Feeding the world: the future role for minerals

According to the Food and Agriculture Organization (FAO) of the United Nations (UN), the world’s population will reach 9bn by 2050, an increase of 2bn over the current total. Most of this increase will occur in developing countries. 

Because of this rapid expansion, a significant increase in crop production is required to 

meet food and energy demands for future generations. The FAO predicts that global crop production will need to increase by 70% by 2050 to meet the projected demands from rising population, diet shifts and rising biofuels consumption.

Challenges to overcome include:

• Dealing with climate change. More incidences of drought and heat stress are predicted in some regions; other problems include increased levels of weeds, pests and disease.

• A smaller rural labour force. The UN currently estimates that 50% of the world’s population is rural, but this is expected to decrease to 30% by 2050, as most growth will be in urban areas.

• Feeding a larger population of livestock.

• Meeting demand for the growing biofuel market including ethanol and bio-diesel. Global biofuel production may increase from the current volume of 155bn litres to 192bn litres by 2018 according to the UN, depending on the price of crude oil and policies regarding the need for food versus fuel.

• Tighter legislation covering the use of fertilisers and pesticides to prevent contamination of land and water systems and potential harm to human health.

Most of the required increase in agricultural production is expected to come from raising yield per acre, rather than a significant expansion in agricultural land area. 

Excessive application and run-off of nutrients can cause eutrophication of water bodies, killing aquatic plants and fish.

Nutrient Provision

Nutrients improve crop yield, growth rates and quality. They can be divided into the following categories:

Primary nutrients: nitrogen, phosphorus and potassium. 

Secondary nutrients: calcium, magnesium and sulphur.  

Micronutrients: boron, copper, iron, chloride, manganese molybdenum and zinc. 

Nutrients are provided naturally from the air, soil and water. They can also be provided artificially as chemical fertiliser, manure or as natural minerals, usually crushed into powder. Nutrients must be in a soluble form, digestible for conversion to a soluble form by bacteria, or available via cation exchange from clays or humic matter in the soil in order to be taken up by plants. Nutrients contained within the crystal lattice of minerals are insoluble and unavailable.

Primary Nutrients

Nitrogen

Nitrogen is part of the chemical structure of cells, proteins, enzymes and chlorophyll, the green pigment responsible for photosynthesis. It aids plant growth, improves the quality of crops and increases seed and fruit production. It is supplied naturally from the air or by application of fertilisers and manure.

Phosphorus

Phosphorus contributes to photosynthesis and helps to form oils, starches and sugars. Another major role is the transfer of energy, which helps stimulate early plant growth and hasten maturity.

Phosphate is supplied from apatite, a calcium phosphate mineral, which occurs mainly in sedimentary rocks (around 80%) but also in igneous and metamorphic rocks (around 20%).  Methods used to refine phosphate rock include, grinding, flotation and drying. Guano was a significant source in the past.

To produce phosphate in a soluble form, apatite is reacted with sulphuric acid to produce single super phosphate (SSP) which contains 16-21% P₂O_5.   If phosphoric acid is used, triple super phosphate (TSP) is formed, which contains more P₂O₅ at 43-48%. Both products contain useful calcium and SSP also contains sulphur. Monoammonium phosphate (MAP) and diammonium phosphate (DAP) are also used.

In 2014, the US Geological Survey (USGS) estimated world phosphate reserves at 67bn tonnes, 75% of which occur in Morocco and the Western Sahara region.

Annual global phosphate rock production in 2014 was estimated by the USGS at approximately 220m tonnes. This is produced by more than 30 countries, with the top 12 supplying over 90% of the total. China, the US and Morocco are the largest producers with The Mosaic Co. in the US and Yuntianhua Group in China being the highest producing companies. Other major producers include Vale SA of Brazil, and Potash Corp. of Saskatchewan (PotashCorp.) and Agrium Inc. of Canada. Office Cherifien des Phosphates (OCP) in Morocco controls the largest reserves. 

More than 80% of all phosphate production is used in fertilisers. The global market is worth about $30bn according to consultancy Integer Research.

Farmers are under pressure to increase crop yields in order to feed a growing global population.

