Agricultural Science I: Agro-ecology, Soil, Plant Nutrition, Land Preparation, and Irrigation

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The Agricultural Science syllabus (grades 12 and 13) covers crop production and animal husbandry through a set of themes: agricultural development in Sri Lanka; environmental impact on crop cultivation; the effect of the soil environment; plant nutrition; land preparation; irrigation and drainage; plant propagation; plant breeding; control of environmental conditions; plant physiology; pest management; post-harvest technology; home gardens and paddy farming; animal husbandry; food and nutrition; environmentally friendly agriculture; and agricultural economics (NIE, n.d.-f). Students who choose this subject are preparing for degrees in agriculture, food science, animal science, and related fields (Brady & Weil, 2016).

Agriculture in Sri Lanka

Agriculture contributes to food security, rural employment, exports (tea, rubber, coconut, spices), and national development. Subsectors include crops (paddy, vegetables, fruits, plantation crops), livestock, fisheries, and forestry. Issues include small farm sizes, climate variability, post-harvest losses, ageing farmers, and market access. Institutions: the Department of Agriculture, research institutes, universities, farmer organizations, and private companies.

Climate and Agro-ecological Zones

Sri Lanka has four climatic seasons: the first inter-monsoon (March to April), the south-west monsoon (May to September), the second inter-monsoon (October to November), and the north-east monsoon (December to February).
The main climatic zones are the Wet zone (annual rainfall above about 2500 mm), the Intermediate zone (about 1750 to 2500 mm), and the Dry zone (below about 1750 mm), each with subdivisions by elevation. The Northern Province, including Kilinochchi, lies in the Dry zone, where the Maha season (north-east monsoon) is the main cultivation season and irrigation from tanks such as the Iranamadu tank supports a second season.
Climatic factors: temperature, rainfall, light, humidity, and wind; their effects on crop choice, and the impact of climate change (droughts, floods, shifting seasons).

Soil Science

Formation: parent material, climate, organisms, topography, and time; weathering of rocks; the soil profile (O, A, B, C horizons).
Components: mineral particles, organic matter, water, air, and organisms.
Physical properties: texture (proportions of sand, silt, and clay; the textural triangle; loam), structure (granular, blocky, etc.), bulk density (dry mass ÷ total volume), particle density (about 2.65 g cm⁻³ for mineral soils), porosity = (1 − bulk density/particle density) × 100, colour, and consistency. Example: bulk density 1.325 g cm⁻³ gives porosity (1 − 1.325/2.65) × 100 = 50 percent.
Soil water: saturation, field capacity, permanent wilting point; available water = field capacity − permanent wilting point; sandy soils drain fast, clays hold more water.
Chemical properties: pH (most nutrients are available at pH about 6 to 7), cation exchange capacity (CEC), salinity and sodicity, organic matter.
Soil biology: earthworms, bacteria, fungi, mycorrhizae; decomposition and nutrient cycling.
Sri Lankan soil groups include Reddish Brown Earths, Non-calcic Brown soils, Red-Yellow Podzolic soils, Low Humic Gley soils, Alluvial soils, and Latosolic soils; learn where each occurs and its suitability for crops (NIE, n.d.-f; Brady & Weil, 2016).
Soil degradation and conservation: erosion (water and wind), compaction, salinization, acidification, and loss of organic matter; measures include contour cultivation, terracing, cover crops, mulching, conservation tillage, organic matter addition, and agroforestry.

Plant Nutrition and Fertilizers

Essential nutrients: macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl); deficiency symptoms (Chapter 7).
Sources: organic manures (compost, farmyard manure, green manure, biofertilizers) and inorganic fertilizers (urea, about 46 percent N; triple superphosphate, about 46 percent P₂O₅; muriate of potash, about 60 percent K₂O) (Havlin et al., 2013).
Fertilizer calculation: to apply 100 kg N per hectare as urea, the amount needed is 100 ÷ 0.46 ≈ 217 kg of urea.
Principles: apply the right nutrient, at the right rate, time, and place; split nitrogen applications; avoid losses by leaching, runoff, and volatilization; integrate organic and inorganic sources; use soil tests and recommendations from the Department of Agriculture.

Land Preparation

Tillage (primary and secondary) loosens soil, controls weeds, and incorporates residues; conservation tillage (minimum, zero tillage) protects soil. For paddy, wet land preparation involves puddling (which reduces percolation but destroys soil structure). Raised beds, sunken beds, and contour bunds suit particular crops and zones.

Irrigation and Drainage

Crop water requirement: ETc = Kc × ETo, where ETo is the reference evapotranspiration and Kc a crop coefficient (Allen et al., 1998).
Methods: surface (furrow, basin, flood), sprinkler, and drip (high water-use efficiency); choose by crop, soil, water, and cost.
Sri Lanka's irrigation heritage: the ancient tank cascade systems of the dry zone, and modern schemes such as the Mahaweli development.
Drainage removes excess water and prevents waterlogging and salinity.
Water management issues: scarcity, efficiency, quality, and competition among uses.

Common Mistakes

Using a single soil property to judge suitability.
Forgetting to convert fertilizer rates between nutrient and product weights.
Ignoring the zone when recommending crops.

CHAPTER 22