Excess phosphate fertilizer, key cause of algal blooms


Dr. Sarath Amarasiri and Mr. Upali Yapa

High phosphorus in waterbodies may lead to algal blooms and make the water unsuitable for drinking .

An algal bloom in a water body in southern Sri Lanka

The writers are the former Director General of Agriculture and former soil scientist, Department of Agriculture respectively.

If extremely large quantities of phosphate fertilizer continue to be added to the farmlands of Sri Lanka, the quality of the drinking water in the country may be affected adversely in the not too distant future owing to eutrophication.

The word eutrophication which came into common usage in environmental circles in the early 1960s, is derived from the Greek words eu and trophe meaning well nourished. It may be defined as the nutrient enrichment process of a water body that can lead to unusually high growth of aquatic biota, an example being algae. Increase in the concentration of phosphorus (P) in many waterbodies has led to algal blooms in diverse parts of the world, mostly during the last 50 years. Excessive growth of algae may discolour the water, make washing, bathing, swimming, and other recreational activities not possible, hinder navigation, reduce significantly the visits of birds, prevent wallowing of buffalo, lower the levels of oxygen in the water, lead to fish kills, bring about unpleasant smells and produce algal toxins that are highly poisonous and make water unsuitable for human drinking even after boiling.

Under natural conditions plant nutrients enter a water body mostly through weathering of rocks, dissolution of sparingly soluble soil minerals and rain. However, nutrient flow to a water body from these pathways is very small, the effects hardly noticeable, and eutrophication extremely scarce.

Algal blooms began to appear on a macro scale in the 1960s mainly owing to the entry of large quantities of phosphates to waterbodies through detergents and fertilizers. The calamity of the pollution of Lake Erie, and in particular, the catching fire of the river Cuyahoga, one of the dozens of rivers bringing water to the lake, on June 22, 1969, resulted in a social uproar in Canada that triggered the withdrawal of phosphates from laundry detergents in North America and Europe. Many countries began replacing phosphates in detergents with Zeolite-A, an aluminium silicate that does not contain phosphorus. With phosphorus in detergents gradually moving out of the picture, many environmental scientists have now identified fertilizer P as the major cause of eutrophication worldwide.

Triple superphosphate (TSP), chemically known as mono calcium phosphate is a water soluble phosphate fertilizer used widely for rice and for other short term crops such as potato, tomato, cabbage, maize, chilli and onion in Sri Lanka. When TSP is added to the soil it will react with it to form a number of less soluble compounds. The surface soil enriched with phosphorus from added fertilizer can move out of the land by erosion, enter waterbodies, sink to the bottom and release much of its phosphorus to the water under the anaerobic chemistry in the depths. The soil surface runoff resulting from rain or irrigation also carries phosphorus to waterbodies. It can be said that excess fertilizer addition could convert waterbodies that were oligotrophic (low P) for hundreds of thousands of years to eutrophic (high P) status in less than a decade as has happened in a number of countries.

Soil scientists during the past 60 years have developed more than half a dozen soil tests to assess the P fertility status of a soil. The test used most commonly in the world today and in current use in Sri Lanka for short term crops, is that developed in 1954 by Dr. Sterling Olsen, a US soil scientist.

After years of research soil scientists have arrived at what is termed an agronomic soil critical P value for a particular crop-soil-climatological combination that is used as a guideline in determining the amount of fertilizer to be added to a crop. The Department of Agriculture (DOA) has set this value at 30 ppm Olsen P for potato. Corresponding values in other countries are; 20 to 30 in India, 20 in Manitoba (Canada), 26 in England, 30 ppm in New Mexico (USA) and 40 ppm in Ireland. Addition of P fertilizer to lands that have higher P content than the agronomic critical value is not likely to be profitable.

While the recommendation of DOA for potato is only about 70 kg TSP for lands which have a history of fertilization, a vast majority of farmers currently add nearly 850 kg per hectare of this fertilizer. As the potato crop removes only a small fraction of the phosphorus added, a soil phosphorus build-up results. This situation applies to cultivation of other short duration high yielding crops as well.

