temp

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.
• In water shortage conditions the yield and quality of vegetable crops suffer. Hence, irrigation is essential for higher yield and good quality vegetables.
• Vegetable crops grow fast, hence, they require frequent and more water.
• A sufficient amount of water in the roots is a pre-requisite for better yield and quality produce.
• Irrigation reduces dependence on rainfall because it can be done as and when required.
• If irrigation is scheduled properly, it can save water and minimise weed problems.
• Irrigation helps in growing more crops in a year in the same field.

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

1. pH of water being used for irrigation should range between 6.5 to 8.5.
2. 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.
3. 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 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.
   1. Addition of gypsum in low calcium soil + leaching
   2. Addition of sulphur + lime + leaching
   3. More frequent irrigation
   4. Avoid the sprinkler method of irrigation
   5. Avoid using fertilisers containing chloride and boron
   6. Select tolerant crops
5. 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 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 inserted into the solution the display unit shows the pH value. Ideally, the pH of the soil and water has to be 6-6.5 and that of the nutrient solution should be 5.6-6.5. For accurate data collection, the pH meter, like all other equipment, should be calibrated beforehand. It is more accurate than the pH colour strip. (Fig. 1.3)

Electrical conductivity (EC) meter is used to measure the total dissolved salts in irrigation water. It is reported in terms of millimhos per centimetre (mmhos/cm), deci Siemens per metre (dS/m), micro Siemens per centimetre (μS/cm) or milli Siemens per centimetre (mS/cm), which gives information on the degree of salinity in water. Micro Siemens per centimetre is the standard unit to represent EC value of freshwater measurements. They are all similar on numerical count. The numerical value remains the same per unit area, only the reference varies. Electrical conductivity of irrigation water is more when it contains more soluble salts and vice versa. The temperature of water affects conductivity and it is usually reported at 25°C. The EC measurement is the easiest and a rapid method to analyse the salinity level of water, but it is non-specific. It measures only the combined effect of all ions present and cannot distinguish between the different types of ions (Fig. 1.4).

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:

1. Tubewell or handpump
   • Start the tubewell or handpump and let it run for about 15-20 minutes. It is necessary to drain out all the water retained in the pipe of the well or pump to avoid ‘pipe effect’ (metals, salts deposited in the pipe).
   • Take a water sample in sample bottles (500 ml–1 litre just before the water falls into the channel.
   • To analyse the quality of water from a tubewell or handpump, never collect the water sample once it falls into the channels because it affects the water quality.
2. Ponds or tanks
   • Sample water from a pond or tank should be taken at least 5-10 metres inside the boundary area to avoid boundary effect.
   • Take a properly washed plastic container for sampling.
   • Displace the surface water of the pond or tank gently and take the sample from the intermediate depth.
3. Collect the water in a sample bottle immediately and close the bottle cap tightly.
4. Label the sample by writing name, address, source, place and date of sampling.
5. Submit the collected samples to the water quality testing laboratory within 2-3 days.

Precautions

• Avoid possibility of any external contamination.
• Don’t wash the bottle with detergents or soap.
• Don’t take water from the pond surface because it may contain organic material and affect the correct representation of water quality.
• Gently shake the pond surface to collect the water sample more accurately.

Activity 2

Measure pH by using litmus paper or pH meter With litmus paper or pH paper
Material required
Litmus or pH paper strip, water from different sources, writing material, practical file, etc.
Procedure
1. Collect the water from different sources and places.
2. To observe the pH of the water sample, take the sample in a beaker (100 ml).
3. Dip the litmus paper in the sample and observe the colour change.
4. The litmus or pH paper changes colour based on the pH of the water sample.
5. Match the colour change with the help of a colour strip.
6. Note down the pH of water.
With a pH meter
Procedure
1. Collect the water sample.
2. Take 40 ml (5 ml more or less) of the water sample in a beaker.
3. Stabilise the temperature of the water sample.
4. Insert the pH meter electrode in the water beaker and turn the beaker to adjust for good contact between the pH meter electrode and water.
5. Before taking a recording, stabilise the pH reading of the sample for 20-30 seconds (automatic pH meter provides signals).
6. After reading, wash the electrode with distilled water to remove any film on it.

Precautions

• Calibrate the pH meter by using a pH 7 buffer solution before noting the observations.
• Insert the electrode into water. It should not touch the bottom of the beaker.
• During the electrode storage, keep it in pH 7 buffer.

Check Your Progress

A. Fill in the blanks

1. Water is an essential _______ for plant life.
2. Vegetable crops grow fast, hence they require _______ and _______.
3. The untreated water from urban-industrial areas is _________ in quality.
4. _________ pH range of water is considered safe for irrigation.

B. Multiple choice questions

1. Irrigation is a practice of_________.
(a) only artificial application of water
(b) watering through rainfall
(c) recharging groundwater
(d) storing rainwater
2. Irrigation water suitable for most of the crops contains _________ boron.
(a) below 1.0 ppm
(b) 1.0 ppm - 1.5 ppm
(c) 1.0 ppm - 2.0 ppm
(d) above 2.0 ppm
3. The most common surface water source is a _________.
(a) tube well
(b) dug well
(c) river
(d) bore well
4. Total dissolved salt in water is measured with a _________.
(a) Hygrometer
(b) Lux meter
(c) pH meter
(d) EC meter
5. Electrical conductivity in irrigation water should be_________.
(a) below 1500 micromhos/cm
(b) 2000-3000 micromhos/cm
(c) 2500-3500 micromhos/cm
(d) above 3000 micromhos/cm

C. Short answer questions

1. Define irrigation and enlist the role of irrigation water.
2. Point out the criteria for suitability of irrigation water.
3. When is water suitable for irrigation?
4. Classify irrigation sources with proper examples.
5. How is water testing helpful for a farmer?

D. Match the columns

A	B
1. Quality test	(a) Mulching
2. Poor quality water	(b) Frequent watering
3. Increase water pH	(c) Deposit salts in root zone
4. Increase irrigation interval	(d) Suitability of water
5. Shallow rooted crops	(e) Carbonate and bicarbonate

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 (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 we need to give 6 mm of water to the crop. In field condition practically, it is not possible so it can be given as 30 mm for every 5 days or 60 mm for every 10 days. The frequency of irrigation varies with the growing season, types of crop and types of soil and its condition.

Water Requirement of Vegetable Crops

• Tomato: 600-800 mm
• Chili: 450-500 mm
• Brinjal: 1000 mm
• Potato: 500-700 mm
• Onion: 640-700 mm
• Watermelon:500 mm
• Pea: 350-500 mm
• Cauliflower: 350 mm
• Bean: 300-500 mm
• Cabbage: 380-500 mm

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.
• 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.

Vegetable crops are divided into three major categories depending upon the rooting depth.

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