Nitrogen fertilisers are blamed for causing disease and polluting the
environment. To find out how true this is we need to know more about what
happens to them in soil
NITROGEN fertilisers have revolutionised agriculture in most of the world.
They have helped us to gain more food from less land. But some people blame
them for causing disease and polluting our rivers and seas. Finding out
whether fertilisers really are to blame proves trickier than might at first
appear. Farmers may add nitrogen fertiliser to the land but soil has a secret
life, and can produce much more nitrogen. How and when it does so depends on
the biology, chemistry and physics of soil, and on how we choose to treat the
soil.
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Nitrate from fertilisers finds its way into ground water, rivers, lakes and
seas. Some medical researchers think that too much nitrate in drinking water
causes disease. In fresh- and salt-water, nitrate can disrupt the ecology so
much that some species of plants and animals die.
Yet, conventional agriculture cannot do without nitrogen fertilisers.
Throughout the world in 1950, farmers used 14 million tonnes of fertiliser. By
1985, the figure had rocketed to 125 million tonnes. In the 24 Western
industrialised countries that are members of the Organisation for Economic
Cooperation and Development, farmers used 32 per cent more nitrogen
fertilisers in 1985 than in 1970. The rest of the world, mostly poor by
comparison, bought and used 120 per cent more nitrogen fertilisers in 1985
than in 1970.
Plants need nitrogen to make protein, as do all living things. They cannot
grow healthily on soils that have too little of it. Lack of nitrogen causes
them to become stunted. Their leaves turn pale, and they yield less produce.
About 78 per cent of the Earth鈥檚 atmosphere is made up of nitrogen gas, and it
was from this that the planet鈥檚 first soils gained their nitrogen. Violent
reactions, fired by lightning in the intensely volcanic early planet, brought
about the formation of ammonia and oxides of nitrogen. These soluble compounds
would be rained out of the atmosphere and onto the Earth鈥檚 surface. Later, as
life evolved, nitrogen became converted into other, more complex compounds.
Time and the processes of weathering created our primitive soils: nitrogen
enriched them.
Plants cannot use nitrogen directly from the air. Instead, they take it in
through their roots in the form of ammonium ions (NH4+)
and as nitrates (NO3鈥�), which are both soluble in water.
Water percolating through the soil tends to remove, or leach, nitrate. Over
the centuries, farmers have learnt to counteract depletion of nitrogen from
their soils by adding farmyard manure, and by growing crops in a certain order
(鈥渞otating鈥� them). In recent decades, they have taken to adding manufactured
nitrogen fertilisers to the soil.
Microorganisms, such as some species of bacteria, and algae can convert, or
鈥渇ix鈥� molecular nitrogen (N2) from the air into ammonium ions and
thus make it available for producing protein. The best known of the nitrogen-
fixing bacteria is the species of Rhizobium that colonise nodules on the roots
of legumes, such as beans or clover. Farmers make good use of the fact that
legumes are natural fertilisers, and sow pastures with a mix of grass and
clover. By planting peas and beans before they plant cereals, they can depend
on the nitrogen-fixing bacteria helping them to save on manufactured nitrogen
fertilisers.
Nitrates and health
Blue-baby syndrome
TOO MUCH nitrate in drinking water can cause a blood disorder in babies
younger than three months. The disorder is called blue-baby syndrome. The
infant鈥檚 lips and body take on a marked blue hue. The cause is that bacteria,
either in an unsterilised feeding bottle, or within the child鈥檚 gut, convert
nitrate into nitrite (NO2鈥�). The haemoglobin in the
baby鈥檚 blood takes up the nitrite instead of oxygen: the result is that the
baby suffers severe respiratory failure.
In Britain, the last reported case of blue-baby syndrome was in 1972, but the
World Health Organization reported 2000 cases between 1945 and 1986. One
hundred and sixty of these babies died. In most cases the babies had drunk
water with more than 25 milligrams of nitrate per litre from private water
sources. Of far greater significance was that their mothers had not sterilised
the feeding bottles.
Some researchers believe that nitrites may cause cancer of the stomach and
windpipe in adults. More recent studies have not confirmed this. In 1984, the
British Medical Association reported that stomach cancer was becoming rarer in
the very areas in which nitrate levels are high.
Statistics from other countries are conflicting. In China, in the early 1980s,
140 out of every 100 000 males died from stomach cancer. Areas in which the
death rate was high also had higher than average levels of nitrate in drinking
water and vegetables. However, researchers at the International Institute for
Environment and Society in West Germany now question whether between 200 and
300 milligrams of nitrate per day are really a relevant health hazard.
