HEALTHY soil is a combination of minerals, rock, water, air, organic matter (plant and animal residue), micro-organisms, including bacteria, fungi, protozoa and a variety of insects and worms. In healthy soil this intricate web carries out a process that continually replenishes the soil and maintains long-term soil fertility.
Author: Abdul Waheed, A. Hannan & Dr A.M.Ranjha
Before the use of chemical fertilisers, the earth and animals worked together to enhance fertility of the soil. Through the decomposition of raw, natural proteins such as bone, blood, fish and feathers, soil received the nutrients needed to maximise fertility. When used with reference to fertilisers, the word organic generally means that the nutrients contained in the product are derived solely from the remains or are a by-product of an organism. Cottonseed meal, blood meal, fish emulsion, manure and sewage sludge are examples of organic fertilisers. Urea is a synthetic organic fertiliser.
There has been much controversy over organic versus inorganic fertilisers. It is important to realise that plants do not differentiate between organic and inorganic fertilisers. Their tiny root hairs can absorb only nutrients that have been broken down into inorganic, water-soluble forms. It makes no difference to your tomato plant if the atom of nitrogen it is absorbing has come from a compost pile or a fertiliser factory. There are, however, advantages and disadvantages to each form of fertiliser, organic and inorganic. The primary advantage of using packaged commercial fertiliser is that nutrients are immediately available to plants. As well, the exact amounts of a given element can be calculated and given to plants.
Commercial fertiliser, especially nitrogen, is easily washed below the level of the plant’s root system through leaching of rain or irrigation. An application which is too heavy or too close to the roots of plants may cause “burning” (actually a process of desiccation by chemical salts in fertiliser). As well, heavy applications of commercial fertilisers can build up toxic concentrations of salts in the soil, thus creating chemical imbalances. If organic materials are readily available and cheap, the expense of the commercial fertiliser should also be considered.
There is less danger of over-fertilisation by adding decomposed organic material to a garden. It provides a slow release of nutrients as micro-organisms in the soil break the organic material down into an inorganic, water soluble form which the plants can use. The addition of organic material improves soil structure or “workability” immensely. It also vastly improves the water-holding capacities of sandy soils, a distinct advantage in arid climates such as ours.
Organic fertiliser is not immediately available to plants. As noted above, this “slow- release” feature can be an advantage. However, if there is an immediate need for nutrients, organic fertiliser cannot supply them in a hurry. Furthermore, information on the amount of nutrients and the exact elements in an organic fertiliser such as manure is not readily available to the home gardener. In contrast, when you apply manufactured inorganic fertiliser you know the kinds and amounts of the elements it contains, and this allows you to be more precise in meeting a plant’s nutritional needs.
The possibility of nitrogen depletion is another drawback of organic fertilisers. Because of complex bacterial action, the addition of a large amount of organic material can cause a temporary nitrogen depletion in the soil and therefore in plants.
Compared to synthetic fertiliser formulations, organic fertilisers contain relatively low concentrations of actual nutrients, but they perform important functions which the synthetic formulations do not. They increase the organic content and consequently the water-holding capacity of the soil. They improve the physical structure of the soil which allows more air to get to plant roots. Where organic sources are used for fertiliser, bacterial and fungal activity increases in the soil. Mycorrhizal fungi, which make other nutrients more available to plants, thrive in soil where the organic matter content is high. Organically derived plant nutrients are slow to leach from the soil making them less likely to contribute to water pollution than synthetic fertilisers.
Some organic fertilisers are high in one of the three major nutrients (nitrogen, phosphorus, or potash,) but low or zero in the other two. Some are low in all three macronutrients. A few organic products can be purchased “fortified” for a higher nutrient analysis. The ingredients used to fortify organic fertilisers are organic materials; for example, rock phosphate to increase phosphorus, or greensand to increase potash.
