Davy. Although he first identified the element, its use as a fertilizer can be dated back to the Romans and possibly even earlier (Davy, 1836). There is ample evidence that different calcium-containing materials were often used to help plants grow. Why this was the case was not understood at the time, but things from bones and shells to waste materials were often used. It was not until modern science developed that true research started to identify plant essential elements (Russell, 1950). Calcium, regardless of its use and form, originated as rocks and minerals found in the Earth’s crust. The most common fertilizer materials for calcium are also the most common natural sources. Lime as a fertilizer or pH-raising, neutralizing, material is either calcite or calcium carbonate (CaCO3) or calcium, magnesium carbonate (dolomite, CaMg(CO3)2). In addition, in semi-arid regions gypsum (CaSO4, calcium sulfate) can be a dominant mineral. Gypsum has the unique property that it supplies sulfate and calcium as a neutral salt, thus not affecting pH when applied. The major use for calcium fertilizer is for pH correction in acid soils. The ability of calcium (and magnesium) to neutralize pH is due to its reaction with hydrogen ions in the soil solution system (active acidity) as well as on the soil exchange sites (reserve acidity) (Tisdale et al., 1985).
The speed and effectiveness of calcium in neutralizing acidity is controlled by two factors, fineness and purity. In the United States fineness is determined by evaluating the amount passing thru a nest of sieves. Material not passing through an eight meshes to the inch sieve is deemed as not effective. Material that passes thru that yet remains on a 60 mesh sieve is deemed 50% effective and that passing through 60 mesh is considered 100% effective at neutralizing acidity. The measure of purity is often referred to as the calcium carbonate equivalent (CCE). This is a weight percentage of pure calcium carbonate, which is set at 100%. Because of this calculation there are some liming materials which can have values over 100%. Dolomitic lime (since it contains magnesium) has a value of 109%. By combining the fineness and purity scores, fertilizer liming materials can be compared by ECC, or effective calcium carbonate. Therefore, limes may be coarser but have a higher purity or finer and a lower purity and have the same ECC in the soil and thus same rate application (Sparks, Page, Helmke, & Loeppert, 1996).
Calcium fertilizers also exist in other compounds and formats. Calcium nitrate, as discussed in the “Nitrogen” section, was one of the first fertilizer materials commercialized. This material is used extensively in specialty crop production, mainly in liquid form. Other forms include calcium oxide (unslaked lime or quick lime). Its CCE is 179% so it is very effective liming material but has many handling issues. It is typically a fine powder which makes large-scale applications difficult and its caustic properties further complicate handling. Calcium hydroxide (slaked lime, hydrated lime) has a CCE of 136. It has some of the same handling issues as its unhydrated precursor, calcium oxide. These materials often can be added to other fertilizer materials or suspended in liquids for easier application. In addition to these materials, chelated calcium has been shown to improve yields of some crops (Tisdale et al., 1985).
Magnesium (Mg) and calcium were once thought to be the same element. In 1755 Joseph Black, a Scottish chemist, proved the two were different but magnesium was not isolated until 1808, once again by Davy (Magnesium, 2012). Natural soil magnesium comes from the weathering of rocks, so several soil minerals and soil colloids/ secondary clay minerals are typically found together. Magnesium fertilizer sources include dolomitic lime, potassium-magnesium sulfate, and magnesium sulfate (Epsom salts). In addition, less-used fertilizers may contain magnesium as magnesium nitrate, magnesia (magnesium oxide) and, in liquid form, as magnesium chloride and magnesium thiosulfate. Additionally, chelated versions are also available and used in situations where antagonisms exist or where supplemental nutrition or correction of deficiencies by foliar applications are warranted. The liming aspects of magnesium are also similar to calcium (Tisdale et al., 1985).
Micronutrient Industrial Fertilizer
Micronutrients (boron, copper, manganese, zinc, and iron) are the most recent discoveries in plant essential elements and hence fertilizer development. Discoveries did not start to occur until the 1920s, with industrial fertilizers following (Russel & Williams, 1977). Traditionally, trace amounts of micronutrient metals were present in various agrichemical applications and in other industrial fertilizers as a result of manufacturing processes. This has changed; now, improved and regulated manufacturing has all but eliminated these trace amounts.
