by Professor W. A. J. M. De Costa
Senior Professor and Chair of Crop Science Department of Crop Science, Faculty of Agriculture University of Peradeniya
Why use fertiliser on crops?
Fertilisers are used for two purposes.
One purpose is to provide essential plant nutrients that are required for crops to produce an economically-important product (i. e. food for humans, feed for animals, a variety of industrial products, etc.). Just as people require food, crops require nutrients for producing what is expected from them.
When a crop is harvested and its yield taken away, a large amount of nutrients is taken out of the system (i. e. the soil). Therefore, continuous cropping of a land leads to the depletion of nutrients in the soil. Application of fertilisers to such a soil replenishes its nutrient pool and makes continuous cropping possible. This is the second purpose of using fertilisers.
A natural ecosystem like a forest does not require an external input such as fertiliser because nutrients are not taken out of the system. Nutrients in dead leaves, branches, trunks and roots are recycled back to the soil. It is a ‘closed’ nutrient cycle, as opposed to the ‘open’ system in an agricultural crop.
Inorganic vs organic fertilzers
Inorganic fertilisers (normally called chemical fertilisers) contain nutrients in a concentrated form (i.e. fraction of the nutrient in a unit weight of the fertiliser is high). They are produced via industrial processes or by refining mined minerals containing the nutrient. Three major plant nutrients, viz. nitrogen, phosphorus and potassium are supplied as inorganic fertilisers, either individually (‘straight fertilisers’) or in a mixture (‘compound fertilisers’).
Organic fertilisers (organic manures) are raw materials of plant, animal or human origin. When applied to the soil, they decompose and release their nutrients. In comparison to inorganic fertilisers, the fraction of nutrients in a unit weight of organic manure is much lower. Therefore, to give a crop/soil the same amount of a nutrient, a much greater quantity of organic manure than inorganic fertiliser has to be applied. All organic fertilizers are ‘compound fertilisers’ in the sense that they contain a mixture of nutrients though in a diluted form.
When applied to the soil, the inorganic fertilizers release their nutrients quickly. In recent times, nano-scale materials have been used to slow down the release of nutrients from inorganic fertilisers (i.e. called ‘nano-coated slow-release fertilisers’). When applied to the soil, organic fertilisers release their nutrients slowly, because the organic raw material has to decompose to release its nutrients. Natural decomposition is done by naturally-occurring soil microorganisms. Formulations of microorganisms are used to accelerate decomposition and nutrient release from organic fertilisers.
Why ‘modern’ agriculture uses large quantities of inorganic fertiliser?
Global population currently stands at ca. 7.7 billion and is projected to reach 8.5 billion by 2030 and 9.7 billion in 20501. Land area suitable for growing crops is shrinking continuously because of a variety of reasons. Some of the productive lands are lost for urbanisation (i.e. population pressure) while some are converted to alternative non-agricultural uses (e.g. industrial purposes). On the other hand, a portion of lands available for crop production is gradually, but continuously, lost because they become unproductive and economically non-viable due to climate change (e.g. temperatures becoming too warm, rainfall becoming insufficient, etc.) and soil degradation (e.g. loss of fertile top soil due to erosion, loss of soil fertility due to continuous cropping and removal of nutrients without adequate replenishment, development soil problems such as salinity, acidity and accumulation of toxic material).
Increasing population and decreasing arable land area means that we are continuously challenged to increase crop yields per unit land area (usually called ‘crop productivity’) to fulfil the increasing demand for food, feed and the variety of products from agricultural crops. To produce a greater amount of yield from the same unit of land, a crop requires a greater quantity of essential nutrients—there is no such thing as a free lunch in nature— in particular nitrogen (N), phosphorus (P) and potassium (K). A crop has to obtain this increased nutrient requirement either from the soil (which may contain some amount of nutrients naturally) or via fertiliser applied to the soil. Except the soils in virgin lands, soils in the large majority of agricultural lands do not contain naturally the amounts of essential nutrients in quantities required by crops to achieve the productivity levels to meet the continuously increasing demand. Hence, the need to add large quantities of nutrients to the soil. This has to be done every season as most nutrients added during the previous season are removed as crop yield. Because inorganic fertilizer contains nutrients in a concentrated form, the required quantities of the three major nutrients can be supplied with a manageable quantity of inorganic fertiliser. Supplying of the same requirement with organic fertiliser would require substantially larger quantities, which are either not possible to find due to insufficient raw material or difficult to manage. Hence, the widespread use of inorganic fertiliser in commercial agriculture. Organic agriculture where crops are grown exclusively with organic fertilisers represents a small fraction of global agriculture (a very optimistic estimation would put it at < 5%).
