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.
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Excessive application and run-off of
nutrients can cause eutrophication of water
bodies,
killing aquatic plants and fish.
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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% P2O5.
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.
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Farmers are under pressure to increase
crop
yields in order to feed a growing global
population.
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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.
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SEM and TEM images of HNT.
Source: I-Minerals Inc.
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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
(NO3-) 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.