Nitrates also act as fertilisers for aquatic plants. If rain washes nitrates
out of soil into streams, rivers, lakes and then into the sea in excessive
quantities, they can boost the growth of algae and other aquatic plants. This
enrichment is called eutrophication, from the Greek eutrophos, meaning 鈥渨ell
fed鈥�. Eutrophication is increasing in the rivers and lakes in many countries.
Eutrophication sometimes changes the balance of aquatic plants and animals so
drastically that a particular species may be wiped out. The organisms that
survive may grow so well that they clog waterways. Large amounts of nitrate do
contribute to eutrophication but scientists claim that the main culprit in
freshwaters is usually phosphate. (Most phosphate comes from industrial or
domestic sources and not from agriculture.)
Algae, especially green algae, respond quickest to eutrophic conditions. When
they grow rapidly on the surface they prevent light from reaching submerged
plants, which may die as a consequence. Bacteria decompose the remains of any
plants, algae and animals that sink to the bottom. The process uses up
valuable oxygen and a vicious circle develops, drawing in all forms of aquatic
organisms, until rivers, ponds and lakes become devoid of life.
In the Baltic Sea, eutrophication has increased the population of algae and
the number of small plants and animals living on seaweed. Less light reaches
plants and animals living on the bottom. About 100 000 square kilometres of
the Baltic Sea now suffers from a deficiency of oxygen.
Many people are worried about the pollution of the sea near coastlines.
Proliferations of algae, or algal blooms as they are called, can cause great
damage. As a result of nutrients from fertilisers washed into the North Sea in
the summer of 1988, algal blooms almost wrecked salmon and trout farms off the
coast of Norway. Local fish farmers lost an estimated US $200 million.
Toxic blooms were present in the North Sea once again this summer and also off
the coasts of Denmark and Yugoslavia, and in the Irish Sea.
Although scientists have clearly shown that nitrates, and not phosphates, are
responsible for algal blooms and plant growth in the sea, European countries
continue to dump into the North Sea more than 1.5 million tonnes of nitrogen
every year 鈥� two-thirds of it from rivers carrying agricultural runoff.
Natural nitrogen reserves
Microbes hard at work
FOR MORE than a century, a succession of researchers from all over the world
have joined in a exhaustive set of experiments on soil nutrients, at
Rothamsted Experimental Station, in Hertfordshire. They have treated Broadbalk
Field, an experimental site at Rothamsted, with nitrogen fertiliser in the
same way since 1843. They have also developed drainage gauges to compare the
amounts of water and nitrate leaching from plots, left bare for more than a
century, with those from other ones planted continuously with a wide range of
crops.
On unfertilised crops the researchers found that soils still leach 20
kilograms of nitrate nitrogen per hectare of land every year. Even under
spring barley, which leaves the soil bare and more prone to nitrate leaching
during the subsequent wet winter months, they found that only 7 per cent of
the 80 or 120 kilograms of nitrogen they had applied was leached.
They conclude that if a nitrogen fertiliser is given in the correct dosage for
a given crop it does not cause nitrate pollution. So where does the nitrate,
found in increasing quantities in drinking water, come from? British
researchers traced what happens to the nitrogen in the fertiliser by labelling
it with the isotope nitrogen-15. They found that most nitrate in water comes
from vast reserves of nitrogen that were held naturally in the soil before and
while farmers introduced intensive practices. Researchers in Sweden have
confirmed the British results. They found that whether or not they applied
spring fertiliser to their experimental plots, the soil still released
nitrate.
Undisturbed soils under natural vegetation can have nitrogen reserves of as
much as 5000 kilograms per hectare. Most of this is held as insoluble nitrogen
and comes from decomposed organic matter. Microbes convert this nitrogen to
ammonium and then to the mobile nitrate form, not when crops need it, but when
conditions suit.
Warm temperatures, moisture and aeration during cultivation encourage
microorganisms to make nitrates. There is no evidence that fertilisers
directly affect the amount of nitrate in water, but British researchers
believe there are indirect effects. They think that adding nitrogen in
fertilisers stimulates microbial activity. This, in turn, releases some of the
nitrogen that is bound onto organic matter and otherwise not available for
leaching.
How much is leached?
The answer lies in the soil
THE FATE of nitrate depends very much on the type and state of a soil, the
influence of vegetation and the amount of rainfall infiltrating and
percolating through the top layers.
Soils are a mix of differently sized mineral particles and a rich diversity of
microflora and fauna. Sand, silt, clay, organic matter, water and air spaces
or pores make up the non-living and dead ingredients of soil. The relative mix
of sand, silt, clay and organic matter determines its texture and consequently
the way it behaves when water moves through the soil.
