Adominant agrarian economy has long existed in Sri Lanka and has played a significant role in ensuring national food security, providing livelihood for rural poor and earning foreign exchange from product export. However, with an ever-increasing human population, the land to man ratio, the extent of land per person in the country, was 1.8 hectares in 1900 and reduced to 0.3 hectares in 2014.
More recently, per person availability of agricultural lands has remained at 0.14 hectares. These statistics stress the need for increasing the agronomic efficiency of agricultural land uses. For this purpose, a significant strategic approach includes proper supply of the principal farming inputs such as quality seed and planting material, modern farm machinery, irrigation water, fertiliser, agrochemicals and hired labour. Among such inputs, fertiliser as plant food plays an important role.
Fertiliser can be defined as a substance containing one or more recognised plant nutrients which is designed for use, or claimed to have value, for promoting plant growth but does not include un-manipulated manures of plant and animal origin.
In common with all living beings, plants need food for their growth and development. Man and other animals can only subsist on food in organic form, that is, on food derived from the plant or animal products. On the other hand, plants can build up organic tissues directly from inorganic materials. They live, grow, and reproduce by taking up water and mineral substances from the soil, carbon dioxide from the air and energy from the sun to form plant tissues. Fertiliser supplements are used to correct inadequacies of nutrients in mineral substances in farming soils.
This article, written in consultation with a senior Soil Scientist in Canada, is intended to make farmers, planters, and other stakeholders in agriculture aware of the need for a scientific basis for judicious fertiliser use in Sri Lanka. Application of a supplement of required nutrients for crops will ensure the production of enough food for the country’s people, the livelihood of rural poor, the earnings of foreign exchange from exports and the agro-ecosystem health in a sustainable manner.
Essential plant nutrients
Of the large number of elements that have been identified as occurring in plant tissues, only 16 have been found to be indispensable for their growth, development, and reproduction. These essential elements are referred to as nutrients. To be recognised as essential, an element must meet the following three criteria:
1). A deficiency of the element makes it impossible for the plant to complete the vegetative or reproductive stage of its life.
2). The deficiency symptoms of the element in question can be prevented or corrected only by supplying the element.
3). The element is directly involved in the nutrition of the plant quite apart from its possible effect in correcting some microbiological or chemical conditions in the soil or culture medium.
The essential elements are carbon, hydrogen, and oxygen, which are derived from the air and soil water, and nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, zinc, manganese, copper, boron, molybdenum, and chlorine, which are supplied from the reserves in the soil. Naturally occurring plants in ecosystems receive all nutrients from the environment but high yielding crop varieties may need an additional supplement of mainly nitrogen, phosphorus, and potassium to get expected high yields. This supplement is mostly provided through urea for nitrogen, triple super phosphate for phosphorus and muriate of potash for potassium.
Urea
Urea is a concentrated solid form of nitrogen fertiliser, contains 46 percent nitrogen in most cases, and has, therefore, advantages in storage, transport, and handling.
For almost all crops, urea is incorporated into soil as a basal dressing prior to crop establishment with a few top dressings applied during the crop growing and development periods.
Urea, white in colour, is usually free from toxic elements such as heavy metals. However, it may contain trace amounts of biuret, an impurity toxic for many crops.
The biuret is formed by the elimination of ammonia from two molecules of urea when urea is heated to temperatures above 140o – 170o during manufacturing. Hence, monitoring of commercial importation of urea is required to ensure that the biuret content remains below the permissible maximum level stipulated by the local authority.
In the local market, complaints have been received about the adulteration of urea by the addition of materials such as ammonium sulfate, white sand, common salt, and granulated muriate of potash in some cases. This is another matter to be investigated under the legal provisions in the Regulation of Fertiliser Act No. 68 of 1988.
Triple super phosphate
Triple super phosphate, popularly abbreviated as TSP, contains 44 to 52 percent phosphorus in the form of P2O5, almost entirely in water soluble form. The granulated form has excellent storage and handling properties.
This fertiliser material is incorporated into soil prior to crop establishment as a basal dressing in seasonal crops. In place of this expensive and imported fertiliser material, for perennial crops such as guava, rambutan, mangosteen, coffee, clove, pepper, nutmeg, cocoa, areca nut and tea, it can be replaced with locally manufactured Eppawala rock phosphate.
Triple super phosphate, ash in colour, may contain trace amounts of toxic elements such as cadmium. Scientific risk assessments have shown that the cadmium level in this phosphorus fertiliser is safe and does not pose any risk to human health. However, commercial supplies should be monitored lot by lot, to ensure that the content of toxic elements inclusive of cadmium is below the permissible maximum level stipulated by the local authority.
Another issue to be addressed, as with urea, researchers have reported the adulteration of triple super phosphate in the local market by adding materials such as single super phosphate and in some cases Eppawala rock phosphate.
Muriate of potash (potassium chloride), popularly abbreviated as MOP, contains about 60 percent potash (K2O). It does not normally pose any problem in handling and storage.
Entire crop requirements are applied as a basal dose but in sandy soils, high rainfall areas and for wetland rice, split application may be more beneficial. For crops such as tobacco, where the leaf quality is affected by excess chloride, potassium sulfate (sulfate of potash), popularly abbreviated as SOP, perform better as premium-quality potash.
