UNIT 5: Irrigation

Success Criteria

• Explain the meaning of irrigation.
• Explain the importance of irrigation.
• Describe systems of irrigation.
• Explain the advantages and limitations of drip irrigation.
• Identify factors that affect the frequency and time of irrigation.
• Explain how each factor affects frequency and time of irrigation.
• Discuss how to irrigate vegetables in a school garden.
• Select an irrigation system.
• Establish an irrigation unit.

Meaning and Importance of Irrigation

Irrigation is the artificial application of water to land to support plant growth.¹ It is primarily used in arid and semi-arid regions, or as a supplement to unreliable rainfall, ensuring consistent water supply for crops.² In Malawi, irrigation is extensively practiced during the dry season to maintain agricultural productivity.³

Importance of Irrigation:

• Increased Cropping Seasons: Enables multiple harvests (double or treble cropping) within a year.
• Ensures Food Security: Provides consistent food supply throughout the year, reducing reliance on seasonal rainfed agriculture.⁴
   
• Regular Fresh Yields: Guarantees a continuous supply of fresh produce from the farm.
• Year-Round Income for Farmers: Allows farmers to generate income consistently, improving their economic stability.
• Mitigates Drought Impact: Reduces the adverse effects of drought on agricultural production.⁵
   
• Land Reclamation: Facilitates the productive use of land in semi-arid and desert regions that would otherwise be unsuitable for agriculture.⁶
   
• Employment Generation: Large-scale irrigation projects create numerous employment opportunities.
• Pest Control: Overhead irrigation can help control certain pests, such as aphids.⁷
   

Systems of Irrigation

Irrigation systems are broadly categorized into Surface Irrigation and Piped (Overhead/Drip) Irrigation.

Factors to Consider When Choosing an Irrigation System:

• Topography:
  • Surface Irrigation: Best suited for flat land due to gravity flow.
  • Piped Irrigation: Can be implemented effectively on varying topographies, including sloping land.⁸
     
• Source of Water:
  • Surface Irrigation: Requires a larger and more reliable water supply due to higher water usage.
  • Piped Irrigation: More economical with water, suitable for limited water sources.
• Type of Crop:
  • High Water Demand: Vegetables and rice thrive under irrigation.⁹
     
  • Lower Water Demand: Fruits generally require less irrigation.
• Value of Crop: High-value crops are ideal for irrigated farming, as the increased yield and quality can offset the investment in irrigation infrastructure.
• Capital Availability:
  • High Capital: Overhead and drip irrigation systems are expensive to establish and maintain.
  • Low Capital: Surface irrigation methods are generally cheaper to set up.
• Type of Soil:
  • Surface Irrigation: Best for soils with high moisture retention capacity (e.g., heavy clayey soils).
  • Piped Irrigation: Adaptable to any soil type, considering drainage and water-holding capacity.
• Technical Skill: Piped irrigation systems (overhead, drip) require technical knowledge for installation, operation, and maintenance (e.g., pipe layout, repair of breakages).¹⁰
   

1. Surface Irrigation:

Water is applied and distributed over the soil surface by gravity. This is a traditional method widely used by farmers.

• Types:
  • Flood Irrigation:
    • Description: Water covers the entire field, saturating the soil.
    • Advantages: Easy and cheap to establish; ensures high soil moisture content; helps control non-aquatic weeds; can improve soil fertility through alluvial soil deposits.
    • Disadvantages: Requires large, reliable water supply; wasteful of water; can lead to soil salinity and waterlogging (suffocating roots); causes soil erosion on light (sandy) soils (best for heavy clayey soils); may not uniformly reach all parts of the field.
  • Furrow Irrigation:
    • Description: Water is applied at one end of parallel furrows and flows by gravity.¹¹ Pipes can be used to supply water from the source.¹²
       
       
    • Advantages: Cheaper to establish; irrigates large areas; requires less skill.
    • Disadvantages: Requires large volumes of water; makes field difficult for machinery due to slippery conditions; construction of furrows is time-consuming and expensive; uneven water distribution; moving water washes away nutrients from furrows.
  • Basin Irrigation:
    • Description: A basin-like structure (earthen embankment) is constructed around individual plants or leveled ground (common in rice paddies) where water is applied.¹³
       
