Common bean (Phaseolus vulgaris) is known under different names (French bean, kidney bean, snap bean, runner bean, string bean). It can be grown as a vegetable crop for fresh pods or as- a pulse crop for dry seed. World production of dry beans is about 16.7 million tons from about 23 million ha and green beans 4.7 million tons from 0.7 million ha (FAOSTAT, 2001).

Common bean grows well in areas with medium rainfall, but the crop is not suited to the humid, wet tropics. Excessive rain and hot weather cause flower and pod drop and increase the incidence of diseases. Optimum mean daily temperatures range between 15 and 20°C. The minimum mean daily temperature for growth is l0°C, the maximum 27°C. High temperatures increase the fibre content in the pod. Germination requires a soil temperature of 15°C or more, and at 18°C germination takes about 12 days, and at 25°C about 7 days. Most bean varieties are not affected, by daylength. The length of the total growing period varies with the use of the product and is 60 to 90 days for green bean and 90 to 120 days for dry bean.

The crop does not have specific soil requirements but friable, deep soils with pH of 5.5 to 6.0 are preferred. Fertilizer requirements for high production are 20 to 40 kg/ha N, 40 to 60 kg/ha P and 50 to 120 kg/ha K. Bean is capable of fixing nitrogen which can meet its requirements for high yields. However, a starter dose of N is beneficial for good early growth. The crop is sensitive to soil-borne diseases and should be grown in a rotation; in the subtropics in the USA wheat, sorghum, onion and potato are common rotation crops, whereas in tropical Africa and Asia maize, sweet potato and cotton are common.

Normal sowing depth is about 5 to 7 cm. Spacing depends on variety. Bush types (erect) normally have a plant and row spacing of 5 to 10 x 50 to 75 cm, while pole-type (climbing) are 10 to 15 x 90 to 150 cm. Pole beans are also often grown on hills spaced 90 to 120 cm apart. Other spacings are possible, and these depend on the method of harvest.

Common bean is sensitive to soil salinity. The yield decrease at different levels of ECe is:0% at ECe 1.0, 10% at 1.5, 25% at 2.3, 50% at 3.6 and 100% at ECe 6.5 mmhos/cm.

The graph below depicts the crop stages of bean, and the table summarises the main crop coefficients used for water management.

Water requirements for maximum production of a 60 to 120-day crop vary between 300 and 500 mm depending on climate. The water requirement's during the ripening period depend very much on whether the pod is harvested wet or dry. When grown for its fresh product, the total growing period of the crop is relatively short and during the ripening, which is about 10 days long, the crop evapotranspiration is relatively small because of the drying of the leaves. When the crop is grown for seed the ripening period is longer and the decrease in crop evapotranspiration is relatively greater. The growing period depends on the number of pickings, and when 3 or 4 pickings are taken the harvest period is 20 to 30 days. Crop coefficient (kc) relating reference evapotranspiration (ETo) to water requirements (ETm) for different development stages is, for common bean, green: during the initial stage 0.3-0.4 (15 to 20 days); the development stage 0.65-0.75 (15 to 20 days); the mid-season stage 0.95-1.05 (20 to 30 days); the late-season stage 0.9-0.95 (5 to 20 days) and at harvest 0.85-0.9. For common bean, dry, the kc value is: during the initial stage 0.3-0.4 (15 to 20 days); the development stage 0.7-0.8 (15 to 20 days); the mid-season stage 1.05-1.2 (35 to 45 days); the late-season stage 0.65-0.75 (20 to 25 days); and at harvest 0.25-0.3.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period are shown in the figure below.

This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.

Water supply needed for maximum yield for both fresh and dry produce is similar during much of the growing period but varies during the ripening period. For green beans supply is continued just prior to the last picking, but for dry bean it is discontinued about 20 to 25 days before crop harvest. With one picking only the harvest period is concentrated. This can be achieved to some extent by the timing of water supply so as to induce a slight water deficit to the crop during the ripening period and a soil water depletion to about 50 percent of the total available water may hasten the onset of maturity. Concentration of the harvest period is more easily achieved for bush than for pole types. The former normally have a more uniform ripening period.

However, a severe water deficit during the vegetative period (1) generally retards plant development and causes non-uniform growth.

During flowering (2) and yield formation (3) frequent irrigation results in the highest 'response to production, although excess water increases the incidence of diseases, particularly root rot. When nitrogen is supplied to the crop in the form of mineral fertilizer, irrigation should be accompanied by adequate application of nitrogen fertilizer in order to maximize yield.

When water supply is limited, some water savings could be made during the vegetative period (1) and, for dry bean, also. during the ripening period without greatly affecting yield, provided water deficits are moderate. Total production is higher when full crop water requirements are met over a limited area then when the cultivated area is extended under limited supply conditions.

The tap root of the bean plant may reach a depth of 1 to 1.5 m. The lateral root system is extensive and is mainly concentrated in the first 0.3 m. At emergence, the rooting depth is about 0.07 m, at the start of flowering 0.3 to 0.4 m, and at maturity 1 to 1.5 m. Water uptake occurs mainly in the first 0.5 to 0.7 m depth (D = 0.5-0.7 m). Under conditions when ETm is 5 to 6 mm/day, 40 to 50 percent of the total available soil water can be depleted before water uptake is affected (p = 0.4-0-5).

When the bean crop is grown with supplemental irrigation, water supply should be directed toward meeting water requirements during the establishment period (0) and the early part of the flowering period (2). When the crop is grown under full irrigation, the soil water depletion during the flowering (2) and yield formation (3) periods should not exceed 40 to 50 percent of the total available soil water (p = 0-4-0.5). When the crop is grown for dry seed the depletion level during the ripening period (4) should not exceed 60 to 70 percent. Water stress in the plant can be detected by eye because the leaves turn dark bluish-green in colour.

Water deficit during the yield formation period (3) gives rise to small, short discoloured pods with malformed beans. Also, the fibre content of the pods is higher and seeds lose their tenderness. Good commercial yield in favourable environments under irrigation is 6 to 8 ton/ha fresh and 1.5 to 2 ton/ha dry seed. The water utilization efficiency for harvested yield (Ey) for fresh bean containing 80 to 90 per-cent moisture is 1.5 to 2.0 kg/m3 and for dry bean containing about 10 percent moisture, 0.3 to 0.6 kg/m³.