Clay particles are invisible to the naked eye. They are held together by
strong chemical forces, which is why clay feels sticky. Silt particles are
smooth and silky, like talcum powder. They range from 0.002 millimetres to
0.06 millimetres in diameter. Sand particles are large enough to see. Their
size ranges from 0.06 millimetres to 2.0 millimetres. Larger ones are classed
as stones. Organic matter in the soil is made up of the residues of plant and
animal remains. It is usually concentrated in the top 10 centimetres of
undisturbed soils, or to the depth they are cultivated by ploughs or other
equipment.
A soil that contains sand, silt and clay in equal proportions is called a
loam. The surfaces of particles of clay are negatively charged. They attract
positively charged compounds and ions that move around in the soil solution.
These positively charged species include the ammonium ion, calcium
(Ca2+), potassium (K+) and hydrogen (H+).
Nitrate stays in solution, free to move around within the soil.
Many tropical soils have a net positive charge which holds much of the nitrate
in soil water. The amount of nitrate lost after rainfall depends on the number
of positively charged sites, the speed of the reaction, the amount of water
and its rate of movement. Tropical soils of the right texture and structure
can be less of a leaching risk than temperate soils. Researchers at the
University of Reading calculate that it takes up to five times more rain to
leach nitrate from a tropical soil than from a temperate soil with similar
physical characteristics. But leaching is still important: reduced mobility of
nitrate is offset by more rain.
Not just cultivation
Feel the texture
CULTIVATING crops has an enormous effect on nitrate leaching. Growing plants
take up water and nitrate, thus tending to counteract leaching. However, at
low temperatures in winter, or early spring, because they are not growing and
transpiring, plants cannot use nitrogen. So, given sufficient rainfall, the
soil is likely to lose more nitrate. In contrast, freezing hinders the flow of
water movement through the soil and thus temporarily prevents leaching.
There are also big differences in how well crops keep nitrates in soils.
Potatoes, for example, have shallow roots and need a lot of fertiliser.
Farmers also need to apply water (irrigate) if they are to produce high
yields. So, given the right type of soil, growing potatoes could lead to loss
of much nitrate through leaching.
Wheat sown in the winter usually produces good deep roots and uses not only a
lot of applied nitrogen but also much of the nitrate produced during the
autumn by microorganisms acting on nitrogen reserves in the soil. Winter wheat
also protects the soil from being washed away with the rains.
Soils vary in their ability chemically to hold different forms of nitrogen.
The amounts of sand, silt and clay plus the effect of different types of
cultivation determines the soil structure, or the size and shape of the soil
building blocks. The arrangement of such clods tends to determine how easily
water passes through the soil, taking nitrate deeper and deeper.
Sandy soils, because they have little chemical bonding, are usually weak. They
are easy to pull apart, or dig, and their clods can be broken down easily into
smaller units. Sand particles are rather like ping-pong balls in a tank which
are difficult to squash together and are separated by large gaps. Water can
pass quickly through the gaps, and so leaching is rapid in sandy soils.
Clay soils are tightly packed and dense, like chunks of jelly in a tank. Water
moves through them much more slowly. Leaching is therefore slower and water
tends to form pools on the surface of clay soils, so nitrates often end up in
surface waters rather than in ground waters.
Heavy downpours can lead to waterlogging, in which water displaces air from
the pores within the soil. The resulting lack of oxygen in the soil encourages
anaerobic organisms to convert nitrates to ammonium forms or to nitrogen gas.
Clay soils hold water and offer less nitrate for leaching than do sandy soils.
French researchers calculate that a clay soil loses seven to eight times less
nitrate than a sandy soil.
It is difficult to measure the effects of different underlying rock types on
nitrate leaching, but some things are obvious. For example, the size and shape
of pores and fissures in the rock govern the rate at which water moves into
the zones of ground water from which it is extracted. (Ground water is the
source of springs and wells, hence much of our drinking water.)
Nitrate levels in temperate countries including Britain, West Germany and
France, are increasing. Tempting though it may be, we cannot blame fertilisers
for all nitrate pollution. Even so, fears about the adverse health effects of
consuming nitrates have prompted politicians to restrict the nitrate allowed
in drinking water, rather than waiting for conclusive evidence.
Drinking nitrates: the legal limits
In 1975 the EEC issued a 鈥淒rinking Water Directive鈥�. It set a legal limit of
not more than 50 milligrams of nitrate per litre of drinking water. This
finally came into force in 1985. But almost all EC member countries fail the
standard in some areas.
