1. Micronutrient applications for crops
2. Products
3. Methods of application
4. Preparation of fertilisers containing micronutrients
5. Reactions of micronutrient materials with fertilisers
6. Dry mixing
7. Bulk blending
8. Incorporation of micronutrients in granular fertilisers
9. Coating of micronutrients on granular fertilisers
10. Liquid fertilisers
11. Other useful elements

Deficiencies of micronutrients in crops have been increasing throughout the world. Some reasons include higher crop yields, use of high analysis NPK fertilisers containing less ballast (=micronutrients), and decreased use of animal manures on many agricultural soils. Also, micronutrient deficiencies have been verified in many locations through increased use of soil testing and plant analyses.

Micronutrient applications for crops

When micronutrient deficiencies are compared with regard to the worldwide importance of various crops, deficiencies of zinc, manganese, and copper are most important, especially for cereal crops. Boron is of greatest concern for some legume crops, cotton, and root crops such as groundnut and sugar beets. Iron deficiency is of greatest concern for crops grown on calcareous soils (iron chlorosis). Molybdenum is especially important for legumes because of its role in nitrogen fixation.

Boron – Recommended boron application rates range from 0.5 to 2.0 kg/ha and should be carefully followed [10]. Crop species vary considerably in their boron requirement as well as in their tolerance to over applications. Residual effects of applied boron also vary with soil conditions, with the lowest effects found on acidic sandy soils in areas of high rainfall. Some crops with high boron requirements are alfalfa, cotton, peanuts, irrigated corn, root crops, soybean, sugar beet and some fruits and vegetables.

Copper – Most copper deficiencies are found on organic soils and on sandy soils. Recommended copper application rates range from 1 to 10 kg/ha. Residual effects of copper are very marked, with responses being noted for up to 8 years after application, so annual applications usually are not necessary [3]. Soil tests should be used to monitor the copper build up in soils where copper is applied. Some crops with high copper requirements are cereals, corn, clover, and some fruits and vegetables.

Iron – Although iron is the most abundant element in soils, levels of available iron often are limiting in calcareous soils, and to a much lesser extent in sandy soils in high-rainfall areas. Soil applications of some iron sources are not effective for crops, so foliar sprays are the recommended control method [3]. Also good results using slow release vivianite (iron phosphate) are known. Plant species vary considerably in their susceptibility to iron chlorosis. Growing iron-tolerant species or varieties within a species is one of the main methods of controlling iron chlorosis on low-iron soils.

Manganese – Deficiencies of manganese are found mainly on well-drained neutral or calcareous soils or organic soils. Recommended manganese application rates range from 2 to 20 kg/ha_ There are no residual effects from applied manganese because of oxidation of the available divalent form to the tetravalent form, which is unavailable to plants [3]. Some crops with high manganese requirements are cereals, cotton, peanuts, soybean, sugarcane, and fruits and vegetables.

Molybdenum – Deficiencies of molybdenum are found on acidic soils and may be corrected by liming soils to pH 6.0. Because the amounts needed to correct molybdenum deficiencies are very low (supplying 60-120 g/ha), seed treatment or the use of compound (N)PK’s with a small amount of molybdenum are the recommended s [3]. Deficiencies are mainly found in legume crops because molybdenum is essential for nitrogen fixation. However, some vegetable crops also are sensitive to molybdenum deficiencies.

Zinc – Deficiencies of zinc are more widespread than those of the other micronutrients. Deficiencies occur in many soil types with pH levels> 6.0, especially in soils with low organic matter [11]. As with copper, residual effects of applied zinc are substantial, with responses found at least 5 years after application. Recommended zinc application rates range from 1 to 10 kg/ha [3] Some crops with high zinc requirements are corn, citrus, field beans, rice, and some fruits and vegetables.

Products

Micronutrient sources vary considerably in their physical state, chemical reactivity, cost, and availability to plants [12]. There are four main classes of micronutrient sources – inorganic, synthetic chelates, natural organic complexes, and fritted glass products (frits).