Why the drive towards reduction of inorganic fertiliser use in agriculture?
While providing the required amounts of the three major plant nutrients to sustain crop yields to ensure food security and maintain soil nutrients at levels required for continuous cropping, application of inorganic fertilisers has caused adverse environmental and human health impacts.
Because nutrients are released readily from inorganic fertilisers, a considerable fraction of those added to the soil gets leached into groundwater and water bodies (i.e. rivers, lakes, reservoirs etc..). The consumption of water from such polluted sources has been linked to a variety of human health issues.
Inorganic fertilizers have been shown to contain toxic substances (e.g. heavy metals such as lead, arsenic, mercury, etc.) as impurities remaining in them after their mining and industrial manufacturing process. The accumulation of these toxic substances in the soil and water sources has been linked to certain human health issues. However, it should be noted that organic fertilizers, especially those of plant and animal origin, are not entirely free from toxic substances.
Alteration of the soil environment by adding concentrated nutrients alters the naturally-occurring community of soil microorganisms who perform many important functions in the soil to ensure its fertility.
In economic terms, inorganic fertilisers, most of which are produced in industrialised developed countries by multi-national companies, are prohibitively expensive to farmers in the developing countries.
Because of the above reasons, there has been a drive towards reduction of the use of inorganic fertilisers and a part-replacement of them by organic fertilisers. Such movements have begun in developed countries (as well as in some developing countries) since the1980s and gathered momentum during the last two decades. During certain periods, some countries and regions of countries have been forced to produce their crops largely on organic fertiliser because of circumstances (mainly political) (e.g. Cuba, Northern Province of Sri Lanka during the ethnic conflict).
Current situation in Sri Lanka
The present situation in Sri Lanka has arisen following a gazette notification by the government to ban the import of inorganic fertilizer and synthetic agrochemicals (i.e. insecticides, fungicides, herbicides, etc.) with immediate effect. The pollution of the water bodies and perceived links to human health issues, such as the Chronic Kidney Disease of Unknown Aetiology (CKDU) are cited as the reasons for the ban. While there have been a longstanding discussion at many levels of the Sri Lankan society on the role of inorganic fertilizers (and agrochemicals) in causing the above issues and calls for ‘toxin-free food’, the total and immediate ban came ‘out of the blue’ without any consultation (to my knowledge) with any of the relevant stakeholders (e.g. the Department of Agriculture, academia, the plantation sector research institutes, farmer organizations, growers of a wide range of crops or their organizations, private sector organizations in the supply and marketing chain etc.). Apparently, the President/government was acting on the advice of a few university academics (who are either advisors or political appointees as heads of public-sector institutions) and longstanding activists (e.g. Ven. Athuraliya Rathana, Dr. Anuruddha Padeniya et al).
Currently, all relevant public sector institutions have been directed to seek how alternatives to inorganic fertilizer (i.e. organic fertilizer) could be produced and supplied to farmers and growers in adequate quantities required during the Yala season which is already started and beyond. It has been stated in the media that any shortfall for the current season (and probably beyond until adequate quantities can be produced locally) will be provided through imported organic fertiliser. A similar strategy has been proposed for synthetic agrochemicals for which the principal alternative is pesticides of biological origin (i.e. Biopesticides).
