Organic matter in soils; a buffer against harmful effects

Chamal Rajapaksa

FERTILISER: Organic fertiliser is plant or animal matter that is gone far into decay. All organic fertilisers have plant or animal origins and; therefore, are safe and environment friendly when used in agriculture. The use of organic matter is as old as agriculture itself, but inorganic fertiliser is only a few decades old.

When inorganic fertiliser became common in agriculture, people realised that it had many advantages over organic fertilisers. Inorganic fertilisers have very high nutrient contents that are soluble and readily available to plants, causing them to respond very rapidly.

Inorganic fertilisers need to be applied in comparatively small quantities due to their high nutrient contents; therefore, their application becomes easy. Small quantities can also be transported and stored with ease, while organic fertilisers are needed in large quantities.

Because of these advantages, farmers became overly dependent on inorganic fertilisers and the long-term and cumulative benefits of using organic matter, fell by the wayside.

However, the inorganic fertilisers cannot in any way help to create the physical conditions of the soil (physical fertility) that are equally important, if not more important, than supplying the chemical nutrients to the soil.

This is not as much as to say that the role of inorganic fertilisers is less important, but organic fertilisers while adding comparatively small quantities of chemical nutrients, impart many other benefits to the soil that help it to preserve its physical structure and physical fertility without allowing it to deteriorate with time.

Soil fertility is the sum total of its physical, chemical and biological properties. Physical fertility takes into account its texture, structure, water holding capacity, aeration and soil depth. If these factors are favourable, then conditions exist for proper plant growth. Lack of physical fertility precludes high crop yields and the physical fertility of a soil can only be improved with the addition of organic fertilisers.

The capacity to hold the essential nutrient elements of the soil, and the soil reaction can be considered the most important chemical properties of a soil. While the clay minerals present in a soil help to keep the nutrients in place, the humus produced by decaying organic matter also helps the soil to hold the nutrients present in the soil or that are added to the soil by way of fertilisers.

Organic matter makes the nutrients change into forms that are readily available to plants. Inorganic fertilisers cannot improve the nutrient holding capacity of the soil or the nutrient availability to plants.

The soil reaction determines the amount of plant nutrients that are present in the soil in soluble form. The soils of the wet and intermediate zones of Sri Lanka are mostly acidic. Adding inorganic fertilisers to such soils, season after season, makes them more acidic. On the other hand adding organic matter enables the soil to keep the soil reaction from changing because organic matter acts as a buffer against such changes.

Even though, the soil has a store of almost all the necessary plant nutrients, it cannot supply all the nutrients needed by plants all the time; therefore, it becomes necessary to supplement the soil with plant nutrients in the form of fertilisers. Inorganic fertilisers are very effective in supplying plant nutrients but several forms of inorganic fertilisers are required to supply all the nutrients needed by plants.

Organic fertilisers by contrast have virtually all such nutrients, but their amounts are relatively low. Because of this the major nutrients quickly needed by plants such as nitrogen, phosphorus and potassium have to be supplied in the form of inorganic fertilisers.

However, nutrients of secondary importance and micro-nutrients required by plants in very small amounts are best supplied through the use of organic fertilisers. In local agricultural practices, secondary nutrients and trace elements are not normally supplied as inorganic fertilisers and; therefore, applying organic fertilisers becomes important.

it is also important to develop the micro-biological fertility of the soil. Various reactions taking place in the soil are often government by micro-biological activity. Since the soil micro-organisms feed on organic matter, enriching the soil with organic fertilizers will also enrich the soil's microbial activity.

As well as microbial activity, activities of large organisms such as earth worms increase soil fertility. Earth worms thrive in soils well supplied with organic matter. Inorganic fertilisers do not contribute to increasing the populations of soil organisms.

Organic matter also reduces the incidence of plant parasitic nematodes. Poultry manure, particularly that of layers has been found most suitable to control root-knot nematodes in various crops. Since organic fertilisers increase the vigour of plants, they are better able to resist diseases and insect pests.

