Efficient water management is a prerequisite to nitrogen management.
Nitrate nitrogen leaching can be minimized by matching irrigation applications to evapo-transpiration (ET) need. In cauliflower, for example, water is needed throughout the crop life, but is most effective at the onset of curd formation (Salter, 1961; Wiebe, 1981). Improved product quality is greater on soils with higher water-holding capacity, but soil type has less effect than nitrogen fertilizer (Nilsson, 1980). Increasing nitrogen from 150 to 300 kg N/ha significantly increased yield. Yields increased up to 500 kg N/ha. Polish growers add nitrogen up to 500 kg N/ha with irrigation (probably accompanied by huge leaching losses into groundwaters and associated serious pollution), and this correlated with increasing nitrate nitrogen in the curds. At these levels of application, there was a linear increase in nitrate nitrogen in cauliflower leaves and curds (Kaniszewski and Rumpel, 1998).
Cabbage is intermediately susceptible to water stress, with the head formation stage more sensitive than the preceding growth periods (Smittle, 1994). Critical periods for water stress are in the 3–4 weeks before harvest.
Yields of vegetable crops, including cabbage, are reduced when soil water tension is >25 kPa. Crops irrigated when the soil moisture tension is <25 kPa at 10 cm produced the highest total and marketable yields. This regime requires more water to be applied, but the water use efficiency rate is similar to that for cabbage irrigated at 50 and 75 kPa.
Several methods exist for measuring ET from climatic data. The modified Penman and the Jensen–Haise methods use combinations of solar radiation, temperature, humidity, wind velocity and vapour pressure measurement to estimate ET for a reference crop. This then requires a crop coefficient to adjust the values obtained for the reference crop to estimate the ET of the crop to be irrigated. The crop coefficient values (ET of the irrigated crop/ET of the reference crop) are multiplied by the ET values estimated by the specific method to estimate the ET of the irrigated crop.
Pan evaporation (Ep) incorporates the climatic factors influencing ET in a single measurement and has been used to schedule irrigation for several
crops. The single crop factor value (ET/Ep) usually results in applications of excessive irrigation during some growth phases and water deficits in others. A generalized curve was developed to describe crop factor value changes during crop growth, but the generalized curve lacks precision. Smittle (1994) have developed regression equations to calculate the daily crop factor values during the growth of several vegetables and have incorporated these equations to estimate ET from Epdata into irrigation scheduling models.
Grieve et al.(2001) determined the effect of salinity and timing of water stress on leaf ion concentration in the field. Using Pak Choi (B. rapaL. Chinensis group), Tatsoi (B. rapa L. Narinosa group), kale (B. oleracea L. Acephala group), cooking greens (B. rapa L.) and mustard greens (B. juncea L., Czerniak), the experiment used saline solutions to simulate the high-sodium and high-sulphate drainage waters typical of the San Joaquin Valley of California, USA. The mineral ion concentrations in leaves were significantly affected by increasing salinity of irrigation water. The stage at which salinity was applied, however, had little effect. With increasing salinity, calcium ions and potassium ions decreased in the leaves of all species, whereas sodium ions and total sulphur significantly increased. Magnesium also rose in the leaves of brassicas with increasing salinity;
there was also an increase in chloride ion content. The use of moderately saline irrigation water did not adversely affect crop quality as rated by colour, texture and the mineral nutrient content available to consumers.
In many areas of the world, water is now the most valuable and scarce resource, and this shortage is set to become more acute. Areas of intensive vegetable production include parts of: the USA, i.e. California and Florida;
southern Europe, i.e. southern Spain, Portugal, Italy and Greece; and the Middle East, i.e. Israel and Egypt. In each of these areas, irrigation is an essential ingredient for Brassicacrop production. The yield response gained by the use of irrigation exceeds 200%. Water used for irrigation on crops, parks and golf courses accounts for 80% of consumption in the USA for example.
Irrigated vegetable production accounts for 1.9 Mha (7.5% of the irrigated area), and Arizona, California, Florida, Idaho, Nebraska, Oregon, Texas, Washington State and Wisconsin account for 80% of this production.
Originally, irrigation was supplied through surface and seepage systems, and these are currently employed on 45% of crops with a water use efficiency of 33%. Sprinkler or overhead systems were developed in the 1940s. At present, they are used on 50% of irrigated land with a water use efficiency of 75%. Since the late 1960s, microirrigation using drip or trickle methods has been developed. Currently these are used on only 5% of US field vegetables, but this is likely to increase substantially as their water use efficiency is between 90 and 95% (Locascio, 2005). As water becomes increasingly scarcer and more expensive, methods that increase the efficiency of its use will become ever more important.