Possible impacts of an absence of inorganic fertiliser in Sri Lanka
It is highly likely that in the absence of inorganic fertilisers, the productivity (i. e. economic harvest per unit land area) of some of the major crops in Sri Lanka (e. g. rice and tea), which are crucial to national food security and economy, will decline significantly leading to a decline in the total production (i.e. productivity × cultivated area). At present, Sri Lanka does not have sufficient sources of readily-available organic fertiliser nor does it not have the infrastructure in place to produce organic fertilizers in adequate quantities to fulfil even the minimum nutrient requirement of these two major crops considering the scale on which they are grown.
The prognosis would be the same for a majority of the other annual crops (e.g. cereals, pulses, vegetables, industrial crops, etc.) and floriculture plants (i.e. cut flower and foliage), which are grown on a smaller scale. Some crops such as rubber and coconut may not show an immediate decline in their harvest but will begin to show declines in the medium-term, depending on the existing fertility status of the soils on which they have been established and the overall management status of the plantation and its trees.
Why is Sri Lankan agriculture so reliant on inorganic fertiliser?
The scientific reasons
Soils in Sri Lanka are, by nature, relatively poor in the amounts of essential nutrients (i. e. the three major nutrients, nitrogen, phosphorus, potassium plus magnesium, sulphur and calcium, which are also needed in relatively large quantities) that they make naturally available for crops growing on them. The natural supply of nutrients from a soil comes when the parent material of the soil (i.e. rocks and minerals) undergoes a very slow, gradual decomposition process called ‘weathering’. The plant nutrients are part of the minerals contained in the parent material and are released to the soil when the minerals weather due to the action of rain and other climatic factors such as temperature. Because of the high rainfall and temperature regime associated with the tropical climate in Sri Lanka, its soils have been highly-weathered over a long period of time (over several millennia) so that the existing soil minerals (the source of natural supply of nutrients) are considerably (if not severely) depleted of nutrients. Because of the high rainfall regime (especially in the wet zone and the Central Highlands and to a lesser extent in the dry and intermediate zones), a substantial portion of the nutrients that are released from minerals via the weathering process are leached and lost to the soil, further depleting its natural fertility.
Furthermore, most of the lands on which crops are currently cultivated in all climatic zones of Sri Lanka have been under cultivation for a long period of time. As explained earlier, long-term cultivation of a soil leads to depletion of its nutrient reserves.
Soils in the Central Highlands and those on sloping terrain in other parts of Sri Lanka are further degraded due to soil erosion caused by high-intensity rainfall. Erosion takes away the top layer of the soil and a substantial amount of nutrients naturally available along with it.
Because of the reasons outlined above, neither the grain yield levels of rice that are required to fulfil the annual national demand nor the green leaf yield levels of tea that would bring the expected level of foreign exchange could be sustained on Sri Lankan soils without providing the required quantities of the three major nutrients via inorganic fertilisers.
It is likely that in the absence of the recommended inorganic fertiliser (especially nitrogen fertilizer) inputs, yield reductions would become detectable in the current Yala season in rice and within a matter of a few months in tea. This is because of the specific physiology of these two crops. Nitrogen is critically-essential for early growth of rice and the leaf growth of tea. Therefore, a shortage of nitrogen to these crops would be felt almost immediately as a retardation of early growth of rice (which would be reflected as a substantial reduction in grain yield) and the weekly green leaf harvest in tea.
Similar to what happens in rice and tea, the retardation of growth and yield is likely to happen with a shortage of nitrogen fertilizer in all short-duration annual crops and commercial plants. Leguminous pulse crops (e. g. soybean, mung bean, cowpea, black gram, common bean, etc.) could be an exception because of their ability to utilise atmospheric nitrogen.
Impacts of a shortage of nitrogen fertiliser are likely to be delayed for a few years (as stated earlier) in coconut and rubber because of their specific physiology where the nut yield or latex (rubber) yield is not as dependent on an immediate nitrogen supply as the grain and leaf yields of rice and tea respectively. However, a shortage of nitrogen will cause a reduction in the internal processes of these plants, which will be reflected in a few years’ time, as a reduction in the processes leading to the production of nuts and latex in coconut and rubber respectively. Recently-planted and younger coconut and rubber plantations will show a retardation of tree growth which will delay the commencement of nut and latex production.