Organic fertilisers depend on soil organisms to break them down to release nutrients; therefore, most are effective only when soil is moist and warm enough for the micro-organisms to be active. Nutrient release by microbial activity, in general, occurs over a fairly long time period. One potential drawback is that the organic fertiliser may not release enough of their principal nutrient when the plant needs it for growth.
Cottonseed meal is a by-product of cotton manufacturing. As a fertiliser, it produces a somewhat acidic reaction. Consequently, it is frequently used for fertilising acid-loving plants such as azaleas, camellias, and rhododendrons. Formulas vary slightly, but generally, cottonseed meal contains seven per cent nitrogen, three per cent phosphorus, and two per cent potash. Nutrients are most readily available to plants in warm soils, but there is little danger of burn.
Blood meal is dried, powdered blood collected from cattle slaughterhouses. It is a rich source of nitrogen, so rich, in fact, that it may burn plants if used in excess. Gardeners must be careful not to exceed the recommended amount suggested on the label. In addition to nitrogen, blood meal supplies some essential trace elements, including iron.
Fish emulsion, a balanced, organic fertiliser, is a partially decomposed blend of finely pulverised fish. A strong odour is associated with most brands of fish emulsion fertiliser, but the smell dissipates within a day or two. Recently, deodorised brands have been developed. Fish emulsion is high in nitrogen and is a source of several trace elements. Contrary to popular belief, too strong a solution can burn plants, particularly those growing in containers. In the late spring, when garden plants have sprouted, an application of fish emulsion followed by a deep watering will boost the plants’ early growth spurt.
Manure is a complete fertiliser, but low in amount of nutrients it supplies. Manures vary in nutrient content according to animal source and what the animal has been eating. A fertilizer ratio of 1-1-1 is typical. Commonly available manures include horse, cow, chicken and sheep (one should avoid using pig, dog or cat faeces because of the problems involved with internal parasitic worms which may be transferred to human beings).
The highest nutritional concentration is found in manure when it is fresh. As it is old, exposed to weather, or composted, nutrient content is reduced. However, most gardeners prefer to use composted forms of manure to ensure lesser amounts of salts, thereby reducing the chance of burning plant roots. Because of its low concentration of plant nutrients, manure is best used as a soil conditioner instead of a fertiliser. Typical rates of manure applications vary from a moderate 70 pounds per 1000 square feet to as much as one ton per 1000 square feet.
Sewage sludge is a recycled product of municipal sewage treatment plants. Two forms are commonly available—activated and composted. Activated sludge has higher concentrations of nutrients (approximately 6-3-0) than composted sludge. It is usually sold in a dry, granular form for use as a general purpose, long lasting, non-burning fertiliser. Composted sludge is used primarily as a soil amendment and has a lower nutrient content (approximately 1-2-0).
There are some questions about the long-term effects of using sewage sludge products in the garden, particularly around edible crops. Heavy metals such as cadmium, sometimes present in the sludge, may build up in the soil. Possible negative effects vary with the origin of the sludge and with the characteristics of the soil where it is used.
When packaged as fertilisers, organic products have the fertiliser ratio stated on the package label. Some organic materials, particularly composted manures and sludge, are sold as soil conditioners and do not have a nutrient guarantee stated on the package, although small amounts of nutrients are present.
Whereas positive effects of use of fertiliser on environment are often overlooked, attention now a days is focussed on negative aspects. Mineral and organic fertilisers are considered harmful.
Mineral and organic fertilisers accumulate dangerous or even toxic substances in soil from their constituents, e.g. Cd from mineral phosphate fertilisers or from town or industrial waste products;
They cause eutrophication of surface water, with its negative effect on oxygen supply (damaging fish and other forms of animal life);
Nitrate accumulation in ground water, thus diminishing the quality of drinking water;
They cause unwanted enrichment of atmosphere with ammonia from organic manures and mineral fertilisers, and with N2O from de-nitrification of excessive or wrongly placed nitrogenous fertiliser.
As to contamination of soils with toxic heavy metals, it can easily be shown that mineral fertilisers make only a rather small contribution in comparison with, for example, town wastes. 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.