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Vegetable Crops

Vegetables form an important component of human diet, supplying major amounts of vitamins A and C and numerous nutrients, protein and fat. Vegetable crops are grown over a wide variety of soils and climatic conditions. One or the other vegetable crop is grown at any time of the year, depending upon the availability of irrigation water. The important monsoon season (kharif) and summer vegetable crops are lady finger, tomato, brinjal, clusterbean, sweet potato, and cucurbitaceous crops. The important winter (rabi) vegetables are cabbage, cauliflower, kholkhol, beetroot, radish, turnip, carrot, potato, onion and leafy vegetables like lettuce and spinach. Most of the vegetable crops are of short duration and quick growing in habit and need frequent irrigations for their maximum production. The irrigation requirements will vary, depending upon the duration of the crop and the season when grown.

Vegetable crops include perennials such as asparagus (Asparagus officialis) which have extensive and deep root system and hence are draught resistant. Another perennial vegetable crop is globe artichoke (Cynara scolymus), which also has extensive root system but requires frequent irrigations throughout the year. Soil moisture deficiency, high temperature particularly during the time buds are forming, results in loose buts which are not marketable. The crop does not tolerate waterlogging. Almost all other vegetable crops are seasonal. They include potherbs or greens, including spinach, and kole which are shallow rooted. The salad crops include lettuce, celery, endive, and parsley which are relatively long season crops with shallow root system's. Soil crust on the soil surface adversely affects the emergence of seedlings of lettuce. The other group of important vegetable crops are the crucifers, namely cabbage, cauliflower, broccoli and brussels sprouts. Important root crops amongst the vegetable crops are beets, carrot, radish and turnip which are rapid growing crops which readily respond to irrigation. Bulb crops include onions and garlic. Beans and green peas are the prominent legumes amongst the vegetable crops. Solanaceous vegetable crops include tomatoes which are deep rooted with high water requirements and high yields. Cucurbits include cucumbers, muskmelon, watermelon, pumpkin and squash which are crops with medium or deep root systems.

Adequate soil moisture availability in the root zone is of prime importance in the cultivation of vegetable crops. In arid and semi-arid climates irrigation is a basic requirement in the cultivation of vegetable crops. Supplemental irrigation is common in most climatic regions, including humid and sub-humid regions. In surface irrigated crops, furrow irrigation is usually preferred. Under conditions of limited water availability both the drip and sprinkler methods of water applications are suitable, though during the recent times drip irrigation is preferred in many situations.

The water requirement of vegetable crops and the frequency of irrigation vary with the type of crop, soil and climatic conditions. However, as a general rule, applicable to most of the vegetable crops, the available soil moisture in the crop root zone should be replenished when 50% off it has been depleted. Specific characteristics of some of the common vegetable crops influencing irrigation are presented in the following pages.

Cabbage: Cabbage (Brassica oleragea) requires a cool climate for growth. The total growing period depends on the growing season. Spring season crop usually has a growing period of 90 days, while autumn crop has an extended growth period up to 200 days, depending on climatic conditions, the variety of the crop and the date of planting. Normally, the growing period ranges from 120 to 140 days. Most varieties can tolerate short periods of frost. Optimum growth occurs at mean day time temperatures around 27°C. Mean relative humidity may be in the range of 60 to 90%. In general, heavy loam soils are preferred. Under heavy rainfall conditions, sandy and sandy loam soils are preferable as they have better drainage properties.

Cabbage has an extensive shallow root system. The majority of the roots are in the top 40 to 50 cm of the root zone. The transpiration of the crop increases with the increase in the growth, with the peak towards the end of the growing season. During the vegetative period, the development of the crop is slow and the water requirements low. During the yield formation stage, which is the period of rapid growth, the availability of adequate moisture in the root zone is important. Depending on the climate, stage of development of the crop and soil type, the frequency of irrigation varies from 3 to 12 days.

Carrots: Carrots grow best in deep sandy and sandy loam soils. Heavy, compacted and cloddy soils hinder smooth development of carrots. The crop requires adequate soil moisture throughout the growing season.

Pea: Pea (Pesum sativum) is grown both for fresh and dried seed. The crop varieties range from tall, climbing plants to small bush types. Bush type plants have shorter growings-period. Pea prefers cool climate with Optimum mean daily temperatures around 17°C, with a minimum of 10°C and a maximum of 23°C. Young plants can tolerate light frost, but flowers and green pods are injured by frost. Peas are grown on well- drained soils with pH in the range of about 5.5 to 6.5. Peas have tap root systems with many lateral roots. Root depths range from 1 to 1.5 m. However, moisture uptake is usually limited to the 60 cm to 1 m depth of the rootzone. The sensitive periods for soil moisture deficits are flowering and yield formation. For high yields soil moisture depletion should not exceed 60% of the total available moisture and 40% during flowering and yield formation periods.

Radish: Radishes are very short season crops, maturing in 4 to 6 weeks. They are. grown on a variety of soils. Since the crop is very shallow rooted, frequent irrigations are required. The top 15 cm of root zone should be maintained at about 0.3 atmosphere tension.