A basic scientific fact which should have been noted by the advisors to politicians, if not the politicians, is that a shortage of nitrogen affects the fundamental plant process, photosynthesis, which is responsible for growth and yield formation of crops2. Shortage of nitrogen, along with shortages of phosphorus, potassium and magnesium, decreases the rate of photosynthesis, which is translated in to a reduction of growth and yield of any crop, which may happen over different time scales in different crops. It is unlikely that in the absence of inorganic fertilisers, organic fertiliser applications would be able to prevent the resulting decrease in growth and yield of a large majority of commercial crops in Sri Lanka.
A few spice crops such as cloves, cardamoms and nutmegs, but not cinnamon and pepper, may escape yield reductions due to a shortage of inorganic fertilizer because they are largely present in homegardens in the Central Province which are generally not fertilized.
Out of the three major fertilizers, containing nitrogen, phosphorus and potassium, a shortage would be most immediately felt for nitrogen fertilizer. The impact would be delayed for phosphorus fertilizer and it would be intermediate for potassium fertilizer. The scientific reasons are that nitrogen is the nutrient that is most critically-needed for a large majority of plant processes and is the most mobile nutrient in the soil, which makes it the most susceptible for leaching losses; phosphorus is the least mobile nutrient and therefore, can remain in the soil for
2 Evans, J. R., & Clarke, V. C. (2019). The nitrogen cost of photosynthesis. Journal of Experimental Botany, 70(1), 7-15. An expert review that was published in a highly-recognized scientific journal in plant sciences. Although most of its content is aimed at specialists in Plant Physiology, there are a few paragraphs (highlighted) from which an educated ‘layman’ reader could gather useful insights in to why nitrogen fertilizer is of such crucial importance for crops. a reasonable period of time and can be released to plants slowly; potassium is a nutrient which is intermediate in terms of its mobility in the soil and criticality of its need for plant processes.
What has been the response of the stakeholders?
This is only a snapshot from my perspective based on discussions with professional colleagues and contacts. An overwhelming majority of academics, research officers, extension officers, commercial growers and farmers do not agree with this immediate and total ban of inorganic fertilizers. A minority of stakeholders in the agriculture sector and an overwhelming majority of environmental activists (who unfortunately have no clear idea of how large-scale agriculture to feed a nation differs from growing a few pots of plants at home) have welcomed the ban. A powerful argument of this minority of stakeholders in the agriculture sector is that organic agricultural products (e.g. organic tea) fetches a higher price in the global market and will offset any loss of foreign exchange due to reduced total production. This argument ignores the decline in yield and total production of locally-consumed food (including the staple food, rice), the wide-ranging implications of which cannot be compensated by a higher price (which is unlikely to happen in the highly-volatile local market for agricultural produce).
Where do we go from here?
While disagreeing with a total and immediate ban on inorganic fertilizer, a majority of academics, research officers and extension officers, but not commercial growers and farmers, acknowledge that there is scope for an appreciable reduction in the quantities of inorganic fertilizer (relative to the levels that have been in use before the ban) without incurring a yield reduction. Farmers have been applying the inorganic fertilizers at rates which are above those recommended by the Department of Agriculture, because inorganic fertilizers had been made available to them at a highly-subsidized price.
Research on a range of different crops over several seasons across a range of locations carried out by my research group has shown that 25% of the recommended amount of nitrogen fertilizer can be reduced without incurring a yield reduction.
Therefore, a phased-out reduction of inorganic fertilizer along with a gradual increase of the contribution of organic fertilizer to supply the nutrient requirement of crops is a viable pathway that a majority of stakeholders agrees on. Increasing the contribution of organic fertilizer requires: (a) up-scaling of organic fertilizers that have been developed in Sri Lanka using microorganisms isolated from local soils; (b) developing infrastructure to produce such organic fertilizers at commercial scale; (c) changing farmer/grower perceptions and attitudes on the total dependence on inorganic fertilizers and start using organic fertilizer as a part-replacement via a concerted extension effort. (The agricultural extension service in Sri Lanka, which was acknowledged as one of the best in Asia in the 1980s, have been severely downgraded during the last three decades); (d) initiating a concerted programme to increase the organic matter content of Sri Lankan soils, which would enable them to retain a higher fraction of the nutrients applied to them via both inorganic and organic fertilizers and thereby minimize leaching losses.