Potassium

Potassium is vital for the suppression of disease, protein production, and photosynthesis, all of which contribute to better plant and fruit quality. 

It is supplied naturally from clay minerals in the soil and fertilisers produced from potash-containing minerals.

Most of the potassium contained in clay minerals is locked within the structure of layered crystal sheets and is not available to plants. Some will become available due to weathering on the crystal surface, which results in a slow release of soluble material.  A small amount is also available as exchangeable cations, loosely held on the surface of clay particles. 

Some large agricultural areas of the world are deficient in potash availability, including 75% of the paddy soils of China and 66% of the wheat belt of Southern Australia, according to a paper entitled "Importance and application of potassic biofertiliser in Indian agriculture", published in the International Research Journal of Biological Sciences. Soils inherently low in potash are often sandy, waterlogged, saline or acidic. 

The potash-bearing minerals of economic importance are evaporates, with sylvine (potassium chloride) being by far the most important. This is often associated with halite (sodium chloride), from which it can be separated by thermal dissolution, flotation or electrostatic beneficiation.  Refined sylvine can be used directly or chemically converted to other potassium salts such as sulphate or nitrate for application as a fertiliser. 

The chloride content of sylvine is itself an important micronutrient thought to increase resistance to disease and improve yields.

Annual potash shipments were estimated at 61m tonnes in 2014 by PotashCorp. with the majority of production coming from 12 countries. The six main producers are Canada, the Russian Federation, Belarus, Germany, Israel and Jordan. Canadian extraction is centred in Saskatchewan where Mosaic, PotashCorp. and Agrium operate. In Israel and Jordan, potassium and other salts are extracted from the Dead Sea by allowing saline water to evaporate in shallow ponds. As the salts crystallise out they are harvested for refining.

The largest importers are China, India and Brazil.

SEM and TEM images of HNT. Source: I-Minerals Inc.

Eutrophication

Eutrophication results from the overuse of fertilisers containing nitrogen and phosphorus, which leads to the pollution of streams, rivers, lakes and coastal waters. 

Nitrogen and phosphorus encourage the growth of algae, with excessive growth producing algal blooms. Algal blooms prevent light from penetrating the water’s surface and deplete oxygen levels, resulting in animals and plants either dying or leaving the polluted zone. Dead organic matter becomes food for bacteria that decompose it. With more food available, the bacteria increase in number and use up dissolved oxygen in the water.

Some algae are toxic, for example blue green algae, which contain cyanobacteria, harmful to both humans and animals. Shellfish such as mussels and oysters can take up biotoxins, which can potentially poison humans. Nitrate in drinking water sourced from contaminated ground waters can be harmful, particularly to infants. Coastal areas badly affected are known as 'dead zones’ and include the northern Gulf of Mexico, which has been polluted by run off from farms in the Mississippi watershed. Lake Erie and the Caspian Sea are also considered dead zones. 

The Global Phosphorous Research Initiative (GPRI) estimates that 8-15m tonnes phosphates are lost to sea every year, as run-off. 

Eutrophication can be controlled by the following, nonexclusive measures:

• Using slow release techniques to supply the nutrients. 

• Limiting amounts of fertiliser applied and the frequency of application.

• Preventing run-off into water systems.

• Planting vegetation along riverbeds to reduce erosion and absorb nutrients.

• De-nitrification by bacteria to convert nitrates into harmless molecular nitrogen.

Secondary Nutrients

As deposits of calcium, magnesium and sulphur are relatively common throughout the world, there are a large number of suppliers.

Calcium

Ground limestone or chalk used for agriculture is often referred to as 'lime’. 

It neutralises soil acidity, thus reducing soluble aluminium, which is toxic to some root systems. Additionally, calcium strengthens cell wall structure, which protects the plant against diseases from fungi and bacteria. It also promotes the uptake of nutrients, helps in the prevention of heat stress and improves fruit quality.

Sources include: exchangeable Ca⁺⁺ ions adsorbed onto clay minerals and organic matter in the soil (this is small and insufficient to supply complete needs); crushed limestone; chalk or dolomite; gypsum; calcareous sands; and SSP.

Magnesium

Magnesium is contained within chlorophyll and is therefore essential for photosynthesis. It also plays an important role in activating enzymes for growth.

