AGROARTICLES

Drip irrigation refers to frequent application of small quantities of water on or below the soil surface as drops, tiny streams through emitters of pre-determined discharge placed along a water delivery line i.e., lateral or emitting pipe. It embodies the philosophy of irrigating the plant (root zone) instead of entire land, as done in conventional surface irrigation methods. It consists of a head control unit, water carrier system and water distribution system.

Advantages
Adoption of drip fertigation in banana is technically feasible, economically viable and
beneficial in many ways:
1. Better establishment of suckers/plantlets and early vigorous growth,
2. Uniform flower initiation and shooting fruit development
3. Improved fruit development contributing to increased hands/bunch, fingers/hand and
bunch weight,
4. Earliness and uniformity in harvesting leading to reduction in crop cycle’s duration (Plant crop + 2 rattoon in 30 months)
5. Successful utilization of saline water for irrigation due to micro-leaching effect in the wetted volume.
6. Saving in water up to 51.8% (Fig. 16) (ICID, 1994) contributing to higher water productivity Efficient use of fertilizers due to fertigation (Fig. 17) (Rasker, 2003)
7. Energy conservation (2434.42 kwh/ha) over surface method,
8. Improved weed control and saving in labor due fewer plant protection & harvesting
operations,
9. Less leaf spot disease incidence and
10. Higher yield (Fig.18) and fruit quality viz., weight, size and color (INCID, 1994; Young
et.al., 1985)

Drip design configurations
As a guideline the key design parameters followed in different countries for adoption of drip irrigation system in banana plantations are presented below in this Table:
Table: Drip design guidelines for Banana in different countries

Banana bunch weight and bunch yield as influenced by number of laterals per row is
presented in Fig. 20 (Lahav & Lowengart, 1998). The data clearly reveals that 3 drip lines per row is significantly superior over two Driplines per row.

Water management:

Rooting characteristics
a) Roots constitute the link between the plant and the soil thereby providing anchorage and guaranteeing nutrient and water uptake.
b) Root system is adventitious and fleshy from the beginning.
c) A healthy rhizome may produce about 500 – 1000 primary roots
d) The primary roots are about 5 to 8 mm in thickness and are white when new and healthy. Later they turn grey or brown before eventually dying.
e) From each primary root a system of secondary and tertiary roots develop
f) The proportion of secondary and tertiary roots in bananas was found to be 22% and 77%, respectively.
g) Functional life of Cavendish subgroup primary roots was estimated to be from four to six months and that of secondary and tertiary roots was about eight weeks and five weeks respectively.
h) Towards flowering, new primary root emergence from the parent rhizome ceases and
sucker roots become predominant.
i) The vertical root zone is very shallow with about 40% of the root volume in the top 10cm and 85% in the top 30cm.
j) Spatial (vertical and horizontal) root distribution of banana is shown in Fig. 22 (Araya,
2003)
k) Root distribution, both horizontally and vertically, is strongly influenced by:
• Soil depth, soil texture, soil compaction; and internal drainage
• SAR and ECe
• High water table, frequent water logging & flooding
• Phytopathological factors.

Effective crop root zone depth
One of the essential pre-requisites for scientific irrigation scheduling is knowledge of
effective root zone depth of the banana crop. The term “effective” could be described as the depth within which approximately 80% of the feeder roots are located. It is also the depth from which the crop meets its 90% of the water requirements and it is the depth considered for calculating irrigation water requirements. Irrigation water depth when scheduled below this rooting depth can result in wastage of both water as deep percolation and leaching of nutrients.
Nearly 88% of the roots exposed, were naturally located within 30 cm of the soil surface and 97% within 40cm. It is recommended; therefore, that even under conditions allowing unimpeded vertical root distribution, banana irrigation should be scheduled to wet only 30cm of soil depth, but to ensure that within this zone, the soil does not dry out beyond 25% depletion of available soil moisture. This correlates closely with water extraction patterns in that 87% of total water extracted by roots came from the same vertical zone. In case of drip irrigation where roots are concentrated in to a confined
wetting pattern, which is predetermined according to soil type, more frequent (daily) irrigations are required to prevent excessive drying of these concentrated (bulb) root zones.
Root distribution both horizontally and vertically, however, is strongly influenced by soil
type, compaction and drainage. Heavy, compact or poorly drained soils severely limit root extension and yields are depressed accordingly. Conversely, lighter soils which are well drained and which have been ploughed to below 50 cm, induce more and healthier roots, and there is a good correlation between bunch mass and root volume. A banana adventitious root system is very spreading, and horizontal extension of primary roots is commonly between 1 to 2 m.

Water supply and crop yield
1. Banana requires an ample and frequent supply of water; water deficits adversely affect crop growth and yields. The establishment period and the early phase of the vegetative period determine the potential for growth and fruiting and adequate water and sufficient nutrient supply is essential during this period.
2.. Water deficits in the vegetative period affect the photosynthetic rate (Eckstein,
1994) and rate of leaf development, which in turn can influence the number of flowers in
addition to the number of hands and bunch production.

