Modern agricultural practices require the use of large amounts of fertilizers and other soil additives. Some of these are washed off the lands through irrigation, rainfall and drainage, into rivers and streams where they can seriously disturb the aquatic ecosystem. Depletion of dissolved oxygen caused by excessive algal growths can spell disaster or death to fish and other aquatic biota. Recent observations have indicated that excessive and indiscriminate application of inorganic fertilizers often leads to accumulation of nitrates in water. When such waters are drunk by cattle (or humans), these nitrates are reduced to the toxic nitrites by intestinal bacteria. Nitrites can cause a serious disease known as methaemoglobinemia which can inflict serious damage to respiratory and vascular systems and may even cause suffocation. Of course, when used judiciously, fertilizers certainly help increase agricultural yields. But indiscriminate and excessive use can also have serious and adverse ecological consequences, especially on aquatic ecosystems and ground water resources.
Since the world’s ecosystems form a sort of continuous and interlinked network, the materials lost by one ecosystem may spell a gain for its neighbours. This is especially true for fertilizers which, when applied in excess, leach out from crop fields into water bodies, thereby affecting the down-stream aquatic life. One important ecological consideration in the use of fertilizers is the question whether or not a given ecosystem should at all be fertilized in order to transform or modify it. It is now being generally realized that agriculture may be intensified only in such a manner as to ensure increased resources and long term socio-economic benefits. Short term planning will only lead to more or less irreparable and long term damage to the environment. Known beneficial effects of fertilizer use on ecosystems include increases in food production, improvements of soils in temperate areas, checking of soil erosion, conservation of soil and water, and enhancement in water use efficiency of crops.
Some adverse effects of fertilizer use are: (a) changes in mobility status of nutrients in soils, (b) deterioration of water resources caused by eutrophication, (c) stimulation of weed growth in crop fields, and (d) disturbance in the ionic balance and equilibrium in soils, often leading to high acidity, nutritional imbalance, shortages of certain trace elements, and molybdenum or selenium toxicity. Even though nitrogen is one of the most highly mobile elements, excessive applications of nitrogenous fertilizers to soils can lead to its accumulation to such a stage that the plants begin to absorb excess amounts and even then some of the excess amounts present in the soil get leaked off through the soil into groundwater or into streams and springs. On the other hand, the virtually immobile phosphorus tends to accumulate in the surface soil and very little of it is carried away by runoff. Certain scavenger plants such as Lucerne and elephant grass have deep root systems which enable them to trap, retrieve and recycle some of the nitrogen leached beyond the shallower root systems of most crop plants.
One of the major lacunae in our knowledge concerns the effects of fertilizer use on the structure and composition of plant, animal and microbial communities in natural ecosystems. Unfortunately, some of the most serious cases of polluted waters are those that cannot be seen or smelled. The water may outwardly appear quite clean and yet it may be seriously polluted. Even in some of the domestic drinking water supplies the organic matter content may still be sufficiently high to exert a protecting effect (against chlorination) on water-borne pathogenic organisms.
Similarly, even though the municipalities and water works engineers may filter the raw waters before supplying them for drinking purposes, there are several kinds of industrial-chemicals which cannot be removed through filtration. The concentrations of some of these industrial or agricultural chemicals have now reached dangerous levels in most rivers, lakes and oceans, and they have also contaminated the ground waters by seepage and diffusion. Ironically, their potential or actual toxic effects, both immediate and delayed, have, in most cases, not so far been fully determined.