Inorganic sources include oxides, carbonates, borates, molybdates, and metallic salts such as sulphates, nitrates, and chlorides (Table 1). The sulphates are the most commonly used inorganic sources and are sold in crystalline or granular form. Oxides of zinc and manganese also are commonly used and are sold as fine powders and in granular form. Because oxides are water-insoluble, their immediate effectiveness for crops is rather low in granular form unless they are granulated into an acidic fertiliser like Single superphosphate (SSP) or Triple superphosphate (TSP) where the available free acid turns the oxide into a plant available sulphate/phosphate. Also, the divalent form of manganese will oxidise to unavailable forms in soil, so there is no residual availability of manganese fertilisers.

Synthetic chelates are formed by combining a chelating agent with a metal through coordinate bonding. Stability of the metal-chelate bond affects availability to plants of the chelated micronutrients (copper, iron, manganese, and zinc). An effective chelate is one in which the rate of substitution of the chelated metal for cations in the soil is quite low; thus, the metal is maintained in chelate form. The relative effectiveness for crops per unit of micronutrient as soil-applied chelates may be two to five times higher than that of the inorganic sources, while relative costs may be 5 to 100 times higher.

Natural organic complexes are made by reacting metallic salts with some organic by-products of the wood-pulp industry or related industries. Several classes of these compounds are the lignosulfonates, phenols and polyflavonoids. The type of chemical bonding between the metal and the organic compounds is not well understood. Although these complexes are less costly per unit of micronutrient than the chelates, they are also usually less effective than the chelates.

Fritted micronutrients (frits) are glassy products in which solubility is controlled by particle size and matrix composition. Frits generally are used only on sandy soils in regions of high rainfall where leaching occurs. This class of micronutrients is more suited for maintenance programs than for correcting severe micronutrient deficiencies. Therefore, frits have only a very small share of the market.

Industrial by-products now are being used as micronutrient fertilisers because they are less costly per unit of micronutrient than products manufactured specifically for the purpose [13]. Oxides of zinc and manganese are the most common by-products used. Because these oxides are dusty and difficult to handle, they are partially acidulated with sulphuric acid and are called oxysulphates, Acidulation results in formation of water-soluble sulphates, which are more available to plants. The percentage of water-soluble zinc or manganese in these oxysulphates is directly related to the degree of acidulation of the oxide by sulphuric acid. Research results have shown that at least 40% of the total zinc or manganese in granular oxysulphates should be in a water-soluble form to be effective for crops [11]

Methods of application

Methods of micronutrient application include application directly to soil, in foliar sprays, as seed treatment or incorporated into (N)PK’s. The most common application method for crops is soil application [12]. Because recommended application rates usually are less than 10 kg/ha (on an elemental basis), it is difficult to achieve uniformity when individual micronutrient sources are applied separately in the field. Therefore, both granular and liquid fertilisers are commonly used as carriers of micronutrients. Including micronutrients with mixed fertilisers is a convenient method of application, and it allows a more uniform distribution with conventional application equipment. Costs also are reduced by eliminating a separate micronutrient application.

Micronutrients can be combined with NPK fertilisers by dry mixing, bulk blending with granular fertilisers, coating onto granular fertilisers, incorporation during the manufacturing process, and mixing with fluid fertilisers [12]. In blending special attention has to be given to the micronutrient form since highly concentrated microelements can cause local burning or local intoxication to plants due to the high local concentration from a single micronutrient particle. To avoid this risk a micronutrient on a carrier (SSP, potash, etc.) can be used.

Preparation of fertilisers containing micronutrients

Micronutrients can be applied in many ways including foliar application, seed treatment, root dipping of transplanted seedlings, and application with pesticides. However, the most popular method is to apply the micronutrients in admixture with the primary nutrient fertiliser that the farmer regularly uses. The first reason for this popularity is convenience; the farmer does not have to devote extra labour to micronutrient application if the micronutrient carrier is a fertiliser that he is already using. A second reason is that uniformity of application is much easier when the micronutrient material is mixed with a much larger volume of fertiliser. The purpose of this section is to describe some of the methods that are used to produce fertiliser mixtures containing micronutrients and to discuss their advantages and disadvantages.