Even if all the above are successfully implemented (which will take time especially in the current context), an agriculture sector, which is totally based on organic fertilizer—the first such country in the world according to the President—is unlikely to produce enough food (e. g. rice) to ensure food security in Sri Lanka or generate other agriculture-based products that fetch foreign exchange and support local manufacturing industries (e. g. rubber). Therefore, it is inevitable that a balance needs to be struck between the reduction of inorganic fertilizer (from the levels that were practiced before the ban) and a viable level of organic fertilizer as a part-replacement to provide the full nutrient requirement that a higher crop yield demands.
As a medium-term solution, research on a more balanced form of agriculture (i.e. an optimum combination of inorganic and organic fertilizer) within the climatic and soil conditions that are prevalent in Sri Lanka (while taking in to account their possible changes as part of global climate change) needs to be encouraged via increased funding. Currently, Sri Lanka invests only 0.11% of its GDP in Research and Development (in all disciplines including agriculture), which is one of the lowest even in Asia. Therefore, there is little room for optimism in this regard.
Importation of organic fertilizers
Importation of organic fertilizers is being promoted as a short-term measure to supply the nutrient requirement to agricultural crops during the period when Sri Lanka is expected develop its local capacity to produce organic fertilizers in quantities sufficient to meet the full nutrient demand of the crops. It is said that the quality of imported organic fertilizer will be assured via strict quality control procedures which conform to, for example, the EU Standards. Only time will tell whether this will actually materialize and provide a solution. A few points of major concern are as following:
Experienced Soil Scientists and fertilizer experts are of the opinion that concentration of nutrients in organic fertilizers is such that large quantities need to be imported (subsequently transported to fields and applied) to fulfil the nutrient demand to produce the crop yields at the required levels to ensure food security and sustain foreign exchange earnings.
Almost all organic fertilizers, being material of plant, animal or human origin, retain a diverse population of microorganisms. Unlike inorganic fertilizers, which are inert material, organic fertilizers are live material. Microorganisms, whether in soils, plants or any other location or entity, are often highly environment-specific. Introduction of such alien microorganisms to Sri Lankan soils could cause all types of unforeseen interactions with local microorganisms. Some of these interactions could have environmental repercussions, which are irreversible as once released to the soil, these alien microorganisms cannot be ‘recalled’. Therefore, it is always advisable and safer to develop organic fertilizers locally rather than importing.
Sterilization of imported organic fertilizer to kill all alien microorganisms via a process of fumigation after importation is suggested as a solution to this problem. However, the large quantities of organic fertilizers that are required to be imported and the toxicity levels
of the chemicals that are used in fumigation could lead to environmental issues that the organic fertilizers are aiming to prevent. Recently, the Cabinet Minister of Agriculture went on record saying that only sterilized organic fertilizer conforming to quality standards acceptable to a government-appointed expert committee would be imported. Given Sri Lanka’s poor record of regulation, implementation and enforcement of quality standards on a range of items, both imported and locally-produced and both agricultural and non-agricultural, it remains to be seen whether these promises will be fulfilled.
