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.
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 [email protected] is Research Officer, Weed Science-Allelopathy Research Laboratory, Department of Agronomy, University of Agriculture, Faisalabad