Reactions of micronutrient materials with fertilisers

It should be recognised that when micronutrient materials are mixed with other fertilisers, chemical reactions are likely to occur. Lehr [14] has described many of the numerous chemical reactions. The effect of these chemical reactions may or may not be important. A water-soluble micronutrient source may be partially or wholly converted to a water-insoluble compound. Conversely, water-insoluble sources may be converted to water-soluble forms. The gain or loss of water solubility may or may not be important. Some examples follow.

Soluble salts of copper, manganese, iron, and zinc are likely to become insoluble when incorporated in ammonium phosphates or ammoniated mixed fertilisers. The reaction forms one of several metal ammonium phosphates such as ZnNH₄PO₄. In general, the water solubility decreases with increase in pH of the fertiliser. Loss of water solubility does not necessarily imply loss of effectiveness but may delay it. Sodium borate when incorporated in ammoniated fertilisers containing calcium may become partially or wholly insoluble, presumably because of formation of calcium borate. (The boron in calcium borate is insoluble in cold water but soluble in boiling water) This effect has been noted with nitrophosphate fertilisers and may occur with other formulations.

Many water-insoluble micronutrients (oxides, carbonates, and silicates} become at least partially soluble when incorporated in superphosphates or mixed fertilisers containing unammoniated superphosphate.

Some insoluble oxides become soluble when incorporated in ammonium polyphosphate fertilisers. In practice, the ammonium polyphosphate liquid fertilisers are widely used in some countries, and zinc oxide often is added and dissolved in them. However, solid fertilisers containing ammonium polyphosphate also will convert zinc oxide (or carbonate) to a soluble form.

Zinc oxide (CuO) and cuprous oxide (Cu) become partially soluble when incorporated in ammonium nitrate or nitrogen solutions such as urea ammonium nitrate solutions. The solubility can be increased by adding enough ammonia to keep the solution pH at 7 to 8. The soluble compounds formed are presumed to be zinc nitrate amine and copper nitrate amine. Because the addition of copper compounds to ammonium nitrate is said to sensitise its decomposition, copper is not used in some countries for solid fertilisers containing a high per centage of both ammonium nitrate and chloride. In TVA tests, the addition of zinc to ammonium nitrate did not cause any increase in explosibility.

Coating of micronutrient materials on granular fertilisers would be expected to result in less reaction than would incorporation, but even so, substantial gain or loss of water solubility of the micronutrient has been found to occur with coated materials. Besides that, the surface coating with microelements might lead to accumulation of dust with high microelement concentrations in certain areas (centre pile, bottom bag).

When chelated micronutrients are incorporated in fertilisers, there is less likelihood of reactions that cause loss of water solubility. Some granulation processes, however, involve high temperature and use of ammonia and acids which can cause decomposition or the organic chelate and consequent loss of availability. Frits are ordinarily insoluble and remain so when incorporated in neutral fertilisers, but an increase in solubility was observed when frits were incorporated in superphosphates.

Dry mixing

Perhaps the simplest method for producing mixtures containing micronutrients is to dry mix the micronutrient material with the primary nutrient materials. This method works well with nongranular materials. Segregation usually is not serious when all materials are fine, such as less than 1 mm in particle size. Fine materials, however, are likely to cake, and some micronutrient materials may promote caking. Caro et al. [1] noted increased caking of some formulations of dry-mixed fertiliser when zinc sulphate was added.

Nongranular dry-mixed fertilisers are not popular in most areas but are still used in some.

Dry mixing by farmers may be practical in some countries. Bautista et al. [17] reported that in the Philippines farmers were supplied with fine granular zinc sulphate heptahydrate (22% Zn) in 5-kg bags and were instructed to mix it with the fertiliser used for basal application for rice to provide about 1 kg of Zn per hectare. The paper did not say what the primary nutrient fertiliser was, but prilled urea is the most popular fertiliser in the Philippines.

Bulk blending

Bulk blending is a form of dry mixing in which all of the nutrients are granular (e.g. 2-5 mm). Micronutrient materials are available in granular form for use in bulk blends. One problem is segregation; unless all materials are closely matched in particle size, segregation will occur in piles, in bins, in spreader trucks, or during mechanical spreading. If the blends are bagged from a bin, disparity in particle size will lead to wide variation in micronutrient content from one bag to another. This variation can cause poor results because of oversupply in some spots and undersupply in others.