Insufficient magnesium results in degradation of chlorophyll in old leaves, causing a yellowing effect between the leaf veins, and brown spots.

Similar to calcium, a very small part of the magnesium requirement can be provided by Mg²⁺ adsorbed into clay minerals and organics in the soil. This is supplemented by the addition of crushed dolomite and magnesium sulphate (Epsom salts). Most of the latter is sourced from natural deposits but it can be manufactured by reacting dolomite with sulphuric acid. 

Caustic calcined magnesia (CCM) is another important source. Calcination of ground magnesite (MgCO₃) takes place at 1,100°C to produce CCM containing more than 90% MgO. China, North Korea and Russia are major suppliers.

Sulphur

Sulphur plays a number of important roles including: production of protein; development of enzymes and vitamins; formation of chlorophyll; root growth; seed production; and resistance to cold. All sulphur absorbed by plants is in the sulphate form, normally supplied as chemical fertilisers such as SSP, organic matter such as manure and minerals such as gypsum or Epsom salts. Elemental sulphur is used as a fungicide for crop protection, as toxic hydrogen sulphide evolves from the interaction with living fungal tissue. 

Micronutrients

Metallic nutrients such as copper, iron, manganese, molybdenum and zinc all have important roles to play in encouraging plant health and growth.

Boron aids the production of sugar and carbohydrates and is important for pollination and seed and fruit development. High soil concentrations of more than 1 parts per million (ppm) lead to necrosis in leaves and poor growth, hence application rates are critical. In large concentrations, boron compounds can be used as herbicides, algaecides and pesticides.

Boron is obtained from ulexite, borax, colemanite and kernite, which are all evaporates. Large deposits are found in the US, Turkey, Chile and Argentina. Rio Tinto Borax operates California’s largest open pit mine in Boron, California, which supplies nearly half the world’s demand for refined borates.

Crop Protection

Market research group Ceresana forecasts that the global market for crop protection products (including herbicides, insecticides and fungicides) will generate revenues of $52bn by 2019.

This is a big opportunity for minerals to provide environmental improvements and reduce the risk of contaminating foodstuffs by replacing toxic chemicals or by reducing their application.

Kaolin

There is a multi million dollar global market for specially refined kaolin for crop protection. The most popular product is 'Surround’ which was originally developed by Doctors Glenn and Puterka of the US Department of Agriculture, in co-operation with Engelhard (now BASF), in 1999.

Surround is a highly refined sedimentary kaolin which is calcined and treated with chemicals to aid adhesion to the fruit and spreadability. Typical crops that can be treated include apples, pears, grapes, plums and olives.

The clay is sprayed onto fruit and leaves as a dilute suspension, usually less than 5% solids. Three applications are normally required for adequate coverage and re-applications are required following rainfall. The sprayed products must be washed before sale.

It is marketed by the NovaSource division of Tessenderlo Kerley, based in Phoenix, Arizona, which has met legislative approval in many parts of the world. Sales are mostly to hot countries where fruit is grown in large commercial quantities and both sunburn and insects can be a problem.

Surround has been proven to be effective against many insect species including pear psylla, leaf hopper, olive fruit fly and thrips. Such insects are irritated by the abrasive nature of the clay particles and are less inclined to feed and lay eggs.  

The very bright  clay particles allow helpful photosynthetically active radiation to pass through to the fruit but reduce harmful radiation in the infra red and ultra violet, thus alleviating heat stress and sunburn. NovaSource claim that losses due to sunburn can be reduced by as much as 50% using Surround.

Prices to the end user through third party farm produce suppliers in the US are typically $40/25 lb bag or $3,550/tonne.

Crop Microclimate Management based in North Carolina has developed a modified hydrous kaolin known as Screen Duo. As the name suggests, this has a dual action. 

• Kaolin keeps the crop cooler by reducing damage from excess radiation, while still supporting photosynthesis. Reduced leaf temperatures lead to a reduction in water loss by transpiration, with savings in water estimated up to 25%. 

• Screen Duo contains a naturally occurring compound to stimulate the plant’s inherent stress coping biochemical systems. 

