Excess fertilizer addition may reduce crop yields and pollute environment
BY Dr. Sarath Amarasiri-September 23, 2018, 12:00 pm
Fertilizer recommendations for vegetables in Sri Lanka have been made by the Department of Agriculture from the results obtained from research carried out by them in experiment stations and in farmers’ fields across the country. Likewise, the Tea Research Institute has recommended fertilizers for tea based on their research. The tea estates followed these recommendations and consequently there is no build up of plant nutrients in the tea lands. On the other hand, recent research carried out by the Department of Agriculture and the universities have shown that many vegetable growing farmers in Nuwara Eliya have been adding chemical fertilizers in amounts higher than the recommendations of the Department of Agriculture during the last thirty years. Some farmers have added more than seven to eight times the amount of fertilizers recommended by the Department of Agriculture.
The three major fertilizers imported by Sri Lanka are urea, triple superphosphate (TSP) and muriate of potash. They contain the plant nutrients nitrogen, phosphorus (P) and potassium respectively. This note discusses the harm brought about by adding large quantities of phosphate fertilizer to vegetables in the Nuwara Eliya District.
The direct evidence that farmers add excess fertilizers in Nuwara Eliya comes from the chemical analysis of their soils. While the soil phosphorus content (Olsen method) in uncultivated lands adjoining farm lands is about 12 parts per million (ppm), recent soil analyses by the Sita Eliya Agriculture Research Station have shown that the average soil test value in the region is about 100 ppm, In fact there are some farm soils having values in excess of 400 ppm. Such high values for soil phosphorus are hardly reported from other countries.
The effect of adding increasing amounts of phosphate fertilizer to vegetable crops in Sri Lanka has not been reported. Such studies have been carried out at the Levin Horticultural Research Station in New Zealand on a large number of crops that include beetroot, cabbage, carrot, lettuce, onion, pea, potato and sweet corn. The results of the field experiments are found in the New Zealand Journal of Experimental Agriculture Vol 16:1, 83-90, 1988. All the crops showed an initial increase in yield, thereafter a flattening out of the yield and finally a decrease after the soil phosphorus content increased beyond about 50 ppm as shown in the figure below
If these results are applicable to Nuwara Eliya, where a large number of farms exceed 50 ppm P, the continued addition of high levels of P would decrease the crop yields. This could be disastrous for potato cultivation and perhaps for all other crops.
Intensively cultivated vegetables are grown in many parts of the country totaling about 90,000 hectares. They include Badulla, Matale, Colombo, Gampaha, Kalutara and Kurunegala Districts. Many vegetable farmers use excess fertilizer no matter the location. There is evidence that some of these soils are also having high values of phosphorus. If unchecked, the problem at Nuwara Eliya may arise in other parts of Sri Lanka as well in due course.
Furthermore, when the soil P value exceeds about 60 ppm P much of the added phosphorus is removed from the soil when it comes into contact with water and goes to increase the phosphorus content of rivers, reservoirs and other waterbodies. High P values in water can lead to eutrophication and cause algal blooms. Two years ago there were 742 farms in Nuwara Eliya that exceeded 60 ppm P. This number may be much more today. About twenty reservoirs in Sri Lanka have had algal blooms including Kalawewa, Parakrama Samudraya, Ulhitiyawa, Kotmale and Maduru Oya. In June this year Rajangana reservoir had an algal bloom. Formation of excessive algal growth in reservoirs began to occur about twenty years ago.
There are reports of formation of algal blooms from toxic species that have led to a breakdown of drinking water supplies in Ohio in USA in 2014. Several Sri Lankan research institutes and universities have found the presence of toxic algal species in our reservoirs. Since algae can move from one reservoir to another, presence of toxic algal species in the NCP can become a national disaster.
If P in water is controlled below hazardous levels there will be no algal formation and therefore no toxic algae.
Concluding remarks
It is a fact that vegetable farmers in Nuwara Eliya add excess phosphate fertilzers to the extent that could decrease crop yields and damage the environment. It is also a waste of money for the farmers and a loss of foreign exchange to the country. What happens at Nuwara Eliya can happen in vegetable cultivation in other areas too.
If correct amounts of fertilizer are added there will be no excess P in the soil, as in tea growing lands, and no excess P in the reservoirs and therefore, no algal formation.
One of the reasons for this debacle is the marketing of P containing fertilizers for surface application by the private sector.
Excess fertilizer was added by farmers and no one else. Didn’t they know that they are doing the wrong thing? Didn’t the government authorities in particular the extension workers bring this matter to their attention. Why has the government continued to allow the private sector to market environment damaging fertilizer mixtures ?
In a letter to the District Secretary of Nuwara Eliya dated 29th June 2017, I brought this matter to his attention. I said in the letter:
"To make matters worse, fertilizer dealers in Nuwara Eliya are today marketing 50 kg fertilizer bags containing all three nutrients nitrogen, phosphorus and potassium, an example being the 9-18-12 and 16-7-14 fertilizer mixtures. These numbers are the percentages of nitrogen, phosphorus and potassium respectively of the fertilizer in the bag. Even the farmer who has 490 ppm soil P mentioned above, who does not require any more phosphorus for his land will be further polluting the environment by adding such fertilizers. It is my view that such fertilizer mixtures should not be sold in Nuwara Eliya".
Fifteen months have passed and today such fertilizer mixtures containing phosphorus are still available in the market.
Farmers should get their soil tested once in four to five years and add fertilizer based on the soil test result. This is a legal requirement in some countries.
It is my view that the government should pay much more attention to the problem of excess fertilizer use to prevent loss to the farmer, loss to the country and destruction of the environment.
(The writer is former Director General of the Department of Agriculture)