    • Suitability: Good for fruit production and rice cultivation.
• Maintenance of Surface Irrigation:
  • Repairing broken embankments.
  • Weeding basin inlets/outlets and canals.
  • Dredging (clearing) canals.
  • Repairing sluice gates.
• General Advantages of Surface Irrigation:
  • Cheap to establish.
  • Easy to use, requiring less skill.
• General Disadvantages of Surface Irrigation:
  • Requires a lot of water.
  • Uneven water distribution in the field.
  • Leads to siltation of irrigation canals.
  • Only effective in water-retentive soils.
  • Only suitable for flat land.

2. Overhead or Sprinkler Irrigation:

Water is applied to crops from a height in the form of showers, mimicking rainfall. It reaches the whole plant and is suitable for varying slopes.

• Components:
  • Water Pump: Provides mechanical force to draw and push water through pipes (types: Hydraulic, Reciprocating, Centrifugal).¹⁴
     
  • Main Pipes: Carry water from the pump to the field (often plastic or rust-resistant aluminum).
  • Lateral Pipes: Connected to main pipes, distributing water to different parts of the field.
  • Riser Pipes: Vertical pipes connected to lateral pipes and sprinklers, placed at even intervals.
  • Sprinklers: Release water under pressure in the form of showers.¹⁵
     
    • Types: Continuous rotating sprinklers (small-scale) and spring-loaded sprinklers (large-scale).
• Factors Influencing Sprinkler Intensity/Period: Soil type, crop type, topography, soil moisture content.¹⁶
   
• Pressure Application:
  • High Pressure: Perennial crops.
  • Medium Pressure: Arable crops.
  • Low Pressure: Vegetables.
• Advantages of Sprinkler Irrigation:
  • Uniform water application.
  • Suitable for all types of crops.
  • Does not require land leveling.
  • Does not cause soil erosion, even on sloping land.
  • Ideal for applying fertilizers and pesticides in solution (fertigation/chemigation).
  • Easy to use once pipes are fixed.
• Disadvantages of Sprinkler Irrigation:
  • Expensive to purchase and maintain pipes and pumps.
  • High cost of pumping water (energy).
  • Less efficient during windy days as showers are deflected.
  • Wasteful, as some water may fall away from roots or evaporate from leaves before reaching the soil.¹⁷
     
  • Labor-intensive to move pipes for manual systems.¹⁸
     

3. Trickle or Drip Irrigation:

Water is supplied directly to plant roots through underground pipes with holes adjacent to planting stations, minimizing water loss.¹⁹

• Advantages of Trickle Irrigation:
  • Efficient Water Use: Controls water loss through evaporation as water directly reaches the roots.²⁰
     
  • Efficient Fertilizer Application: Easy to supply fertilizers with water (fertigation).²¹
     
  • Water Scarcity: Ideal for areas with limited water resources.
  • Accessibility: Dry ground between furrows allows easy access for workers and machinery.
  • Weed Control: Prevents weed growth in furrows as water is localized to plant roots.
  • Erosion Prevention: Risks of erosion are avoided as water flows underground or drips slowly.²²
     
• Disadvantages of Trickle Irrigation:
  • Expensive to install and maintain.
  • Requires technical knowledge for operation and maintenance.
  • Requires good quality water to prevent pipe blockages (e.g., from sediments or mineral deposits).

Maintenance of Piped Irrigation:

• Repairing broken parts.
• Replacing broken pipes.
• Occasionally flushing pipes with phosphoric acid to remove salt deposits (especially when using hard water).
• Replacing faulty components.
• Unblocking sprinklers or emitters.

Factors Affecting Frequency and Timing of Irrigation

Optimal irrigation frequency and timing are crucial for plant health and water efficiency.²³

• Type of Soil:
  • Sandy Soil: Needs more frequent irrigation due to low water holding capacity and rapid drainage.
  • Clay Soil: Holds water for longer periods, requiring less frequent irrigation.
• Type of Crops: Different crops have varying water requirements.²⁴ Vegetables, for example, need large quantities of water to prevent wilting, whereas some other crops might be more drought-tolerant.
   
