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Introduction

Vegetable crops require frequent irrigation for better growth and development. Irrigation requirement may vary from crop to crop. If water is the limiting factor, then proper management and conservation practices can be fruitful to grow vegetables round the year.

Conservation, management and use of irrigation water are critical to successful vegetable production, especially when the fields are under drought condition. A well-organised water management plan and irrigation scheduling is the key to water management in vegetable crop production.

Leafy vegetables require frequent irrigation. Fruiting vegetables and root and tuber vegetables have different critical stages of water requirement. This unit will help you understand about the water sensitive or critical stage of vegetable crops.

Role of Water in Plants

• Water is an essential element for plants.
• It helps in the growth and development of plants.
• It helps plants to absorb and transport minerals from soil.
• It is essential for the conduct of biochemical reactions.
• It is integral for making food through photosynthesis.
• It helps plants to manage heat or frost stresses.
• It is necessary for seed germination and seedling establishment.

Sources of Water for Plants

Rainfall and irrigation are the two main sources of water for plants.

Rainfall is a natural source of water and the quality of water is also good. But, it is a limited and unpredictable natural source. Whereas, the artificial application of water to the soil in order to maintain a proper soil moisture regime for plant growth is called irrigation. Irrigation is the practice of planning and applying water artificially to maintain soil moisture. It can be made an assured source.

The irrigation requirement of crop plants depends on:

• The type of vegetable crop: Shallow-rooted crops need light but frequent irrigation as compared to deep-rooted vegetable crops.
• The growing season: Summer vegetable crops need more frequent irrigation than the winter crops. Occasionally the rainy season crops also need irrigation.
• The climate: Crops should be irrigated less frequently during the cool climate and more frequently in tropical or hot climate.
• The soil type: Frequent but light irrigation should be done in sandy soil and deep but less frequent irrigation is required in clayey soil.
• The type of irrigation system: Regular irrigation is needed in the drip system and less frequently in the surface, sub-surface, and sprinkler irrigation system.

Session 1: Irrigation and Water Quality

Importance of Irrigation

• Since vegetable crops contain 80-90% water, they require a large amount of water and frequent irrigation for proper growth and development.

Sources of Irrigation Water

1. Surface Water Sources

Surface water sources are found on the surface of the land. These sources are rivers, canals, ponds, lakes, dams, etc. Generally, the quality of water from these sources is quite good and fit for irrigation.

2. Groundwater

Groundwater is underground water lifted through dug wells, tube wells and bore wells. This water quality varies from poor to good.

Sources of Irrigation Water

Surface Water Sources

• Rivers
• Ponds
• Lakes
• Dams

Groundwater Sources

• Dug wells
• Tube wells/Bore wells

Fig. 1.1: Sources of irrigation water

Do you know?

India has very few water resources and the groundwater level is also depleting at an alarming rate. Therefore, it is essential to conserve rainwater. Also, mulching in crops can save water and increase irrigation interval.

In some areas, poor quality water full of toxins, heavy metals and microbes is used in the cultivation of vegetables. It can be used but only after proper treatment.

The quality of water is as important as the quantity for successful vegetable cultivation. In India, water quality concerns have often been neglected because of the availability of good quality water but nowadays this situation is changing in many areas. Poor quality water from urban-industrial areas and the salinity of groundwater need to be properly treated before using for irrigation.

Good quality water is a crucial factor for soil to remain productive for long. It allows growing of any kind of vegetable crop and also gives a high yield and better quality of vegetable crops.

Various regions in the country use poor quality water to irrigate the crops. Untreated water from urban-industrial areas is of poor quality. In some areas, groundwater is very deep and poor in quality.

Using poor quality water for irrigation may:

• deteriorate the soil health.
• deposit excess salt in the root zone.
• reduce uptake of minerals and affect crop yield.
• reduce soil permeability and increase water runoff.
• show toxicity of metals in some plants.

Criteria of Suitable Water for Irrigation

• pH of water being used for irrigation should range between 6.5 to 8.5.
• Water salinity is an indicator of total dissolved salts present in the water. It is of prime concern for both the soil structure and crop yield.
• Salt concentration is measured by electrical conductivity (EC) in milliSiemens per meter (mS/m) or micromhos per cm.
• Water having EC below 1500 micromhos/cm is good for irrigation.

Water Quality Factors

3. Sodium Adsorption Ratio (SAR)

Sodium adsorption ratio (SAR) is a measure of the relative proportion of sodium (Na⁺) to calcium (Ca²⁺) and magnesium (Mg²⁺) in water. High sodium causes breaking of soil aggregates and sealing of the soil pores. Sodium weakens the binding capacity of soil.

A small SAR value indicates low sodium content in water. It should be below 10 in irrigation water.

4. Residual Sodium Carbonate and Bicarbonate Concentration

Residual sodium carbonate and bicarbonate content in water increases the pH. This can have an alkalising effect and raise the SAR index. Residual sodium carbonate below 1.5 mg/litre in irrigation water is safe. The following measures can be adopted for the management of this water quality:

• Addition of gypsum in low calcium soil + leaching
• Addition of sulphur + lime + leaching
• More frequent irrigation
• Avoid the sprinkler method of irrigation
• Avoid using fertilisers containing chloride and boron
• Select tolerant crops

5. Boron in Water

Boron is the most common element found in toxic concentrations in water. It cannot be easily removed from water. The only remedy is to dilute high boron water. Below 1.0 ppm boron content is acceptable level in irrigation water.

