By K. Thedchanamoorthy
Senior Lecturer Eastern University, Sri Lanka
Crop yields at global level have constantly increased during the last two decades. The average rice yields in Asian countries have gone up from 1.3t/ha to 2.8t/ha Maize and wheat yields have shown an increase of 45% and 70% respectively. The increase in yield has been primarily facilitated by the introduction of high yielding varieties and use of chemical fertilizers and pesticides. The consumption of fertilizers in the Asian countries have also escalated from 17 million metric tons of NKP nutrients to 59 million metric tons over the period. The chemical fertilizers are found responsible by over 50% for the rise in crop yields. Today, the chemical fertilizers have become the major input for agriculture. Farmers appreciate chemical fertilizer for its quick response, ease of handling, easy access and availability, and comparatively low cost per unit of plant nutrients. Inspite of the impressive yield responses by fertilizers the analysis of recent rice yield data in Sri Lanka indicates that total production leveled during the past decade, which reveals of plateauing in yield linked to the level of fertilizer application. The farmers and the Scientists now begin to recognize some of the limitations created by chemical fertilizers.

The new improved crop varieties are essentially high response varieties, bred to respond to high doses of fertilizers. When they are cultivated under conditions of high level of applied fertilizers; water supply and adequate pest and disease control, the new improved and hybrid crop varieties produce very high yields. But, if these conditions cannot be given, the yield losses are higher than with local varieties.

Experiments reveal that the applied fertilizers are not fully utilized by the crops.

The fertilizer use efficiency has often proved to be lower than expected. In the high land 40-50% of the applied nitrogen is found to be lost. The Nitrogen losses through fertilizers are as high as 60- 70% in irrigated rice. Under unfavourable conditions, such as high rainfall, soil erosion, low soil organic content, the losses are even higher.

Continuous use of only Nitrogen, Phosphorus and Potassium(NPK) fertilizers deplete Macro and Micro nutrients in the soil. In continuously cultivated and fertilizer applied crop fields, deficiency of zinc, iron, copper, magnesium, manganese, and boron has been reported. The deficiency of these nutrients can cause malnutrition not only on the growth and development of crops, but also on the animal and human beings consuming them. In an experiment decline in crop production by the deficiency of micronutrients, caused by heavy application of NPK fertilizers has been reported. It was reported that application of 100kg of NPK fertilizers depleted 433g of Cu and 629g of Zn from the soil. The paddy yield in ancient fields (Purana fields) in Sri Lanka was found to decline due to deficiencies of micro nutrients caused by continuous application of NPK fertilizers. In general, only Major nutrients (N,P and K) are applied as fertilizers. The use of Macro and Micro nutrients are very much limited due to cost and other factors such as toxicity at high levels of application. Experiments show that the Imbalance of NPK nutrients in the absence of others affect the growth, yield and quality of crops.

Countries like Sri Lanka do not produce the required fertilizers and solely depended upon importations. At the farm level this will mean rising in price and at the national level draining of foreign exchange. Besides, escalation in the price of fertilizers is associated with the price increase of petroleum products, which is common year after year.

Some fertilizers, especially phosphate fertilizers contain toxic elements such as cadmium, copper and uranium. Long use of these fertilizers may cause toxicity in soil and in animal and human beings. In 1960, a disease called ‘itai- itai’ affecting people in Japan was reported and the causal agent was found to be cadmium which came through phosphorus fertilizers applied to paddy crops.

Phosphatic fertilizers are mostly obtained from non renewable resources such as mineral rocks which may be fully exhausted and become non available like petroleum oil after some years.

Fertilizers have been known to destroy soil fertility. They accelerate decomposition of organic matter, leading to degradation of soil structure, and higher vulnerability to drought by reduced water holding capacity of the soil. Acid type fertilizers such as ammonium sulphate may decrease the soil pH and lower the availability of some plant nutrients and increase vulnerability to some disease attack.

Apart from limitation to crop production, the chemical fertilizers can also create problems in the environment: Only a smaller portion of the applied fertilizers are utilized, by the crop and a greater portion of it is lost by way of leaching, volatilization, immobilization and other processes. The leaching out fertilizers, especially nitrogen fertilizers get their way into water bodies through drainage and into drinking water wells. This may lead to increased levels of nitrates in drinking water. High levels of nitrates (over 45 miligram/litre) causes toxicity problems in human beings, such as urinary problems and still births (birth of blue babies). It is reported that some irrigation wells in Jaffna have nitrate levels 4 times that of safety level. Fertilizers also contribute to global risk arising from the release of nitrous oxide (N2O) to the atmosphere which inturn depletes the ozone layer and causing green house effect.

Accumulation of nitrates in water bodies causes deleterious effects on the growth and development of some living organisms in the water body, leading to imbalance in the eco-system. Phosphorus fertilizers reaching the water bodies cause eutrophication which leads to algal blooming, reduction in oxygen level and toxicity problems. Because of this, the big-diversity in the eco system could be greatly disturbed.

Some beneficial organisms in the soil such as earth worms, nitrogen fixing bacteria, phosphorus solubilizing bacteria, Micorhiza etc can also be affected by chemical fertilizers.

In general, increased levels of applied fertilizers and continuous application affect physical, biological and chemical fertility of the soil.

In the long run, continuous application of heavy doses of fertilizers can make soil lose their potentiality to sustain crop production. Judicious application of fertilizers, based on soil test values, suitable agronomic practices to enhance fertilizer use efficiency, adequate incorporation of organic manures are some of the alternative measures to be promoted to take chemical fertilizers on the correct path.