Another problem with blends is the sparse distribution of micronutrient carriers. For example, only one granule in thirty may be a micronutrient carrier. The larger the granules the less likely that any one plant will have a micronutrient granule within reach of its root system. Also, the higher the concentration of micronutrient material, the fewer the granules required to provide a given application rate. This problem is not unique to bulk blends but would also be encountered in direct application of granular micronutrient materials. One method for coping with the problem is intentional dilution of the micronutrient with either an inert material or a primary nutrient material. For example, micronutrient chelates are granulated with clay or vermiculite to produce granules containing 1%-5% of a micronutrient element or similar percentages of two or more micronutrients. Several products are available that contain 1%- 10% of one or more micronutrients granulated with one or more primary nutrients. These products may be used in blends.

Incorporation of micronutrients in granular fertilisers

In most cases incorporation of micronutrients in granular fertilisers containing one or more primary elements is not technically difficult. However, it is often un-economical to produce small lots of special grades in large granulation plants, so the practice is economical only when there is a substantial demand for micronutrient-enriched fertilisers. ln several countries in Europe, there is a need for boron for root crops such as sugar beets and fodder turnips. Because these crops are important ones, there is sufficient demand to justify an economical scale of operation. The boron often is incorporated in a superphosphate-based PK mixture in amounts to supply 0.2%-0.5% B.

Several companies in the United States market “premium” fertilisers containing small percentages of two, three, or more micronutrients. For example, mixtures containing manganese, iron, and zinc are intended for soybeans. One company markets premium fertiliser containing polyphosphates with zinc and iron incorporated in the polyphosphate. Boron is recommended for cotton in portions of Alabama, Georgia, and Mississippi, and mixed, granular fertilisers are marketed containing 0.2% or 0.3% B. Also, in Africa (Ethiopia, Kenya) NPK’s with boron and zinc are popular because of their beneficial effect on yield (the von Liebig effect). In general, no important technical problem, apart from taking care of a clean working environment and preventing dust inhalation for workers, would be expected in incorporating the small amounts of micronutrients that are commonly used. When larger percentages of micronutrients are incorporated, some problems may occur. TVA has made a series of tests aimed at producing materials with relatively large percentages of micronutrients for use in bulk blending. Concentrated superphosphate was granulated with steam in a conventional drum granulator with addition of anhydrous sodium tetraborate (Na₂B₄O₇); a small amount of ammonia was added to decrease the free acid content of the superphosphate. The scale of the operation was 10 tph. The product after drying contained 47% available P₂O₅ and 3.4% B [19]. No unusual problems were encountered when using the anhydrous borate. When the pentahydrate was used, however, large balls formed in the dryer, and the granules were soft and weak. This condition was attributed to release of water of hydration in the dryer. It was concluded that the anhydrous sodium tetraborate should be used for production of this product.

In pilot-plants tests ammonium nitrate was granulated with zinc oxide, borax, or both, to produce granules containing 2.3% Zn and 0.5% B, or 1.9% Zn and 0. 7%B [20]. The micronutrient materials were added to hot concentrated ammonium nitrate solution (95% NH₄NO₃), and the mixture was granulated in a pan granulator. The granules were dried and screened, and the fines and crushed oversize were recycled to the granulator. The boron content of the products was fully water soluble, and the zinc content was 30%-40% water soluble. The compound 3Zn(OH)₂•NH₄NO₃was identified in the water-insoluble portion of the zinc-containing products. The water-soluble zinc may have been zinc amine nitrate. Tests showed that the physical properties of the products were as good as straight ammonium nitrate and that the sensitivity of ammonium nitrate to detonation was not increased by borax or zinc oxide. In subsequent work, granular ammonium nitrate containing up to 8% Zn was produced by addition of zinc oxide. One company now markets a product containing 30% N and 10% Zn.

Zinc oxide was incorporated in ammonium polyphosphate (15-60-0) during granulation without difficulty at levels of 1%-3% Zn; 80%-100% of the zinc was water soluble [20]. Although the zinc oxide did not react with ammonium polyphosphate in absence of water when the granules were placed in the soil the ammonium polyphosphate and 80% of the zinc dissolved and diffused in the soil. In similar tests with diammonium phosphate-zinc oxide granules, most of the diammonium phosphate dissolved and diffused into the soil while the zinc remained at the granule shell as ZnNH₄PO₄.