The first mention of boron (B) was by a Persian named Rhazes in the late first century ce. He classified minerals into six classes, one of which was boraces. The actual discovery of the element was by French chemists, Louis Jacques Thênard and Joseph Louis Gay-Lussac in the early 1800s (Boron (revised), 2006). Boron as a plant essential element was not discovered until 1923 in England by Katherine Worthington (Russel, 1957). Boron exists as an anion in soils, so leaching of natural boron and fertilizer-based boron can be of agronomic concern. This leads to large areas of the United States being deficient (Figure 3). Boron fertilization is one of the widest micronutrient applications, mainly due to the loss mechanisms. Sodium tetraborate (borax) is the most common source and is the form that it is naturally found in geology. This ore is mined in the United States in California and then is recrystallized; it can be processed into different mesh sizes depending on its use (soil or foliar) and formulation wanted (suspension, broadcast, or solution). Low rates, equating up to 3.4 kg/ha of element, are the general recommendation (Tisdale et al., 1985).
Copper (Cu; cuprum in Latin) was identified as a plant essential element in 1931, even though mining of it has been documented back to 4500 bce. Copper has long had a use as a fungicide. Prevost in 1807 reported the first documented use on cereals to treat smut, but it was not until 1885 that research of its control of downy mildew on grapes in France was published by Professor A. Millardet (Johnson, 1935). Copper deficiencies requiring fertilizer additions are most common on soils high in organic matter (histosols, mucks, peats) and in very sandy soils with extremely low organic matter. Most copper fertilizer is sourced from copper sulfate although copper chelates, copper oxide and copper hydroxide also have uses. Use rates on soil applied fertilizer can go as high as 22 kg/ha of the element (Tisdale et al., 1985).
Manganese (Mn) has been observed since 1860 in examining ash samples of plants but it was not considered an essential element. Opinions started changing when Bertrand in 1897 discovered its role as a component of oxidizing enzymes (Kelley, 1914). Manganese is common in most rocks since it is very frequently found in association with iron, and in high amounts can even impede iron uptake. The most common fertilizer source is manganese sulfate. This is usually used as a foliar spray, while soil applications tend to be chelates. Manganese fertilizers in the soil that are not chelated often have calcium or iron substitute in the compound, which then allows manganese to oxidize to a less reactive and plant-available form. Because of this, foliar applications prevail. Foliar options are chelates and complexes as well as salts of various anions such as sulfate, oxide, carbonate, chloride, and oxides. Manganese oxide is not very soluble so particle size fineness is often used to overcome. Use rates on soil applied fertilizer can go as high as 28 kg/ha of the element (Tisdale et al, 1985).
Zinc (Zn) was discovered as an essential element in plant physiology in 1926 by Sommer and Lipman (Brown, Cakmak, & Zhang, 1993; Sommer & Lipman, 1926). Zinc fertilizer sources tend to be dominated by zinc sulfate. The addition of sulfur as an anion, or application of zinc with sulfur fertilizers, enhances its availability. Other zinc compounds that are used as fertilizer material include carbonate, phosphate, and oxide. Additionally chelates and complexes are used, especially in applications with liquid phosphate fertilizers. Zinc has a strong relationship with phosphorus and these two elements are often applied together. Additionally in high-phosphorus soil environments (manure) additional zinc applications may be necessary. Broadcast use rates of soil-applied fertilizer can go as high as 22 kg/ha of the element (Tisdale et al., 1985).
Iron (Fe) is one of the most abundant elements on Earth, comprising 5% of the crust. Yet even though it is abundant, there are soil and plant circumstances that require fertilizer additions. These circumstances are usually a result of an imbalance with high levels of manganese, in areas of extremely high pH, saturated conditions (usually temporary), or under organic soils. Foliar iron sources can also be chelates or complexes as well as sulfates and oxides (Tisdale et al., 1985). Fertilizer applications in the soil are usually not effective because of the rapid oxidation to less soluble ferric forms of the element from the applied ferrous versions. Because of this chelation is used to protect the iron from reacting. This has been shown to be an effective way of addressing deficiencies from the soil aspect. Most common fertilizer sources used in the soil are EDDHA or EDTA chelates (Goos & Johnson, 2000).
Molybdenum (Mo) was established as a plant essential element in 1939 (Arnon & Stout, 1939). Soils responding to fertilizer additions are often acid and sandy soils, commonly occurring on coastal plains. Fertilization rates are very low, less than 0.35 kg/ha elemental. The industrial fertilizer materials used are dominated by ammonium molybdate and sodium molybdate. Foliar sprays, seed treatments as well as broadcast fertilizer additions are used (Tisdale et al., 1985).