In 1986, Britain鈥檚 Department of the Environment did a survey which found that
82 water supplies, serving 2.5 million people, breached the EC limit on one or
more days. Last year, a nationwide survey in Britain showed that 74 water
supplies, serving 1.6 million people, contained more than 50 milligrams of
nitrate per litre of water, albeit not much more. However, the amount of
nitrate by which supplies exceed the EC limit is increasing. This is
particularly so in the main farming areas of Britain such as Norfolk,
Cambridgeshire, Lincolnshire and Hereford and Worcestershire.
Five to 10 per cent of West Germany鈥檚 drinking water 鈥� largely from boreholes,
rather than rivers as in France and Britain 鈥� contain water with nitrates
above 50 milligrams per litre. Average concentrations, however are rising by
between 1 and 2 milligrams per litre every year in areas that are cultivated
intensively. The highest levels are from water beneath the lighter soils, much
as those of northern Germany around Hamburg and Bremen.
A study carried out for the French Ministry of Public Health revealed that 2
per cent of the population (1.2 million people) consumed water with more than
50 milligrams of nitrate per litre, and 0.6 per cent of the population
consumed water containing more than double the EC limit. A later inventory in
1987 revealed that although fewer French people were drinking water with more
than 100 milligrams nitrate per litre, more water contained nitrate in excess
of 50 milligrams per litre. The wetter, more fertile north of France has the
most nitrate-rich drinking water. Affected areas include the Nord-Pas-de-
Calais, Brittany, the Paris Basin, the Loire country, Poitou-Charentes and
Champagne-Ardennes.
Drainage of some Swedish wetlands has provided the country with some very
productive soils that are rich in organic matter. About 10 per cent of the
arable areas have soils containing more than 20 per cent organic matter and
thus large reserves of nitrogen. Despite that 鈥� but only until recently 鈥�
Swedish farmers applied fertiliser to soils in which reserves of nitrogen
often exceeded the requirements of the crops grown on them. One result was the
eutrophication of surrounding lakes and rivers.
How to save money on fertilisers and protect the environment
ONCE farmers know more about the problems that nitrates can cause, they are
likely to use fertilisers more wisely. Fertilisers are expensive, and it is in
the best interests of farmers to reduce nitrate leaching. To do this, they
need to know more accurately how much nitrogen individual crops need.
They should apply fertiliser only when the crop is most likely to use the
nitrate. It makes more sense to wait until soil warms up in the spring,
because that is when crops need nitrogen from the soil.
Dividing up fertiliser between February, March, April, May and occasionally
June also makes financial sense. A heavy downpour in the early spring, for
example, would lose only part of the total.
Cultivating crops that are sown in autumn would help to keep soil covered as
well as using some of the nitrate naturally released in the soil.
Alternatively, farmers can cultivate during the winter 鈥� and between main
crops 鈥� quick-growing catch crops to take up nitrate produced in the autumn.
The catch crops can then be ploughed under prior to planting the next food
crop in the spring.
Leaving soil undisturbed prevents aeration and microbial activity that would
otherwise encourage the release of nitrates in the soil. At Maryland
University, in the US, researchers found that planting without ploughing
resulted in less nitrate being available for leaching. British researchers
have since confirmed those findings.
Mixing in straw after harvesting instead of burning it provides microbes with
a source of food: the soil provides them with the nitrates they need to make
enzymes to break down the straw.
Organic farming, however wholesome its image, could increase the amount of
nitrate available for leaching. Organic farming demands traditional sources of
nutrients such as farmyard manure. Organic farmers also grow legumes such as
clover to fix nitrogen from the air with the help of bacteria living in the
nodules on the roots. The farmers then plough in the legume/grass mix before
they sow the next crop, to supply the crop with nitrates.
Both traditional and organic systems supply plants with the same form of
nitrogen 鈥� ammonium and nitrate. But research has shown that while farmyard
manure is as effective in producing high yields, it can give as much as 100
kilograms more nitrate per hectare to the soil than artificial nitrogen, and
it is often put on land in the autumn.
Farmers also tend to mix manure containing ammonium nitrogen into the soil in
the autumn. The result is that microbes start to convert ammonium to nitrate
at a time when plant growth is slow and rainfall is high 鈥� conditions that
encourage leaching. The same applies to ploughing in legumes 鈥� once again,
nitrate will be released in a form more prone to leaching. Organic farmers
must exercise skill and care if leaching is not to be increased.
Further reading
Nitrates: The Threat to Food and Water, by Nigel Dudley (Green Print 1990)
provides a review of the nitrates controversy. Fertilisers food production and
the environment is a schools鈥� guide from the Fertiliser Manufacturers
Association (tel 0733 331303). Two features in New 杏吧原创 (8 October 1988
and 29 April 1989) present the latest research findings of the Rothamsted
Experimental Station team on nitrates and leaching.