The Department of Agriculture has analysed more than 1500 soil samples from farmers’ fields in Nuwera Eliya during the period 2003-2008. About 70% of the soil samples had more than the agronomic critical P value. In an analysis of 100 soil samples in potato growing areas in Badulla district, 83% exceeded the critical value.

As the soil P value increases, the rate of P loss to waterbodies also increases. In order to protect waterbodies from high concentrations of P, some countries have introduced an Environmental soil P critical value that should not be exceeded. This stands provisionally at 60 ppm Olsen P.

In the survey of soil phosphorus in the farmers’ fields of Nuwara Eliya referred to above, nearly half of them had soil P values exceeding the environment limit of 60 ppm P. Some farms have soil P values exceeding 300 ppm P. Such high values for soil phosphorus have hardly been reported from other countries.

Some countries have taken legal steps to prevent phosphorus build up in soil. They include mandatory soil P analyses of soils from farmers’ fields for phosphorus content from accredited laboratories at least once in five years, filing a five-year nutrient management plan that has to be approved by relevant government authorities and limiting amounts of phosphorus to be added based on the soil test.

The water flowing from the waterbodies of Nuwara Eliya and Badulla districts carrying high levels of P could travel all the way to the irrigation reservoirs in the Mannar, Anuradhapura and Polonnaruwa districts . Algal blooms have been reported occasionally in reservoirs at Kotmale, Kandalama, Nachchaduwa, Nuwerawewa, Tissawewa, and at Parakrama Samudraya during the last few years.

On January 26, an algal bloom occurred in Ulhitiya and Rathkinda reservoirs. This is probably the first such occurrence reported from there. The phosphorus content of the water was 0.18 ppm. According to the European Union Joint Research Centre Classification made in 2002, the Ulhitiya – Rathkinda water is hypertrophic, meaning highly nutrient polluted. It is reported that many of the other irrigation reservoirs are also hypertrophic, a situation that has arisen in recent years.

If excessive additions of TSP are continued in the Nuwera Eliya farmlands where 50% of the lands are already at P levels beyond the environmental critical limit, and if such poor agronomy is practiced in other parts of Sri Lanka as well,

it would be a matter of time for some of our waterbodies to be filled with algae on a regular basis. There are also disturbing reports that some of the Anuradhapura reservoirs have had algal species that can produce toxins which are poisonous. With water from one reservoir flowing to another in Sri Lanka, spread of such algae to a large number of reservoirs could turn out to be a national disaster.

The challenge to Sri Lanka is clear. No soil shall exceed the environmental critical limit of phosphorus. No waterbody shall reach Eutrophic status.

The government should take immediate action to arrest the excessive addition of phosphate fertilizer in order to prevent eutrophication of the waterbodies. Following action is suggested to protect the waterbodies of Sri Lanka from excess additions of phosphate fertilizer.

1. Make the general public aware of the problem of eutrophication.

2. Initiate programmes to monitor the phosphorus content of farmlands and waterbodies immediately

3. Make farmers aware of the reasons for establishing agronomic and environmental critical phosphorus limits for soil in terms that they can understand, and prevail on them that addition of excess phosphorus does not increase their profits.

4. Make farmers aware of their social responsibility not to over fertilize and endanger the health of people living hundreds of miles away.

5. Formulate laws and regulations to prevent farmlands exceeding the environmental critical soil phosphorus limits.

Dr. Bombastus Paracelsus (1493-1541), founder of the science of pharmacology, Professor of Medicine at University of Basel, Switzerland and well known eccentric in his time, is remembered for his famous saying "All substances are poisons, there is none which is not a poison. The right dosage differentiates a poison and a remedy". Phosphorus is a plant nutrient without which animals, plants, microorganisms and other living beings cannot survive. Excess phosphorus in water is a potential poison.

animated gif
Processing Request
Please Wait...