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Design of a geographic information supported database for the management of pressurised irrigation systems at the plantation du Haut Penja, Cameroon

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par Chick Herman AZAH
University of Dschang - Agric engineer 2009
  

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2.1.2 Ecology

Banana is cultivated principally in the tropics. The major banana-growing regions of the world are situated between the equator and latitudes 20°N and 20°S. The crop has a high water demand and is sensitive to low temperatures and wind. The principal environmental factors which affect the growth of banana are:


· Water

Availability of water is one of the critical factors that determine where bananas should be grown. It is generally considered that bananas require a weekly precipitation of 30-40 mm of rainfall or 1500-2000 mm annually for optimum growth. There is however, overwhelming evidence worldwide (except in parts of the humid tropics) to support the need for supplementing irrigation of bananas as rainfall distribution is seasonal and erratic.

With respect to the use of water, the banana plant has a number of important characteristics (Swennen and Vuysteke, 2001):

- A high evapotranspiration rate due to large broad leaves and large total surface area. Maximum evapotranspiration is estimated at between 5-6 mm/day.

- A shallow superficial root system compared with most tree-fruit crops. In general,
100 % of water is obtained from the first 0.5-0.8 m with 60 % from the first 0.3 m.

- A poor ability to withdraw water from a soil with low moisture content. A depletion of 35 % (management allowable deficit) of the total available water should thus not be exceeded.

- A rapid physiological response to soil water deficit especially in conditions of high evaporative stress. Robinson & Alberts (1987), found that, after 6 days without water, when tensiometers showed 25 kPa, the level of photosynthesis on banana plants reduced by 19 % compared with well-watered plants. When tensiometers showed 70 kPa, the level of PS was reduced by 80 %. At this stage, external wilting symptoms are clearly visible.

· Temperature

The rate of banana growth and development is determined by temperature. On the basis of the mean daily temperatures (maximum + minimum /2), the optimum mean for photosynthesis and flower initiation is 22°C, whereas the optimum mean for plant development and leaf emergence is 31°C (Turner & Lahav, 1983; Robinson & Anderson, 1991). Mean temperature balance required for growth (assimilation) and development (leaf emergence) is 27°C.

· Soils

According to Delvaux (1995), soil physical factors important for banana are porosity and mechanical impedance (compaction), aeration and natural drainage (water logging), water-holding capacity and soil temperature. Plantation longevity and sustained high production is dependent on porous, loose soils which allow unimpeded root extension. Banana root density is thus inversely related to soil bulk density. When using a penetrometer, the soil strength should not exceed 1500 kPa down to 800 mm depth. Studies in waterlogged soils, in South Africa indicate drains should be dug between rows of bananas to assist in the removal of excess water from about 12 m on both sides of the drain,

thus a deep drain every eight banana rows will be a useful insurance policy (Robinson and De Villier, 2007). Minimum soil temperatures of 10°C to 15°C are severely restricting on banana root extension. Hence, the slope aspect which conditions the field exposure to sunlight as well as the planting density will affect soil temperature.

Sandy Clay soils are best for bananas because there is a good balance between the water-holding capacity and the cation exchange capacity on the one hand, and increased aeration, water infiltration and drainage on the other hand. Optimum soil texture should be about 30% clay, 10% silt and 60% sand. The texture thus determines the total available water (TAW). The TAW is expressed in mm water/m soil depth. Light sandy soils will therefore require more frequent watering to maintain field water capacity than will do loam or clay soils.

Soil chemical aspects such as soil acidity and salinity are equally important for plant growth and in irrigation management (Robinson and De Villier, 2007). For optimum plant growth, the soil pH measured in water should be between 5.8 and 6.5. Salinity is usually only a problem in Mediterranean climates, which have saline soils, low rainfall and use poor irrigation water. Thus, soils with electrical conductivities of less than 1mmho/cm are required for good growth.


· Wind

Wind causes different types of damage in banana plantation. At wind velocities of more than 70 km/h, between 50-100% of the plants can be blown down. Winds modify the physiological functioning of the banana plant through its effect on the boundary layer of moist, undisturbed air adjacent to the leaf surface, and by its effect on leaf temperature. If the wind speed is high and humidity low, the boundary layer quickly disperses, leaf temperature rise, stomata close and the plant suffers physiological stress. Propping is usually employed in supporting banana and windbreak and hedges to prevent wind damage.

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