Rational medium- to long-term possibilities for reducing the use of inorganic fertilizer while increasing yields of major food crops at a rate required to keep pace with increasing population and consequently increasing demand
A few medium- to long-term options, based on sound scientific principles, are available and are briefly discussed below:
Genetic modification of crops
In addressing the challenges of increasing crop yields while decreasing their use of nutrients (i.e. increasing the yield per unit nutrient used), scientists have been trying to modify the components and steps involved in the photosynthesis process via genetic engineering. One of their aims has been to produce a plant which achieves a higher photosynthetic rate with the same level of nitrogen used. After about two decades of research effort, a recent research publication in the prestigious science journal Nature reports of such a breakthrough in rice3. Reading through it carefully, I gather that this new genetically-modified rice plant (we call them ‘transgenic’ plants) has the potential to achieve a higher photosynthetic rate and grain yield with the same level of nitrogen as the ‘normal’ plants (which are not genetically-modified). However, this is possible under ‘well-fertilized conditions’ meaning that at the currently-used high nitrogen fertilizer rates4. This particular publication does not indicate whether such higher levels of photosynthesis and yields are possible at lower than ‘well-fertilized conditions’ which are likely to prevail in fields fertilized exclusively with organic fertilizer. Nevertheless, as Professor Stephen Long, a recognized world authority on photosynthesis states, the production of this transgenic rice plant could be a ‘game-changer’ to increase grain yield of rice without a proportionate increase in nitrogen input.
However, it should be noted that a considerable time could elapse from the point of producing a ‘transgenic’ plant to developing a new crop variety that could be released to the farmers for commercial cultivation. Yet, this appears to be a solid step in the right direction.
3 Long, S. P. (2020). Photosynthesis engineered to increase rice yield. Nature Food, 1(2), 105-105. A brief comment by Professor Stephen Long on the recent breakthrough in producing a genetically-modified rice plant which is able to achieve a higher photosynthetic rate and grain yield with the same amount of nitrogen.
4 Yoon, D. K., Ishiyama, K., Suganami, M., Tazoe, Y., Watanabe, M., Imaruoka, S., … & Makino, A. (2020). Transgenic rice overproducing Rubisco exhibits increased yields with improved nitrogen-use efficiency in an experimental paddy field. Nature Food, 1(2), 134-139. The research publication which describes the above breakthrough in photosynthesis and nitrogen use. Increasing the organic matter content in soils
Soil organic matter (SOM) is a component of the soil in addition to the soil particles. While the soil particles arise from weathering of rocks and minerals of the soil parent material, SOM arises from the decomposition of organic material added to the soil. SOM helps to retain nutrients and water in the top layers of the soil where most plant roots are also present. In addition, SOM helps to improve the aeration and structure in the soil, which are vital physical properties in the soil to facilitate plant growth.
Except the soils in the terraced plateaus of the Central Highlands, soils of almost all arable crop lands in Sri Lanka have inadequate SOM. This means that the ability of these soils to retain the nutrients that are added to them, especially in the form of readily-released inorganic fertilizer, is limited. Therefore, a concerted effort to increase the SOM status in Sri Lankan soils will enable reduction of leaching losses of nutrients and associated environmental consequences such as pollution of water sources. Increased SOM will also enable reduction of the amounts of inorganic fertilizer applied without causing a shortage of nutrients to the crops as a greater fraction of the applied fertilizer remains in the soil to be absorbed by the plants.
Therefore, while the total and immediate ban of inorganic fertilizer and replacing them with organic fertilizer will not provide the required nutrients in sufficient quantities, the large-scale application of organic fertilizer, if it happens as envisaged, will serve to increase the SOM of Sri Lankan soils in the medium- to long-term. This will make the Sri Lankan Agriculture sector less-reliant on inorganic fertilizers. However, this will have to be a gradual, phased-out transition rather than a sudden, unplanned total ban on inorganic fertilizers. Such a transition should be towards achieving an optimum balance of inorganic and organic fertilizers, which will ensure food security while protecting the environment. This is an endeavour that has been undertaken in many parts of the world, which include both the developed and developing countries, and is termed ‘Sustainable Intensification of Agriculture’5.
5 Baulcombe, D., Crute, I., Davies, B., Dunwell, J., Gale, M., Jones, J., … & Toulmin, C. (2009). Reaping the benefits: science and the sustainable intensification of global agriculture. The Royal Society. A very useful, concise, but comprehensive description of the salient features of sustainable intensification of agriculture written by a group
of experts from the Royal Society, UK. Can be accessed at https://royalsociety.org/topics-