• Weather Conditions:
  • Dry and Hot Weather: Requires more frequent irrigation due to high evapotranspiration rates to prevent wilting.
  • Cool Weather: Requires less frequent irrigation.
• Moisture Content of the Soil:
  • Irrigation should aim to bring the soil to field capacity (optimal balance of water and air).
  • Excessive water beyond field capacity leads to waterlogging, which removes air from the soil and can choke roots.²⁵ Waterlogging must be avoided.
     
• Stage of Plant Growth:
  • Older Plants: Generally require more water, especially during critical stages like flowering and fruiting, for cell turgidity and dissolution of food reserves (e.g., maize, Irish potato).
  • Young Plants: May require less water.
  • Approaching Maturity: Irrigation should typically be reduced as crops near harvesting.
• Time of the Day:
  • Avoid Morning Irrigation: Water can be lost rapidly through evapotranspiration due to the scorching sun.
  • Best Time: Afternoon (4:00 – 5:00 PM): Allows moisture to remain in the soil for longer periods, even until the following day, as evaporation rates are lower.

How to Irrigate Vegetables in a School Garden (Practical Application)

To effectively irrigate vegetables in a school garden, consider the following steps, focusing on water efficiency and plant needs:

1. Assess Soil Type and Crop Needs:
   • Determine if the garden soil is sandy, loamy, or clayey to estimate water retention.
   • Identify the specific water requirements of the vegetables being grown (e.g., leafy greens generally need more frequent watering than root vegetables).²⁶
      
2. Select an Appropriate Irrigation System:
   • For a school garden, a drip irrigation system is highly recommended due to its water efficiency, direct root zone delivery, and ability to prevent weed growth.²⁷
      
   • Alternatively, a simple watering can or hose with a gentle spray nozzle for small areas, or furrow/basin irrigation if the garden is flat and water is abundant. Sprinklers can be used but are less efficient for small-scale vegetables.²⁸
      
3. Establish an Irrigation Unit (if applicable):
   • Drip System:
     • Water Source: Connect to a tap or a water tank.
     • Main Line: Lay a main hose from the water source along the garden rows.
     • Drip Lines/Emitters: Attach smaller drip lines with emitters (or punch holes) near the base of each plant.²⁹ Ensure proper spacing.
        
     • Filter: Install a simple filter to prevent clogging if water quality is an issue.³⁰
        
     • Pressure Regulator: May be needed if water pressure is too high.
   • Basin/Furrow System:
     • Create small basins around individual plants or form shallow furrows between rows.
     • Use a watering can or hose to fill the basins/furrows.
4. Determine Frequency and Timing:
   • Start with Visual Inspection: Feel the soil. If it feels dry about an inch or two below the surface, it’s time to water.
   • Weather: Water more frequently during hot, sunny, and windy days. Reduce frequency during cool, cloudy, or rainy periods.
   • Plant Stage: Young seedlings need consistent moisture.³¹ Mature plants (especially fruiting ones) often have higher water demands. Reduce watering as vegetables approach maturity.
      
   • Best Time: Water in the late afternoon (around 4-5 PM) to minimize evaporation and allow water to soak into the soil overnight. Avoid midday watering.
5. Application Technique:
   • Deep and Infrequent (for established plants): Encourage deep root growth by watering thoroughly but less often. This helps plants become more resilient to dry spells.
   • Gentle Application: Avoid harsh sprays that can damage tender plants or wash away soil.
   • Target Root Zone: Direct water to the base of the plants, avoiding wetting foliage unnecessarily to reduce disease risk.
6. Monitoring:
   • Regularly check soil moisture levels.
   • Observe plants for signs of wilting (indicating insufficient water) or yellowing leaves/stunted growth (which could indicate overwatering or nutrient deficiency).
   • Adjust irrigation schedule as needed based on observations and changing weather conditions.

By following these steps, a school garden can effectively manage its water resources, promote healthy vegetable growth, and provide a valuable learning experience in sustainable agriculture.