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Impact of fertilizers on health and
environment
Positive effects of fertilizer use on the environment are
often overlooked and only the negative aspects are brought
into focus.
Mineral and organic fertilizers are accused of accumulation
of dangerous or even toxic substances in soil from
fertilizer constituents, for example cadmium from mineral
phosphate fertilizers or from town or industrial waste
products; eutrophication of surface water, with its negative
effect on oxygen supply (damaging to fish and other forms of
animal life); nitrate accumulation in ground water,
diminishing the quality of drinking water; unwanted
enrichment of the atmosphere with ammonia from organic
manures and mineral fertilizers, and with N2O from
denitrification of excessive or wrongly placed N fertilizer.
Regarding contamination of soils with toxic heavy metals, it
can easily be shown that mineral fertilizers make only a
small contribution in comparison with town wastes, for
example.
However, as soil fertility must be considered in the very
long term and not only in decades or centuries, the annual
addition should be kept at such a low level that the
enrichment is negligible.
Industrial waste products should always be carefully checked
to determine whether they contain potentially toxic
substances, and appropriate critical limits should be
established.
Nutrient losses from the soil into surface and ground water
(mainly nitrate by leaching and phosphate by erosion) occur
even when fertilizers are not used, but they are increased
slightly but unavoidably even by correct fertilizer use and
are increased substantially by excessive or unbalanced use.
Considerable leaching of nitrate is caused, for example, by:
excess application of organic liquid manure; intensively
fertilized speciality crops; ploughing of grassland;
fertilizer application for over-optimistic yield
expectations which fail to materialize; part of the
correctly estimated N requirement remaining unused because
of other limiting factors not being taken into account -
deficiencies of secondary or micronutrients, for example.
In other words, N losses are mainly due to mistakes in
fertilizer use or crop management, not fertilizer use
itself. Moreover, counter-measures can be taken to prevent
loss of nitrate residues after harvesting (soil must not be
left bare over winter) and to prevent soil erosion.
Nitrogen loss by leaching seems to range from 10kg/ha N to
more than 100kg/ha N; in extreme cases more than 150kg/ha N
depending on the usage of fertilizer and preventive methods.
In Germany the average appears to be far below the
officially (but wrongly) discussed figure of 100kg/ha N, but
for most soils rather in the range of 30-60kg/ha N.
In any case, exaggerated overall averages do great injustice
to farmers who apply fertilizers accurately and spend much
effort in preventing excess leaching. From a scientific
point of view, much more attention needs to be given to the
enigma of N balance sheets before drawing premature
conclusions on N losses.
Loss of phosphate by leaching (<1kg/ha P) is negligible,
while loss by erosion is due to bad soil management rather
than fertilizer use.
Atmospheric pollution by ammonia is mainly due to primitive
methods of storing and spreading organic manure. N immission
(involuntary intake from the air) ranges, in Central Europe,
from 10 to 15kg/ha N, with over 40kg/ha N recorded in the
vicinity of intensive animal husbandry.
Of the mineral fertilizers, only urea and ammonium sulphate
might cause significant NH3- volatilization losses,
especially if not incorporated (grassland, topdressing of
cereals, for instance). To minimize these losses,
incorporation into the soil or application before rain or
irrigation is recommended.
The contention that agriculture contributes considerably to
N2O production via denitrification, as a result of excessive
or wrongly applied N fertilizer, is a serious problem. This
gas contributes to the destruction of the ozone layer in the
stratosphere which protects the world against ultra-violet
radiation.
Official estimates, derived mainly under artificial
conditions or by the difference method, showing losses of
approximately 15 per cent or more of the applied N, are not
really substantiated. Total denitrification losses in the
range of 5-10 per cent of the applied N, of which only about
10 per cent is as N2O, seem to be more realistic, especially
for soils under normal moisture conditions.
Since pollution of the environment should be minimized,
governments are trying to control the avoidable negative
influences by special laws.
Under conditions of food shortage, the major goal of
fertilizer use is a high crop yield giving a lower priority
to food quality and possible negative influences on the
environment. However, when production efforts have resulted
in meeting the food demand or even in a surplus, the quality
aspect and the potential pollution effects on soil, water
and air receive the same or more importance as the crop
yield itself.
This topic should be considered in a broad sense.
Fertilizers can influence quality.