Substantial percentages of iron, manganese, and copper were soluble when appropriate compounds of these elements were incorporated in granular ammonium polyphosphate.

Coating of micronutrients on granular fertilisers

When small percentages of micronutrient materials are to be added to granular fertilisers they may be added as a coating on the surface of the granules. This method has several advantages. The coating may be added to homogeneous granular compound fertilisers, to bulk blends, or to straight (single nutrient) fertilisers. The coating may be added just before shipment, thereby avoiding storage of additional grades. Each granule is coated with a micronutrient as opposed to bulk blends containing granular micronutrient materials in which only one granule in thirty may be a micronutrient carrier. The operation is simple, and the equipment is inexpensive.

One drawback of the coating method is that the addition of a micronutrient material {whether coated or not) lowers the primary nutrient grade. This is a problem in countries where only prescribed, registered grades can be marketed. Bulk blenders, however, can easily deal with this problem. In areas where they are required to make only registered, whole-numbered grades, the grade is often adjusted by adding a filler such as crushed, screened limestone; thus, dilution by a micronutrient can be compensated for by adding less filler. Another method for grade adjustment is to use two materials of different concentration, such as ammonium sulphate and urea, in varying proportions.

Some micronutrient materials will adhere to some granular fertilisers without a binder. One example is a finely powdered zinc oxide (80% Zn) which was applied as a coating to granular ammonium nitrate in a rotary drum such as is commonly used for coating fertilisers with clay or other conditioning agents. Up to 8% Zn was applied as a zinc oxide coating with up to 93% adherence in industrial-scale equipment [19]. However, adherence decreased to 47% after 6 months’ bagged storage. The addition of steam in the coating drum increased adherence after 6 months’ storage to 72%. More complete adherence was obtained in pilot-plant tests with lower percentages of zinc oxide or by using a small amount of 70% ammonium nitrate solution as a binder. Achorn and Balay [21] have described plant practices for adding micronutrients to bulk blends.

Most micronutrients require a binder for good adherence on the surface of granular fertilisers. The binder may be oil, wax, water, molasses, or fertiliser solution. TVA has developed and studied procedures for coating. A batch method was selected because some bulk blenders use a batch mixer. The batch method consisted of the following steps [22]:

• Charge the mixer with the granular fertiliser to be blended and the powdered micronutrient.
• Blend dry for 1 minute by rotating the drum mixer.
• Add the liquid binder through a spray nozzle while continuing rotation of the mixer (1 minute).
• Continue mixing for 1 minute.
• Discharge the coated material.

This procedure gave a more uniform coating than did the addition of the binder before mixing. Silverberg et al. [23] reported the results of tests with this procedure in some detail. ln general the best results were obtained when the micronutrient material was finely ground (minus 100-mesh or finer). Oils gave good results, but the less viscous oils were absorbed into the more porous granular fertiliser causing some loss of adherence with time. he amounts of binder ranged from 0.5% to 5.0% of the mixture. In some cases, the addition of 2% clay improved adherence when water was used as a binder. Ammonium polyphosphate solution (11-37-0) was an excellent binder for zinc, manganese, and iron oxides. The oxides reacted with the solution to give a hard, smooth coating. As much as 13% of a mixture of these oxides was coated on granular TSP using 3% of the APP solution as a binder. Adherence was 100% even after -2 weeks’ storage Achorn and Balay [21] have described some U.S. plant practices for coating micronutrients on blends.

Use of a 64% solution of ammonium nitrate as a binder for coating micronutrients on bulk blends has been described [24]. Over 97% adherence was obtained.

Coating with oil and clay is a common practice in European plants for both bulk blends and chemically granulated mixtures. The usual proportions are about 0. 5% oil and 1. 0% clay. The coating usually is done in a continuous drum mixer. The oil is specially formulated for the purpose. Several companies in the world produce specially formulated oils for dust control on granular fertiliser [26].

In some developing countries an oil coating is objectionable to farmers because it oils the fertiliser bags which they wish to reuse for grain storage. A possible solution to this problem is to use a waxy material that is a solid at atmospheric temperatures.