In pure form, muriate of potash is a white crystalline salt, but the colour ranges from white to red depending on the impurities in potash minerals. The colour has no influence on fertiliser effectiveness.
In the local market, similar to other fertiliser, scientific evidence highlighted that adulteration of muriate of potash had occurred by the addition of materials such as white sand and brick powder in some cases. This is another matter to be investigated and monitored.
Nutrient retention in soil
All nutrients in varying quantities are taken up daily by crop plants as needed during the growing and development periods which range from 50 to 120 days for most seasonal crops. However, for efficiency, nutrient supplements are given in two to three split applications during the season. Hence, once fertiliser is added in limited splits, it is of utmost importance to retain applied nutrients until taken up by crop plants to ensure efficient use of this expensive farming input. Once fertiliser is added to the soil, the nutrients are released and adsorbed onto soil colloids (clay and humus). The colloids serve very much like a modern bank where nutrients are stored until taken up by plants. Crops take up daily nutrients from soil colloids through an ion exchange mechanism.
Most soils in Sri Lanka have a very low capacity for nutrient retention and are very low in humus. Added organic manures are quickly decomposed into inorganic components under our tropical hot climate, thus contributing very little to the humus pool development. Given the inherent conditions for local soils and climate, it is recommended that more frequent additions of organic manures to farmlands simultaneously with recommended fertiliser applications to facilitate nutrient retention in soil.
In addition to organic manure application, incorporation of basal fertiliser into soil prior to crop establishment and application of top dressings at correct soil moisture levels also improves retention of added nutrients. In the case of upland soils, two days after cession of rain or crop irrigation and in the case of lowland paddies, the appearance of a thin water film on the soil surface can be considered as an optimum time for top dressing of fertiliser.
Other benefits of organic manure
The principal organic manures in the country are un-manipulated materials such as rice straw, green matter of plants such as Gliricidia and wild sunflower and farmyard manure. An application of a quantity of 10 tons of organic manure for one hectare of land in a season is generally recommended as a blanket dose. However, such a quantity is not readily available for most farmers and, therefore, an application of whatever amount at affordable cost is suggested.
In addition to the supply of a substantial quantity of plant nutrients, the other benefits of organic manures include improvements to soil quality. Soil quality can be improved by provision of energy and body-building constituents for soil microorganisms, facilitating better retention of water in soil during dry spells, enhancing the formation and stability of soil aggregates and reducing the risk of soil erosion. Under continuous application of organic manure, it is advised to test soils in two-year intervals to assess nutrient supply from added organic manures and accordingly make necessary adjustment to supplemental fertiliser use.
In commercial farming, most farmers use fertiliser excessively. For example, in a study in 2006, it was revealed that most farmers apply 430 percent of the recommended quantity of fertiliser for leek, 260 percent for potato and 250 percent for carrot in the Kandapola village near Nuwara Eliya city; 830 percent for red onion, 750 percent for chilli and 180 percent for cabbage in the Kalpitiya peninsula; and 340 percent for brinjal, 240 percent for chilli and 150 percent for big onion in the Punchikulama village in Anuradapura. These excessive rates are very costly. Farmers need more education, knowledge, and awareness of the correct methods and rates to apply fertiliser and reduce the costs of production.
Dual application
Particularly, the vegetable farmers use fertiliser together with un-manipulated organic manure in areas such as Nuwara Eliya, Welimada and Bandarawela. This dual application tends to accumulate nutrients in soil. Therefore, regular testing of soils in those farmlands helps avoid such a nutrient build up in soil which may create deficiencies of other nutrients.
In degraded lands, most farmers apply more than recommended quantities of fertiliser to get rid of the problem of reaping low crop yields.
In this regard, the degradation status of land should be addressed first and once the soil is restored, crop cultivation could be continued with the application of fertiliser at recommended rates.
In some situations, farmers broadcast fertiliser on the surface of soil in dry uplands and immediately irrigate the land which causes significant nutrient losses through water runoff.
Some farmers, particularly, in remote areas depend on cheap fertiliser which in most cases is highly adulterated. Hence, fertiliser should be purchased only from reputable dealers. In case of suspected adulteration, adequate legal provisions are available under the Regulation of Fertiliser Act No. 68 of 1988 to act on.
In Sri Lanka, most fertilisers are imported and costly, perhaps contain traces of toxic impurities, might be subject to local adulteration and may contribute to pollution of the surrounding environment under misuse. On the other hand, fertiliser is required to help sustain profitable crop yields in making the industry viable. Hence, having considered both situations, fertiliser should be used in a judicious manner to minimise adverse effects and reap potential benefits.
In this regard, the responsibility for judicial fertiliser use rests with all stakeholders involved in the industry. They include farmers, planters, fertiliser importers and distributors, wholesale and retail fertiliser sellers, researchers, land use planners and policymakers. Formulation of a National Fertiliser Use Policy and timely updating the Regulation of Fertiliser Act No. 68 of 1988 would help ensure judicious fertiliser use towards regenerating agriculture in the country.
The writer is a Freelance Agriculture Scientist (Natural Resource Management).