They do so indirectly, by improving plant health, especially
resistance to adverse climatic factors, diseases and pests,
or directly, by increasing the content of essential and
beneficial organic and mineral nutrients in human food and
animal feed. They can have negative impacts, through their
incorrect or imbalanced use or by the involuntary addition
of toxic substances.
Examples of the improved resistance of well nourished plants
to adverse climatic factors: (1) resistance to drought: a
better supply of K improves their waterholding capacity, and
P encourages early root growth and so ensures better
survival in dry spells; (2) resistance to frost and cold:
increased by a better supply of K, P and some micronutrients
(for example,Mn, Cu); (4) resistance to ultra-violet
radiation: a good supply of Zn counteracts radiation-induced
destruction of growth regulators.
Clearly, the damaging effects of plant diseases and pests
cannot be completely eliminated simply by supplying abundant
and balanced plant nutrition, but in many cases they can be
contained and reduced to a lower and sometimes negligible
level.
Examples of such instance include: (a) better resistance to
some insect pests resulting from a good supply of K, as a
result of better mechanical protection and a decrease in
cell constituents attractive to insects; (b) better
resistance to fungal attack resulting from a good supply of
boron; (c) improved soil fertility also seems to result in
soil fungi producing a better supply of antibiotics which
protect plants therapeutically against some bacterial
diseases.
Further research is certainly needed in this border area
between plant nutrition and plant protection with the aim of
minimizing the need for protective sprays.
There are two separate aspects of food and fodder quality:
1. Market value: depending on easily recognizable external
characteristics such as cleanness and absence of decay;
furthermore, on the content of protein, sugar, etc, for the
processing industry.
2. Nutritional value: comprising palatability (taste and
smell, difficult to categorize), content of the many
important organic and mineral nutritional constituents, and
absence of undesirable or even dangerous toxic substances.
Nutrient supply affects food quality. for instaqnce:
- Better supply of nitrogen increases amounts of total and
pure protein, protein quality (more of essential
amino-acids), and some vitamins, especially B1; excessive
supply tends to increase amide content, resulting in bad
flavour after cooking, or to raise nitrate content
unacceptably.
- Better supply of phosphorous improves protein quality and
increases the content of some vitamins and of mineral
phosphate, which is an important mineral nutrient; slightly
increased radioactivity due to uranium present as natural
impurity in P fertilizer seems to be of no importance
whatsoever.
Better supply of Potassium increases carbohydrates, and
especially vitamin C; as with P, slightly increased
radioactivity coming from the naturally occurring K40
isotope is of no importance.
Other nutrients: The advantage of having an optimum supply
of all nutrients is obvious. Individual nutrients which may
adversely affect quality when supplied in excess include the
heavy metals such as Zn and Cu. Unwanted contamination with
the toxic heavy metal Cd may arise from the use of town
wastes or, less significantly, from P fertilizers.
Deficiency diseases
Although the fertilizer-induced increase in the content of
essential food constituents does not necessarily signify
that fertilizers improve "health", it seems nevertheless to
be so. Before the advent of fertilizer use, deficiency
diseases in farm animals and humans were widespread, for
example bone weaknesses due to lack of P, vitamin
deficiencies due to inadequate plant nutrition, diseases in
grazing livestock due to deficiencies of Cu and Co.
Furthermore, some virus and bacterial diseases seem to have
diminished in their infective capacity as a result of
improved nutrition. The considerable increase in human life
expectancy must also be attributed in part at least to
having more and better food, stemming in turn from
fertilizers and such.
Even so, it has to be admitted that a significant proportion
of the benefits to food quality are lost in processing, for
instance, in the production of white bread, and may even be
lost during cooking, as with some heat-sensitive vitamins.
In view of the established generally positive effect of
fertilizer use on food quality, it is surprising that
certain groups of consumers in the developed countries are
requesting so-called "natural" food, in the sense of food
produced not only without chemical plant protection, but
also without the use of synthetic mineral fertilizers (quite
apart from food additives such as preservatives and
colourants).
A special market has been developed for such products of
"organic farming" using either organic manure alone or
together with "natural" mineral fertilizer such as rock
phosphate. This is fully acceptable so long as scientific
principles are observed and no unfounded claims of superior
quality are made.
The writer zacheema@pakwatan.com is Research Officer, Weed
Science-Allelopathy Research Laboratory, Department of
Agronomy, University of Agriculture, Faisalabad
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