Bentonite

Research by Agriclay in the US indicates that montmorillonite can adsorb toxins and bacteria, reducing their negative impact. For instance, dilute clay suspensions can control potato scab, caused by Streptomyces scabies. 

Bentonites are also used to improve the viscosity and suspension properties of  aqueous spray applications used for crop protection.

Diatomaceous earth

Diatomaceous earth is used to control pest infestations in harvested grain stored in silos. The razor sharp edges of the diatoms lacerate insects’ legs and other body parts, after which the powdery material absorbs body fluids, bringing death by dehydration. 

Sandy Soils

Zeolites and bentonites can be used to improve plant yield from unproductive sandy soils due to their ability to retain water and nutrients. This proved successful in northeast Thailand where bentonite added at 20 tonnes/km² improved rice yields by 73%, according to the International Water Management Institute (IWMI). Similar trials are taking place in South Africa and Australia.

Acid Soils

The best pH for most crops is 6.5-7.0. Acidic soils below pH 5.5 are most common in areas of high rainfall where base cations are leached out and concentrations of H⁺ and Al³⁺ increase, causing inhibition of root growth.  

At low pH, phosphorus is sequestered into an insoluble form by the reaction with Al³⁺ and Fe³⁺ and becomes unavailable to plants.

Other factors contributing to low soil pH are acid rain and 'nitrification’, the break down of ammonium (NH⁴⁺) from fertiliser to nitrate (NO^3-) and H⁺.

Limestone, dolomite or calcareous sands can be applied to increase pH. 

Calcium hydroxide (hydrated lime), calcium oxide (burnt lime) and chemically derived products like potassium carbonate are also used due to their higher solubility and quicker action.

Nano Technology

Nano particles are defined as having at least one dimension less than 100 nanometres (nm=10^-9 metres).

There is a huge amount of research across the world in nanotechnology, some of which is directed towards agriculture.

Much work has focused on synthetically engineered nanoparticles such as carbon-based fullerenes (which include carbon nanotubes) and metal-based materials such as titanium dioxide (TiO₂), cerium oxide, magnetite, zinc oxide, gold, silver and copper. 

There is concern that with long-term use of some of these nanoparticles they may be retained in the environment, creating problems with plant, animal or human health. 

As an example, research at the University of California found that cerium oxide nanoparticles added to soybean to improve growth rate entered the root system. The roots host bacteria that alter atmospheric nitrogen into a form the plant can use - the cerium nanoparticles prevented the bacteria’s ability to do this. 

Synthetic carbon nanotubes (CNT) provide benefits in many different industries, for example plastics and polymers, but caution is required before their use is considered in agriculture, as they have been linked to respiratory problems and cancer.

However, there is huge scope for naturally occurring nano-scale minerals, asthese do not have the same health and safety concerns. Halloysite nanotubes (HNT) and zeolites are very good examples, both having potential in slow release applications.

Development of slow release applications

The Association of American Plant Food Control defines slow release fertiliser as "containing a plant nutrient in a form which delays its availability for plant uptake after application, or which extends its availability to the plant significantly longer than a reference 'rapidly available nutrient fertiliser’ such as ammonium nitrate, ammonium phosphate or potassium chloride". 

Slow release of nutrients allows for more precise farming procedures giving improved crop yields without worsening pollution and eutrophication. In the case of nitrogen, losses by leaching into water systems and atmospheric emission will reduce, as HNT or zeolite will adsorb gaseous ammonia. Soil quality can be improved by decreasing toxic affects associated with the overuse of fertilisers.

Slow release of pesticides can reduce total requirements by 70-80%, thereby reducing costs and impact on pollution. 

HNT is effective in controlling insects, which collect particles in their hair and later groom and consume the pesticide-loaded tubes.

Specific DNA strains can be transferred into plants by slow release from HNT (genetic modification) to develop insect and virus resistant varieties. There are many other potential benefits, which include fruit that will ripen faster and maize with improved nutritional value.

Zeolites or HNT, loaded with nutrients and blended into a fertiliser, can provide the same plant yield from smaller applications, due to reductions in volatilisation (such as ammonia from manure) and leaching losses. The USGS is currently experimenting with zeolites for this purpose.