Liquid fertilisers

Liquid fertilisers are used in substantial amounts in only a few countries. Two types of liquid fertilisers are recognised:

• Clear liquids are fluids in which all or nearly all of the ingredients are in aqueous solution.
• Suspensions are fluids that contain solid particles. The solids may be soluble salts suspended in their saturated solution, they may be insoluble materials, or both types of solids may be present.

Liquid fertilisers may be used for foliar application or for application to the soil (drip irrigation). Micronutrient sprays are often used for correcting deficiencies particularly for tree crops. A soluble salt or chelate of the micronutrient material may be dissolved in water to form a solution of suitable concentration. Finely powdered insoluble materials also may be used for foliar application. Because some metal salts form acidic solutions, the slightly soluble basic salts may be preferable for foliar application [29]. An example is CuSO₄•3Cu(OH)₂•H₂O, a basic copper sulphate.

It is common practice to combine foliar application of micronutrients with pesticide sprays or with foliar application of primary nutrients. When two or more materials are combined for foliar application, care must be taken to ensure that the materials are compatible. Many pesticides are incompatible with solutions of micronutrients. Incompatibility means that some chemical or physical change takes place that renders the mixture unusable or harmful. In general, micronutrient solutions should be applied separately unless compatibility is shown.

Liquid fertilisers containing micronutrients and one or more of the primary nutrients are suitable for application to the soil or for foliar application when diluted sufficiently to avoid leaf burn. Such liquid fertilisers may be produced merely by dissolving fertiliser salts in water. Dry mixtures of water-soluble fertiliser salts are available that may be marketed as such and dissolved in water prior to use.

In formulating liquid mixed fertilisers, it is not sufficient to know that all ingredients are water soluble. Many reactions may occur that result in the formation of water insoluble compounds of micronutrients.

In general. most micronutrients are soluble in aqua ammonia or in solutions containing ammonium nitrate and urea, provided that proper choice is made of the micronutrient compounds and that free ammonia is present or added to keep the solution pH above 7. Hester [30] describes the formulation and preparation of a solution based on aqua ammonium containing all micronutrients except iron. The solution contained .5% N as ammonia and ammonium sulphate, 0.29% Mo, 1.1 % Cu, 2.0% Mn, 1.9% Zn, 0.9% B, and 0.12% Co. The metallic elements were added as sulphates, but they were present in the solution as complex metal amine sulphates. Boron and molybdenum were added as boric acid and molybdic acid.

Silverberg et al. [23] have discussed the solubility of micronutrients in nitrogen solutions and in various NP and NPK liquid fertilisers. In general, only boron (borax) molybdenum (sodium molybdate) were appreciably soluble in nearly neutral ammonium orthophosphate solutions. Copper, iron, and zinc, however, were soluble in ammonium polyphosphate solutions to the extent of 1%-3%, and manganese was soluble to the extent of 0.2%. The production of liquid fertilisers including ammonium polyphosphate solution, has been discussed by Hignett [31] and in a group of 12 papers in Products and Techniques for Plant Nutrient Efficiency [32]. Several of these papers discuss the potential advantage of liquid fertilisers in tropical countries and the incorporation of micronutrients in liquids.

When micronutrients are incorporated in suspension fertilisers, solubility is not a technical problem although it may be an agronomic problem. Soluble or insoluble compounds of micronutrient elements are added to suspension fertilisers using materials of small enough particle size that they are easily suspended and will not clog spray nozzles. Production of suspension fertilisers containing micronutrients has been described by Silverberg et al. [23].

Other useful elements

Although only 17 elements are considered to be essential for plant nutrition at this time, many other elements are known to occur in plant tissues. Positive effects of some of these elements have been reported by numerous researchers. Whether any of these elements eventually will be discovered to be essential for plant growth remains to be seen. Current criteria for an element to be considered essential are (a) it must be present for the plant to complete its life cycle; (b) its action must be specific and unable to be replaced by that of another element; and (c) its action must be direct [33].

Another term, beneficial elements, has been proposed to describe some elements that appear to have specific functions in plant nutrition but do not meet all of the three criteria required for essentiality as described above. Two such elements (cobalt and silicon) will be discussed here.