Low organic matter content, high acidity, low fertility, low phosphorus content and iron toxicity are the major problems found in Sri Lanka. These problems have arisen in soils due to farming over long periods without using organic fertiliser.

Adding liberal amounts of organic fertiliser to farmland should prevent such problems from occurring. It is well-known that crops cultivated with organic fertiliser have produce of a higher quality and freshness than in crops cultivated with inorganic fertilisers.

Vegetables such as cabbage, tomato and okra treated with organic fertiliser have vitality, freshness and an attractive appearance that is appealing to consumers. Such vegetables also keep their freshness over longer periods.

Seeds of rice cultivated with organic matter are preferred as seed because they tend to be fuller and more attractive than ordinary seeds. Organic matter treated food crops are also known to have a high nutritive value.

When applying organic fertiliser to seasonal crops, it has to be applied as a basal dressing because organic matter should have enough time to decay and be available to crops before they complete their life in a few months.

Perennial crops should receive organic fertilisers about twice a year in recommended quantities. When adding organic fertiliser to perennial crops, care should be taken to incorporate the fertiliser into the soil thoroughly.

The amounts of organic matter that are recommended for various crops are as follows: On a per hectare basis, rice requires about 1-2 metric tons, lowland vegetables about 4-5 metric tons and upland vegetables about 10-15 metric tons while subsidiary food crops about 2-4 metric tons.

These amounts have to be added every season. In farming systems where only organic fertilisers are used (for organically farmed food) enough organic fertilisers have to be applied so that plants do not become short of the necessary nutrients.

If a quick plant response is needed, well rotted organic matter has to be applied to the soil. When rice fields are fertilised with new straw, it has to be done 3-4 weeks before sowing or transplanting. Such early applications will prevent the yellowing of rice seedlings.

When applying organic fertilisers like cattle manure containing viable weed seeds, care must be taken to prevent the weed seeds from germinating in the crop. This can be done by treating the manure with a urea solution before heaping it and covering the heap with black polythene.

Before applying goat manure, the pellets must be first crushed before applying to the field or else the pellets will remain for long periods without decaying.

After applying poultry manure, the soil must be thoroughly wetted for 2-3 days in the absence of rain. This procedure will wash out the chemicals present in the manure that cause the tender leaves of plants to get scorched. The same precaution must be adopted when bat guano is used.

However, all in all, it is always important to supplement organic fertiliser applications with recommended amounts of inorganic fertilisers for best results. Combined applications of both types of fertilisers will result in a number of economic advantages.

Such fertiliser practices are very common in intensively cultivated vegetable and potato systems of the upcountry areas such as Nuwara Eliya, Bandarawela and Welimada. In these regions farmers always add about 10-15 tons of cattle manure per acre before planting potato or vegetables.

Thereafter, they add top dressings of inorganic fertiliser according to recommendations and end up with very high yields. The quality of their produce is also very high, particularly of their leeks, carrots, cabbages and beets.

In areas where organic and inorganic fertilisers have been used over long periods, considerably high residual plant nutrient contents are found in the soils. For such soils the full amount of organic fertiliser and half the recommended amount of inorganic fertiliser is sufficient to grow good crops.

In areas where soil fertility is poor, full recommended amounts of both kinds of fertilisers have to be applied. As well as organic fertiliser, application of inorganic fertiliser to crops in proper amounts is known by the euphemistic term of 'Integrated Plant Nutrition Management'.

Because poultry manure lowers the soil acidity, there is no need to add lime or dolomite for the same purpose. This is particularly important in potato crops grown in the upcountry regions where soils tend to be acidic.

If poultry manure is added to such soils, applications of lime or dolomite on top of it will result in the potato crops becoming more susceptible to the scab disease due to the soil becoming less acidic or even alkaline.

The writer is Minister of Agriculture Development.