Other new techniques, which can solve these problems and compete with HNT and zeolites include:

• Controlling the water solubility of nutrients by semi-permeable coatings.

• Inhibition of the nitrification reaction i.e., oxidation of NH⁴ to NO³, thus extending the residence time of nitrogen in the soil.

Halloysite nanotubes

Halloysite is an inert clay mineral generally recognised as safe (GRAS) by the US Food and Drug Administration (FDA). It occurs in the form of hollow tubes which typically range from 0.5-6.0 microns in length with outer diameters at 50-200 nanometers (nm) and internal diameters (lumen) of 10-30nm. Chemically, it is similar to kaolin (aluminium silicate) but the crystal shape is tubular rather than platy, due to prevailing conditions at the time of formation.

There are numerous deposits of halloysite globally but many are small and of scientific interest only. Major operations which are already commercial or under development include: Matauri Bay in New Zealand, owned by Imerys Ceramics; the Dragon Mine in Utah owned by Applied Minerals Inc.; and the Helmer Bovill deposit in Idaho, owned by I-Minerals Inc. 

The latter two companies are promoting HNT and have engaged in considerable research into their benefits as nanotubes. Long tube lengths and high lumen diameters characterise the Idaho deposit, which could potentially provide good slow release properties.

NaturalNano Inc., based in Rochester, New York State, buys halloysite from different sources and refine the material into high value HNT. The company is highly proactive in researching novel applications, including agricultural.

The hollow tubes of HNT can be loaded by immersion in a concentrated aqueous solution of the required chemical, which diffuses into the tubes according to Fick’s Law. Loading is enhanced by evacuation of air contained within the tubes. 

The loaded chemical is slowly released over a period of hours or days, depending on the morphology of the halloysite and the viscosity of the chemical solution. Lumen diameter can be increased by treatment with acid and tube ends can be capped to increase release time. 

Zeolites

In contrast to other clay minerals, zeolites have a rigid three dimensional (3D) crystal structure similar to a honeycomb, consisting of interconnected channels and cages.  Water molecules and potassium and calcium cations are contained within the crystal. Zeolite has a high cation exchange capacity but only for those of the correct size to fit into the pores, hence the description 'molecular sieve’.

Zeolites can absorb up to 55% of their weight in water according to US-based Zeotech Corp., by effectively providing a slow release reservoir, which can help plants during prolonged dry spells and prevent root rot. It can improve non-wetting sandy soils and increase crop production.

There are many varieties of natural zeolite, including clinoptilite, modenite, analcime, chabazite and natrolite. The former two are the most widely used.

Modenite has been used in Japan and Taiwan for more than 50 years to control moisture content and increase pH of acid volcanic soils, providing benefits in crop yield, particularly rice.

Many other parts of the world are now either using or researching zeolites for agriculture.

Clinoptilite has a pronounced selectivity for large cations such as potassium and ammonium. It is therefore used in the preparation of fertilisers and manures, providing slow release, which improves efficiency by reducing losses as run-off.

Zeolites can also act as a trap for toxic heavy metals present in soil, such as cadmium and lead, preventing their transfer into plants.

Around 3m tonnes zeolite is produced annually, sourced mainly from China, South Korea, Japan, Jordan, Turkey, Slovakia and the US.

Synthetic zeolites are considerably more expensive than the natural varieties but offer some technical advantages, not least larger internal pores. They are manufactured by treating sodium, aluminum and silica with steam or by reacting calcined kaolin with sodium salts and water. The high cost of synthetic zeolites precludes their use for most natural zeolite applications. 

Conclusions

Given the increasing requirement for food and the fact that fertilisers improve crop yields by 30-50%, the demand for minerals required for their production will increase. 

For new ideas it may be a slow and expensive business proving compliance with agricultural legislation before sales can commence. Such legislation varies between different parts of the world and there are often local discrepancies within a country. Successful companies will improve crop yields more efficiently, whilst simultaneously replacing toxic chemicals and reducing pollution.

Acknowledgements:

Prof. Maria DeRosa, Carleton University; Jason C White, Connecticut Agricultural Experiment Station; Yuri Lvov, Louisiana University; Roberta Virta, USGS.

*Frank Hart is technical director at First Test Minerals Ltd.