Worldwide, growers using water for irrigation will be forced to confront the problems of decreased supply and increasing salinity in their irrigation
water and the implications of this for the quality of vegetables made available to the consumer. The consumer is becoming ever more conscious of the health risks associated with nitrate nitrogen loading, especially in leafy vegetables of which brassicas comprise a major portion. One of the on-farm management options is the re-use of agricultural drainage effluents. This strategy is especially attractive because significant amounts of good quality water are preserved and also because the volumes of drainage water that ultimately need to be disposed of are substantially reduced (Sorenson, 2000).
In the suggested drainage water re-use system proposed for the San Joaquin Valley of California, USA, selected crops would be grown and irrigated in sequence, starting with the very salt-intolerant species (V. Rubatsky, personal communication, San Joaquin Valley Drainage Program, 1990).
Drainage effluents from these crops would be used to irrigate crops of higher salt tolerance. At each step in the sequence, the drainage water becomes progressively more salty. The composition of the drainage effluent waters in this region is typically a mixture of salts with sodium > sulphate > chloride >
magnesium > calcium predominating in that order.
Members of the Brassicaceae are relatively tolerant of salinity (see Chapter 1). Even with these crops, however, abnormally high levels of salinity severely limit plant growth. It is essential that brassicas are able to form large vigorous root systems. This requires sufficient oxygen in parallel with adequate water supply. Mylavarapu et al. (2005) identified the importance of subsoiling which allows adequate soil aeration for collard (B. oleracea var.
acephala) in the southeastern states of the USA.
Leafy vegetables are the primary source of mineral nutrients for human diets. Numerous tables of food composition list the major constituents of vegetables and give values for predominant mineral ions (see Table 5.13).
These values are only estimates in so far as the data are based on a limited number of samples and vary due to biological and environmental factors such as maturity, analytical procedures and processing. In addition, the availability, uptake and partitioning of mineral ions within the plant are controlled by numerous environmental factors including the concentration and compo- sition of solutes in the soil solution. Under saline conditions, mineral ion interactions in the external growing medium may affect the internal requirements of elements essential for plant growth and development. These imbalances often influence the growth and nutrition of the crop, which in turn may affect crop quality in terms of colour, texture and nutritive value.
Calcium plays a vital nutritional and physiological role in plant metabolism (see Chapter 8). Under saline conditions, ion imbalances in the substrate or plant may adversely affect calcium nutrition. Substrate levels of calcium that are adequate for plant requirements under non-saline conditions may be nutritionally inadequate and growth limiting when the plant is salt stressed. The calcium status of the plant is strongly influenced by the ionic composition of the external medium in that the presence of other
salinizing ions in the substrate may reduce calcium activity and limit the availability of calcium to the plant. Cations such as sodium and magnesium may disrupt calcium acquisition, uptake and transport. Leaf calcium concentrations in all vegetables tended to decrease as salinity increased despite added quantities being available in the soil. Salinity-induced calcium deficiency can result in physiological disorders in brassica vegetables such as internal tipburn, browning and necrosis of the inner leaves (Chapter 8).
Under conditions of salinity stress, maintenance of adequate levels of potassium is essential for plant survival. High levels of external sodium not only interfere with potassium acquisition through the roots but may also disrupt the integrity of root membranes and alter the selectivity of the root system for potassium as compared with sodium.
Total sulphur content in the leaves of all brassica vegetables increased as sulphate values rose. The brassicas are particularly active sulphur accumulators. Members of the Brassicaceae are among the 15 plant families which biosynthesize significant quantities of sulphur-rich glucosinolates.
Hydrolysis of these compounds yields ‘mustard oils’ that impart the characteristic spicy tastes to these vegetables. Increases in total sulphur accumulation in response to irrigation with moderately saline, sulphur- dominated waters may enhance the flavour, provide benefits to human health and raise the acceptability of brassica vegetables to the consumer.
Table 5.13. Water content and mineral ion concentration (mg/100 g edible portion) of selectedBrassicacrops irrigated with saline water (11 d/Sm) compared with non-saline conditions.a
Water Refer-
Vegetable (%) Ca Mg Na K P S Cl ence
Tatsoi 93.7 123 26 236 323 24 63 208 1
Mustard greens
Vitamin 95.0 106 20 220 262 23 48 490 1
Red Giant 94.1 116 24 61 419 23 52 118 1
89.3 181 29 33 374 46 2
92.6 138 18 220 32 3
Kale 89.0 306 91 222 476 53 175 246 1
89.7 202 45 43 420 55 2
91.2 134 43 221 46 3
Pak Choi 94.0 118 24 248 426 24 53 192 1
93.8 118 19 71 234 39 2
aData from crops grown with saline water are underlined.
1 = Grieve et al. (2001); 2 = Rubatsky and Yamaguchi (1997); 3 = Ensminger et al. (1995).