Chapter 16: Soil, Water, and Crop Production

Climate and agriculture in Sri Lanka

Sri Lanka has three broad climatic zones: the Wet Zone (south-west and central highlands, with high rainfall throughout the year), the Dry Zone (north, east, and south-east, with a pronounced dry season), and an Intermediate Zone between them. Two cropping seasons follow the monsoons: Maha (about October to February, with the north-east monsoon) and Yala (about April to August, with the south-west monsoon). Crop choice, planting dates, and water management must match the zone, the season, and the soil. Major crops include paddy (rice), tea, rubber, coconut, spices, vegetables, fruits, and pulses.

Soil

Soil is a mixture of mineral particles, organic matter, water, air, and living organisms. Texture depends on the proportions of sand, silt, and clay; loam, a balanced mix, is generally the best for crops. Structure is the arrangement of particles into aggregates, which controls aeration and drainage. Key properties: pH (most crops prefer 5.5–7.0), organic matter (improves structure, water holding, and nutrient supply), cation exchange capacity (CEC) (the soil’s ability to hold nutrients), water-holding capacity, and depth (Brady & Weil, 2016). Soil testing (sampling the field in a zigzag pattern, mixing, and sending a composite sample to a laboratory) guides fertilizer and lime use. Soil conservation includes contour cultivation, terraces, cover crops, mulching, minimum tillage, and agroforestry, which are especially important on the steep hill-country slopes where rain erodes bare soil.

Plant nutrition and fertilizers

Plants need macronutrients (N, P, K, with Ca, Mg, and S) in large amounts and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl) in tiny amounts (Taiz et al., 2015). Nitrogen promotes leaf growth, phosphorus roots and flowering, and potassium fruit quality and stress tolerance. Sources: organic (compost, farmyard manure, green manure, crop residues) and inorganic (urea, triple superphosphate, muriate of potash). Biofertilizers (for example Rhizobium for legumes) and crop rotation with legumes add nitrogen. Fertilizer calculation: urea contains 46% N. To supply 90 kg N per hectare: kg of urea per hectare. Apply the right amount, at the right time, in the right place, because over-application wastes money and pollutes water.

Water and irrigation

Crop water requirement is the evapotranspiration of the crop, , where is the reference evapotranspiration (from climate data) and is a crop coefficient (Allen et al., 1998). Example: with mm/day and , mm/day. Since 1 mm of water over 1 ha is , this is per hectare per day.

Irrigation methods: surface (flood, furrow; low efficiency), sprinkler, and drip (water delivered to the root zone with high efficiency, saving water and enabling fertigation). Scheduling uses crop stage, soil moisture, and weather. Sri Lanka’s ancient tank cascade systems are an example of efficient, sustainable water management. Good drainage prevents waterlogging and salinity.

Crop establishment and management

Land preparation: ploughing, harrowing, levelling, bed and drain formation.

Planting material: quality seed (certified), seedlings, cuttings, grafts. Seed rate depends on germination percentage and spacing.

Spacing and plant population: 20 000 plants per hectare yielding 2 kg each give kg/ha.

Paddy: transplanting or direct seeding; water management; weed control; harvesting at the right maturity.

Crop care: weeding, mulching, thinning, staking, pruning, fertilizing, pest and disease control.

Protected cultivation: greenhouses and shade nets protect crops from rain, pests, and heat and allow off-season production; hydroponics grows plants in nutrient solution without soil.

Crop groups: cereals, pulses and legumes, vegetables, fruits, root and tuber crops, plantation crops (tea, rubber, coconut), spices (cinnamon, pepper, cardamom, cloves), and ornamentals.

Common mistakes

Applying fertilizer without testing the soil or calculating the need.

Over-irrigating, which leaches nutrients and wastes water.

Planting at the wrong season or in unsuitable soil.

Practice questions

Calculate the urea required to supply 69 kg N/ha (urea is 46% N). [150 kg]

With mm/day and , find the daily water need in . [36 m³/ha]

Explain why contour cultivation reduces soil erosion.