Onion: Onion is the most important bulb crop grown under a variety of climates from temperate to tropical. The crop flourishes in mild climates without extremes of temperature and without excessive rainfall. During the initial growth period, cool weather and adequate soil moisture are advantageous for the proper establishment of the crop. During the crop ripening period, warm, dry weather is desirable for high yields of good quality bulbs. The mean daily temperature for optimum growth of the crop ranges from 15 to 20°C. The length of the growing period varies with the climate, but usually ranges from 150 to 175 days, from sowing to harvest. The crop is usually grown in the nursery and transplanted after 30 to 35 days. Direct seeding is also practiced. The optimum soil temperature for germination is 15 to 25°C. For bulb production the plant should not flower, as it adversely affects the yield. For the initiation of flowering, temperatures lower than 14 to 16°C and low humidity are desirable. Onion is sensitive to soil salinity. Optimum pH is in the range of 6 to 7. Onion, like most other vegetable crops, is sensitive to soil moisture deficits. The crop is shallow rooted with roots concentrated in the top 30 cm of the soil, requiring frequent and light irrigations. When the soil is kept relatively wet, root growth is reduced and this favours the enlargement of the bulb. Irrigation is discontinued when the crop approaches maturity.

The growth periods of onion crop are the establishment period, reckoned from sowing to transplanting which takes 30 to 35 days, the vegetative period comprising 25 to 30 days, the yield formation (bulb enlargement) period of 50 to 80 days and the ripening period of 25 to 30 days. Onion is most sensitive to soil moisture deficit during the yield formation period, particularly rapid bulb growth which occurs about 60 days after transplanting. It is also equally sensitive during the planting. For a seed crop, the flowering period is very sensitive to soil moisture deficit. However, over-irrigation will reduce growth and yield. Frequent irrigations are required to prevent cracking of the bulb and to ensure good quality bulbs.

Potato: Potato (Solanum tuberosum) is grown in most regions throughout the world, but is particularly important in the temperate climates. The optimum mean temperatures are around 18 to 20°C.

Night temperatures below 15°C is desirable for tuber initiation. Temperatures below 10°C and above 30°C adversely affect growth. Potato varieties can be grouped into early (90 to 120 days), medium (120 to 150 days and late (150 to 180 days). Early varieties are bred for temperate climates with long day lengths of 15 to 17 hrs, while late varieties result in good yields under long and short day conditions. For tropical climates varieties tolerant to short days are required.

Potato is grown in 3 or more year rotations with other crops such as maize, bean and lucerne to maintain soil productivity, to check weed growth and prevent crop damage due to insects, pests and diseases, particularly soil-borne diseases. Potato requires well drained soils with adequate aeration, with pH ranging from 5 to 6. Under irrigations, the crop is grown on ridges. The crop is moderately sensitive to soil salinity.

Potato requires adequate soil moisture for optimum growth. The available soil moisture should not be depleted beyond 30 to 50%. Soil moisture depletion more than 50% reduces the yield of the crop. Soil moisture deficits during the period of stolonization, tuber initiation and yield formation are particularly sensitive to potato. Early vegetative and ripening stages are relatively tolerant to potato. However, there is considerable variation between varieties in crop response to soil moisture availability. Potato has a shallow root system and about 70% of the total moisture uptake is from the top 30 cm of the root zone and nearly 100% from the top 40 to 60 cm of the root zone. Under irrigated conditions, irrigation scheduling should be based on avoiding soil moisture deficits during the periods of stolonization and yield formation. To obtain water savings greater level of soil moisture depletion can be allowed during the ripening period. This will also help in advancing the maturity of the crop.

Tomato: Tomato (Lycopersicon esculentum) is the second most important vegetable, next only to potato. It is a rapidly growing crop with a growing period of 90 to 150 days. The crop is neutral to day length. Optimum mean daily temperature is in the range of 18 to 25°C with night temperatures between 10 and 20°C. The crop is sensitive to frost. The crop can be grown on a wide range of soils but prefers well-drained light loam soils with pH from 5 to 7. Waterlogging induces diseases, particularly bacterial wilt.

The seed is usually grown in nursery plots and seedlings are transplanted after 25 to 35 days. Tomato is moderately sensitive to soil salinity. The most sensitive period to salinity is during germination and early vegetative periods. The plant produces flowers from bottom to top.

The growth period of tomatoes are recognized as follows: nursery stage (25 to 35 days), flowering stage (20 to 30 days), yield formation stage (20 to 30 days) and ripening stage (15 to 20 days). Highest yields are obtained by frequent, light irrigations. Tomato is most sensitive to soil moisture stress during and immediately after transplanting, during flowering and yield formation. Soil; moisture deficit during the flowering, period causes flower drop. Moderate moisture deficit during the vegetative period enhances root growth. Tomato has fairly deep root system. In deep soils roots may penetrate up to 1.5 m. About 80% of the soil moisture uptake is from the top 50 to 60 cm of the root zone soil. For high yields of good quality, tomato needs controlled supply of water throughout the growing period. However, excessive irrigation during the flowering period induces flower drop and reduces fruit set. It may also lead to excessive vegetative growth and delay in ripening. Highest demand for water is during flowering. Irrigation during and after fruit set should be limited to the extent that it will not induce stimulation of new vegetative growth at the cost of fruit development. Furrow irrigation is the most commonly adopted method of surface water application. The crop is highly, smutted to drip irrigation. With drip irrigation even moderately saline water could be used to irrigate the crop. However, tomato is not suited to sprinkler irrigation. Under sprinkler irrigation fruit set may be reduced with an increase in fruit rotting.

 
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