Chlorine (Cl) was first identified as a plant essential element by Broyer in 1954 in tomatoes (Broyer, Carlton, Johnson, & Stout, 1954). It is a common compound in association other macronutrients (i.e., ammonium, calcium, magnesium, and potassium) and its requirement is often satisfied as a result of those macro fertilizer applications. Chlorine fertilizer rates vary widely from zero for some crops up to broadcast applications of over 123 kg/ha (Tisdale et al., 1985).
Nickel (Ni) is the most recently discovered plant essential element. Only proved to be such in 1987 (Brown, Welch, & Cary, 1987). Fertilization is rarely needed, but when applied is usually in a foliar application in a form of sulfates, nitrates, or complexes of lignosulfonates or heptogluconates (Nickel, n.d.).
Cobalt (Co) is an essential element only in legumes and is required by nitrogen-fixing bacteria nodules on roots. It is not officially considered a plant essential element since it has not been shown to be a requirement for non-legumes. Soils that are responsive to cobalt fertilization vary widely across the world. Cobalt sulfate and cobalt chelates are the only two widely used fertilization sources. In some parts of the world a cobaltized superphosphate is also manufactured. Low rates, equating up to 0.21 kg/ha of element are the general recommendation (Tisdale et al., 1985).
Chelated and Complexed Fertilizers
Several micronutrient fertilizers have been made as complexation and/or chelation of cationic metal nutrients (calcium, magnesium, iron, copper, manganese, zinc). Chelation refers to a binding or protection of the metal by an organic cyclic structure. Chelating agents are available to be taken up by the plant but protect the metal from interactions with other chemistries or soil elements that antagonize its use. Complexes are organic structures such as lignosulfates and heptogluconates; they provide some protection but are not fully protective of the metal (Tisdale et al., 1985).
In many places, fertilizers are manufactured that are termed “complete” fertilizers. They contain all three of the major macronutrients (nitrogen, phosphorus and potassium). These fertilizers are listed as an analysis of the elements expressed as elemental and oxide versions (N-P2O5-K2O). The oxide nomenclature for phosphorus and potassium is a holdover from the early 1900s; its origin is not fully understood and there has been debate since the mid-1950s on changing the format to N-P-K, but to no avail. Most of the United States legally requires the oxide forms, while internationally it varies. The oxide version analysis gives many complete fertilizers a 1:1:1 ratio of elements (a balanced complete fertilizer) such as 10-10-10 and 20-20-20. These are most commonly found in bagged forms used for homeowner or specialty crop production systems while single-element fertilizers are used for large-acreage production. These are manufactured and shipped internationally and serve as raw ingredients for complete fertilizers, which can be a blended material or co-formulated in a prill1 form (Soil Science Society of America, 1955).
The development of fertilizer materials is again going through a revolution. In the past decade polymer coating, which can regulate nitrogen movement from a granule form to a solution form, has been introduced. Co-manufacturing multiple elements into one granule is useful as a multinutrient fertilizer; for instance, a phosphorus, zinc, and sulfur product as well as a potassium and boron granule are now widely used. Furthermore, impregnation of liquids onto fertilizer granules and blends of different fertilizer sources is becoming more commonplace, as is the incorporation of elements with humic ore. All of these technologies bring added cost but are being developed for better nutrient utilization and/or environmental control of loss, both of which can translate to improved cost management for the user. This area is a growing experimental area, with research being performed on impregnating or incorporating microorganisms or enzymes into liquid applications, or impregnated on fertilizer granules.
Fertilizer use in the Unites States has remained flat while yields have increased. Crop yields are higher than ever before at an increased efficiency of nutrients but not at the rate of stagnation of fertilizer tonnage. This has led to current soil test values in the United Sates decreasing in large areas, a trend that started almost two decades ago. Yield and demand for food production will continue to increase and that will demand more or better utilization of fertilizer nutrients. Food production must increase by 50% by 2050 and along with that increase, some Third World areas will increase livestock needs for grains by 70%. Current supplies of raw phosphorus and potassium are estimated to last well into the next century and perhaps longer, depending on advances in mining equipment and plant efficiencies as well as advances in fertilizer applications. It is believed that research may still identify nutrients that are essential; however, new discoveries will have minimal impact on industrial fertilizer manufacturing due to the low amounts needed physiologically. The revolution that began in the 1970s and modernized fertilizer manufacturing may have ended. The new frontier will be in enhancing those traditional fertilizers with new technology to enhance their utilization or, more importantly, minimize agricultural impacts on the environment. This will need to be done to continue to increase the living status of the world’s population and to continue to feed that population into the future (CAST, 2013)
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(1.) Prill: a pellet or solid globule formed by the congealing of a liquid.