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.