Water stress effect on banana photosynthetic rate under field conditions
3. The flowering period starts at flower differentiation, although vegetative development can still continue. Water deficits in this period limit leaf growth and number of fruits.
Water deficits in yield formation period affect both the fruit size and quality (poorly filled
fingers). A reduced leaf area will reduce the rate of fruit filling; this leads, at harvest time, to bunch’s being older than they appear to be, consequently the fruits are liable to premature during storage.
4. Regular water supply under drip irrigation produces taller plants, with greater leaf area, and results in earlier shooting and higher yields. Interval between irrigation has a
pronounced effect on yields, with higher yields being achieved when intervals are kept
short as in drip irrigated crop.
5. under conditions of limited water supply, total production will be higher when full crop
water requirements are met over a limited area than when crop water requirements are
partially met over an extended area.
6. the ratio between relative yield decrease and relative evapotranspiration deficit is 1.2 to 1.35, with little difference between different growth periods (Doorenbos and Kassam,
1979).

Irrigation scheduling & Crop water requirement

The goal of an efficient irrigation scheduling programmed is to “provide knowledge on
correct time and optimum quantity of water application to optimize crop yields with maximum water use efficiency and at the same time ensure minimum damage to the soil”.
Thus,
1. Irrigation scheduling is the decision of when and how much water to apply to a cropped field.
2. Its purpose is to maximize irrigation efficiencies by applying the exact amount of water needed to replenish the soil moisture to the desired level.
3. Make efficient use of water and energy. Therefore, irrigation scheduling for bananas involves accurate calculations of the amount of water to be applied at each irrigation, and the interval between irrigation, for each soil-plant-climate combination. The banana is a tropical herbaceous evergreen plant which has no natural dormant phase and which has a high water demand throughout the year, especially at high temperatures. In this respect the important characteristics of the banana plant are:
a) A high transpiration potential due to the large broad leaves and high LAI
b) A shallow root system compared with most tree fruit crops
c) A poor quality to withdraw water from soil beneath field capacity
d) A rapid physiological response to soil water deficit These factors make banana plants extremely sensitive to even slight variations in soil water content, emphasizing the importance of correct irrigation scheduling. With drip irrigation, intervals of irrigation are usually daily irrespective of pan evaporation (Epan), or even in pulses several times per day.

The crop evapotranspiration under standard conditions, denoted as Etc, is the
Evapotranspiration from disease free, well-fertilized banana crop, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic condition. The amount of water required to compensate the evapotranspiration loss from the cropped field is defined as crop water requirement. Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost in evaporation
+ transpiration. The irrigation water basically represents the difference between the crop water requirement and effective precipitation. The irrigation water requirement also includes additional water for leaching of salts and to compensate for non-uniformity of water application.
The crop water requirement for scheduling irrigation is calculated according to the
following formula:

Crop ETc = (Epan x Kpan) Kc

Crop ETc = ETo x Kc

Where,
Crop ETc = Water requirement (mm/day)
Crop ETo = Reference crop evapotranspiration (mm/day)
Epan = Evaporation from USWB Class A Pan evaporimeter (previous day)
Kpan = Pan coefficient
Kc = Experimentally derived Crop factor

The Epan from an USWB Class A Pan evaporimeter reflects the evaporative demand of
the atmosphere for the location in question. While the crop factor (a dimensionless ratio) indicates the combined loss of water from a banana plantation both by transpiration and soil evaporation
(Crop ETc) relative to that lost by evaporation from the USWB Class A Pan evaporimeter over the same period. Experimental estimates of crop factors for different crop growth stages have been worked out gravimetrically with soil samples, volumetrically with drainage lysimeters or physiologically by measurement of transpiration loss by several workers. The daily requirement in millimeters is converted to the equivalent volumetric quantity for the area under drip (1 mm = 10m3/ha). A field irrigation schedule prepared based on crop coefficient approach for irrigating banana grown in Tropical conditions of India (Sample).

Water requirement/ha
Month
2.550 0.60 1.530 15.30 474.30
August 3.8 0.75 2.850 0.70 1.995 19.95 618.45
September 3.5 0.75 2.625 0.80 2.100 21.00 630.00
October 4.3 0.75 3.225 0.85 2.741 27.41 849.71
November 4.0 0.75 3.000 1.00 3.000 30.00 900.00
December 3.3 0.75 2.475 1.10 2.722 27.22 843.82
January 3.7 0.75 2.775 1.10 3.052 30.52 946.12
February 5.0 0.75 3.750 1.10 4.125 41.25 1155.00
March 6.1 0.75 4.575 0.90 4.117 41.17 1276.27
April 6.6 0.75 4.950 0.80 3.960 39.60 1188.00