Quality Testing Instruments

It is important to test the suitability of water quality for its intended purpose. Water testing will help to know whether the quality of water is fit for irrigation or not. If it is not, then one needs to find out what is the specific reason for the poor quality of water.

Generally, the pH and electrical conductivity (EC) are the two most important parameters for water quality analysis. When a pH colour strip is dipped into alkaline or acidic water the colour changes as shown below:

1. pH Meter

A pH meter is an equipment by which we can measure the pH level of any solution. It consists of a display unit and electrode. When the electrode is dipped into a solution, it shows the pH value.

Water Quality Testing Instruments

pH Meter

The pH meter is used to measure the pH value of soil and water. Ideally, the pH of soil and water should be 6.0-6.5 and for nutrient solution 5.6-6.5. The pH meter should be calibrated beforehand for accurate data collection. It is more accurate than pH colour strips.

Fig. 1.3: Digital pH meter

Electrical Conductivity (EC) Meter

The EC meter measures the total dissolved salts in irrigation water. It reports values in:

• Millimhos per cm (mmhos/cm)
• Deci Siemens per metre (dS/m)
• Micro Siemens per cm (µS/cm)
• Milli Siemens per cm (mS/cm)

Micro Siemens per cm is the standard unit for freshwater EC measurements. EC increases with more dissolved salts in water. Temperature affects conductivity, typically measured at 25°C.

EC measurement is fast but non-specific—it shows total salinity but not individual ions.

Fig. 1.4: Digital Electrical Conductivity (EC) meter

Practical Exercise - Activity 1

Collection of water samples for quality testing

Material required:

• Plastic sample bottle (500 ml)

Procedure:

• Start the tubewell or handpump and let it run for 15-20 minutes to flush out stagnant water.
• This avoids the 'pipe effect' where metals or salts deposited in pipes affect the sample quality.

SESSION 2: WATER REQUIREMENT AND IRRIGATION METHODS

Water Requirement

Water requirement (WR) of a crop is the total quantity of water needed for crop growth and yield that may be supplied by rainfall or irrigation or both.

Water requirement varies from crop to crop and soil profile. It is different from irrigation requirement (IR), which is the total quantity of water applied to a cropped field to supplement rainfall and soil profile contribution.

When the entire water requirement is supplied by irrigation, then both WR and IR are the same. It is expressed as the unit of absorbed water required for the production of one unit of dry matter.

Water Requirement Formula

Water requirement (mm) = Evapo-transpiration + Application losses + Special needs

Where:

• Evapo-transpiration (ET): Total loss of water by transpiration from crop and evaporation from soil.
• Application losses: Water loss during the application of irrigation water.
• Special needs: Water required for land preparation, transplanting, leaching, etc.

How Much to Irrigate

If the water requirement of a particular crop is 6 mm per day, it means every day 6 mm of water is needed for the crop. In field conditions, it is practically difficult, so it can be given as 30 mm every 5 days or 60 mm every 10 days.

The frequency of irrigation varies with the growing season, type of crop, soil type, and condition.

Water Requirement of Vegetable Crops

Crop	Water Requirement (mm)
Tomato	600-800
Chili	450-500
Brinjal	1000
Potato	500-700
Onion	640-700
Watermelon	500
Pea	350-500
Cauliflower	350
Bean	300-500
Cabbage	380-500

Source: Reddy, 1999

Points to Understand

• The water requirement of a crop is expressed in mm/season.
• Crops require more water in summer than in winter.
• Long-duration crops have higher water requirements.
• Shallow-rooted vegetable crops need light but frequent irrigation.

Irrigation Guidelines for Vegetable Crops

Key Points on Irrigation

• Timely irrigation means higher yield and better quality of produce.
• Only a few vegetables, such as brinjal, chili, watermelon, and amaranthus, can tolerate partial drought.
• Unlike clayey soil, sandy soil requires low but more frequent depth of water.
• Avoid over flooding as it causes poor aeration and poor germination.
• With regular irrigation, keep the ridges and fields moist but not wet for better crop growth and development.

Rooting Depth Classification of Vegetable Crops

Rooting Depth	Vegetable Crops	Depth (cm)
Shallow-rooted	Onion, cabbage, cauliflower, celery, potato, radish, cowpea, lettuce, broccoli	45-60 cm
Moderately deep-rooted	Pea, muskmelon, cucumber, brinjal, watermelon, tomato, sweet pepper	90-120 cm
Deep-rooted	Asparagus, pumpkin, winter squash, sweet potato, watermelon	More than 120 cm

Source: Swarup

When to Irrigate?

While growing vegetable crops, some stages of the plants are very sensitive to water stress. If irrigation is not done at these stages, it can negatively affect growth and yield. This is known as the critical stage.

Water shortage in early crop stages delays crop maturity and reduces yield, while moisture stress in later stages reduces the quality of the produce. Therefore, maintaining adequate moisture is essential for high yield and good quality produce.

The frequency of irrigation and the amount of water to be given depend on several factors:

• Depth of the root system
• Water use efficiency
• Growth stage of the crop
• Soil type
• Prevailing weather conditions
• Actual consumptive use of the vegetable crops