Cobalt

Cobalt is essential for animal nutrition, but it has not been established as essential for plant growth. Ruminant animals require cobalt for the synthesis of vitamin, B12 by their rumen microflora. This was established about 1935, but an essential role of cobalt in plants was not demonstrated until 1960 [33]. Cobalt has been found to be essential for the growth of legumes which rely on symbiotic nitrogen fixation.

Field responses to cobalt applications by subterranean and clover were reported on sandy soils of South Australia and Western Australia. Later, it was shown that narrow- leafed lupins were even more sensitive to cobalt deficiency. Concentrations of cobalt are less than 1 ppm in many plant tissues, which is even lower than those of molybdenum, a micronutrient. Cobalt application rates also are quite low: less than 1 kg/ha, Although it is possible to prevent cobalt deficiency of livestock by applying cobalt-containing fertilisers to pastures, it may be more practical to supply the cobalt directly to animals in the form of a “cobalt bullet” given orally to young animals In cobalt-deficient areas [33].

Selenium

Most arguments for cobalt are also valid for selenium: it improves animal health and gives better fertility. The quantities needed are very low and toxicity in case of over formulating is high. A direct feed supplement containing selenium seems to be the better route to success.

Silicon

Silicon is the second most abundant element on earth, and there are sufficient supplies of available silicon in most soils. Some plant species absorb large quantities of silicon for example, up to 4% by flooded rice [33]. Grasses grown under upland conditions also absorb significant quantities of silicon. In such crops, silicon is found in leaves and stems and is credited with increasing stalk strength. Sugarcane also is known to have a high silicon requirement.

Highly weathered soils in the tropics and volcanic soils generally contain the least silicon. Growth reductions and “leaf freckle” of sugarcane have been reported on such soils, which are low in extractable silicon. Amending these soils with silicate materials has become a common practice, especially for sugarcane and rice production. Rates of 5 to 30 t/ha have resulted in significant increases in sugar production from sugarcane in Hawaii, for example [34].

Other beneficial effects reported on various plant species include improved resistance to fungus diseases and insect attack. This may be related to deposition of silicon dioxide in cell walls of leaf and stem tissues to give them increased rigidity [33). Reduction in manganese toxicity symptoms in crops grown on acid, weathered soils also has been reported after silicate applications. Reduced pollen viability due to low silicon uptake has been reported on several crops. Further research is needed to better understand the effects of silicon on plant growth and reproduction.

Sodium

Although in many cases unwanted due to unwanted damage to plants, some crops have beneficial effect from a fertilisation by sodium. An example of this is sugar beet.

The most common way of sodium addition is regular salt (NaCl) although in some case sodium sulphate (Na₂SO₄) can be a “low chloride” option.

References

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17. Bautista, A. C., J. C. Bunoan, Jr., and R. Feuer.1980. “Development of the Zinc Extension Component for Irrigated Rice in the Philippine Masagana99 Production Program,” Presented at the Symposium on Paddy Soils, Nanjing, China, October 19-24, 1980.

18. Hoffmeister, G. 1973. “Quality Control in a Bulk• Blending Plant,” IN TVA Fertilizer Bulk-Blending Conference, pp. 59-70, Bull. Y-62, TVA, Muscle Shoals, AL, U.S.A.

19. McCollum, ·J. A, R. S. Meline, and B. V. Durham. 1966. “Adding Nutrients to Fertilizers,” Chem. Eng.Progress) 62(9): 130-132.

20. Hignett, T. P. 1964. “Problems in ‘Production of Micronutrients Enriched Fertilizers,” IN Proc. 14th Ann. Meeting of the Fertilizer Industry Round Table, pp. 50-55.

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22. Young, R. D. 1969. “Providing Micronutrients in 26. Frick, J. 0. 1977. “Petroleum-Based DCA’s to Control Fugitive Dusts,” IN Proc. 27th Ann. Meeting of the Fertilizer Industry Round Table, pp. 94-97.

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Links to related IFS Proceedings

87, (1965), Bulk Blending of Fertilisers: Practices and Problems, T P Hignett

388, (1996), Fertiliser Blending – Technology, J E Leonard

546, (2004), Micronutrient Inclusion in Fertilisers: Safety and Compatibility, H Kiiski