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Drought stress in vegetable production

July 02, 2007

OVER the years, problem of water shortage is well documented for various agricultural uses and natural bounties, vegetables are not an exception. Commercial production of vegetable is not possible without adequate water availability through out the growing season. Drought conditions threaten vegetable production and demand more efficient water use.

There are two main reasons for irrigating vegetable crops. First, most vegetables have shallow roots, which rarely exceed 24 inches in depth. Second, the useful product is sold on the basis of freshness, weight and appearance. These characteristics make vegetables particularly sensitive to water shortage. Other reasons for irrigating vegetables include: allowing flexibility in planting time, establishing a more uniform plant stand, influencing soil temperature, utilising certain herbicides and soil fumigants more effectively, and assuring reliable yields.

Vegetable crops require from around six inches of water in a season for radishes to 24 inches for tomatoes and watermelons. Precise irrigation requirements can be predicted based on consumptive use and effective precipitation values.

Leafy vegetables e.g. cabbage, lettuce, and spinach are generally planted at or near field capacity. Being shallow rooted, these vegetables benefit from frequent irrigations throughout the season. As leaf expansion relates closely to water availability, these crops, especially cabbage and lettuce, are particularly sensitive to water stress during the period of head formation through harvest. Over watering can result in burst heads. Broccoli and cauliflower, although not grown specifically for their leaves, respond to irrigation much as the leafy vegetables do. Cauliflower, in particular, is noted for being sensitive to water stress at all stages of growth, responding to drought with reduced growth and premature heading.

In case of root, tuber, and bulb vegetables e.g. carrots, beets, turnips, sweet potato, radishes, potatoes, and onions, yield depends on the production and translocation of carbohydrates from the leaf to the root or bulb. The most sensitive stage of growth generally occurs as these storage organs enlarge. Carrots require an even and abundant supply of water throughout the season. Stress causes small, woody, and poorly flavoured roots. Uneven irrigation can lead to misshapen or split roots in carrots and early bulb formation in onions.

While cucumbers, water melons, musk melons pumpkins, squashes, peas, peppers and tomatoes are most sensitive to water stress at flowering stage and as fruits and seeds develop. Fruit set on these crops can be seriously reduced if water becomes scare. Regular supplies of water during the period of fruit enlargement can reduce the incidence of fruit cracking and blossom-end rot in tomatoes.

Water is a precious and a costly input which must be managed properly. Efficient irrigation requires application of correct amount of water at the correct time. A number of irrigation practices and strategies are recommended for vegetable crops to make every drop of water count.

Reducing irrigation below the level required for best production can reduce yield and quality of vegetable crops greatly. No advantage is gained in trying to spread a given water supply over too large an area. When irrigation water is in short supply, it may be necessary to take some land out of production. If you have a choice, plant the most productive land rather than marginal land.

Commercial growers weigh economic factors heavily in selecting vegetable crops. However, during periods of anticipated drought, factors such as water requirements assume increased importance. Certain crops and cultivars are less sensitive to short periods of water stress than others. Select these less sensitive crops and cultivars.

Vegetable planting season should be started with adequate soil moisture as pre- plant irrigation benefits many vegetable crops. Such irrigation builds subsurface soil moisture and promotes a deeper root system. Over irrigation should be avoided as it wastes water and can leach chemicals into groundwater supplies.

Proper plant stand should be established as rapid emergence and a uniform plant stand make the most efficient use of soil moisture. Wet soil exposed to sunlight has greater evaporation loss than the soil shaded by a crop.

Proper germination and emergence of vegetables in field require careful water management. Less water and more precise control can often be obtained by using transplants. Once in the field, however, transplanted crops generally develop shallower root systems than direct seeded crops and may require more frequent irrigation.

Vegetable growers should use mulches and row covers to increase temperatures for more rapid plant growth. These also can save water by reducing surface evaporation.

Drip or micro irrigation is an expensive, but efficient system of irrigating high value crops, such as vegetables. Combined use of such systems with mulches and row covers for added efficiency should be taken into consideration.

Good irrigation scheduling is essential for all irrigation systems if growers are to apply the correct amount of water at the correct time. Irrigation scheduling should be given careful attention to monitor soil moisture, climate, and crop growth.

Maintenance of proper soil structure and fertility is a must as proper soil structure permits optimum infiltration and water holding. Proper soil fertility encourages the best plant growth and utilisation of available soil moisture.

Weeds compete with vegetables for soil moisture and decrease yields. In particularly weedy fields, weeds can use more water than the vegetable crops. A good weed control should be ensured as it reduces competition for soil moisture and increases water use efficiency.

Insect and disease damage restrict the growth and water use efficiency of vegetable crops, reducing both yields and quality. Maintaining good plant health is especially important in regard to diseases classified as wilts, which reduce the ability of the crop to absorb water.

Careful attention to irrigation should always be an integral part of vegetable production systems as it will pay off with improved crop quality, a more reliable product, and after all greater profit to the vegetable-growers which is their prime objective.