Crop water requirements
• Requires large quantities of water for maximum productivity
• Depending on prevailing climatic conditions seasonal water requirements of banana range from 1200 to 2690 mm from planting to harvest (Robinson and Alberts, 1989)
• In semiarid Carnarvon, Western Australia the annual banana irrigation requirement was estimated at 2000mm (annual evaporation=2580mm and rainfall= 227mm)
• Daily water requirements vary in the range of 3 – 6 mm/day depending on the combination of LAI, temperature, humidity, radiation & wind (Stover & Simmonds, 1987)
• In the tropics, maximal Kc (ETc/Epan) is high and values range from 1.28 to 1.4 (Israeli and Nameri, 1987)
• In the subtropics, maximal summer Kc is somewhat lower at 0.8 to 1.0, decreasing to 0.6 in winter (Robinson and Alberts, 1989).

Drip irrigated banana
• In Hawaii, India and Israel drip irrigation is reported to be the most successful system for bananas (Young et.al., 1985; Hegde and Srinivas, 1989; Lahav and Kalmar, 1981)
• In Israel banana production is entirely dependent on drip irrigation @ 1050 mm/year in
Western Galilee and 1500 mm/year in the Jordan Valley) since the annual rainfall of 400 – 600 mm falls entirely in winter when no growth occurs.
• Leaf emergence rate was slower in sprinkler irrigated crop in comparison to drip irrigated plants, leading to extended vegetative cycle duration (Fig. 25) (Robinson and Alberts,1987).

• Drip irrigated banana yield and yield components grown under semi-arid conditions were shown to be positively correlated to pan factor that ranged from 0.25 to 1.25 (Fig. 26) (Goenaga and Irrizary, 1995, 1998 & 2000) and 0.2 to 1.8 (Young et.al., 1985).
• Drip irrigated bananas in Hawaii produced double the yield when compared to well
managed sprinkler irrigated banana-plantation (Young et.al., 1985). Likewise in India drip irrigated banana based on pan factor 0.4 to 1.2 gave yield improvement of 52.2% over surface irrigated crop (Singh, 2002).

Fertigation:
One of the most important challenges faced by the banana farmers today is to provide
crops with optimal quantity of water and nutrients in the most cost – efficient manner possible. For intensive crop production, the best answer to this challenge is “Fertigation”, fertilization via the irrigation system. Fertilization via drip irrigation system besides irrigation is the most important management factor through which farmers control plant development, fruit yield and quality. The introduction of simultaneous irrigation and fertilization (Fertigation) opened up new possibilities for controlling water and nutrient supplies to crops and maintaining the desired concentration and distribution of ions and water in the soil. Thousands of banana farmers around the world including Israel, India, Philippines, Brazil, Australia, South Africa, Hawaii, Puerto Rico etc have already learnt to appreciate the advantages of precise water supply to crops through investment in a drip irrigation system. In the last few years a rapidly growing number have also realized that once a drip irrigation system is installed, it is easy to achieve its full benefits through the next natural step – Fertigation. Fertigation ensures that
essential nutrients are supplied precisely at the area of most intensive root activity.

Potential advantages of fertigation
The main advantages of drip fertigation over drip irrigation combined broadcast or banding fertilization can be summarized as follows:
a) Reduced time fluctuation in nutrient concentrations in soil in the course of the growing season, because of the flexibility in delivery of nutrients and water. Theoretically when curves of yield response to nutrients and water are convex and monotonic, time fluctuations of nutrient and water contents in soil cause reduction in yield. The attenuated fluctuations under microfertigation therefore ensure higher and more consistent yields relative to broadcast fertilization.
b) Easy adaptation of the amounts and concentrations of specific nutrients to crop
requirements, according to the stage development and climatic conditions
c) Convenient use of compound, ready-mix and balanced liquid fertilizers with minute
concentrations of minor elements that are otherwise very difficult to apply accurately to the field.
d) The crop foliage remains dry, thus retarding development of plant pathogens and avoiding leaf burn.
e) Precise application of nutrients according to crop demand, thus avoiding excess fertilizer concentrations in the soil and leaching out of the wetted soil root volume
f) Application of water and fertilizers to only a part of the soil volume; the addition of nutrients. Only to wet area, where active roots are concentrated, enhances fertilizer use efficiency (> 85%) and reduces leaching of nutrients to deep underground water by seasonal rains.
g) It is unaffected by wind and causes less runoff than overhead sprinkler irrigation
h) It reduces labor requirement and heavy tractor traffic in the field, associated with the
broadcasting of fertilizers, and allows easy application of nutrients via the water when topdressing is expensive because of plant height

Fertigation methods & equipment
There are two major methods of fertigation viz., quantitative and proportional, which are
Chosen according to soil type and equipment availability.

Quantitative fertigation:
In quantitative method the fertilizer is mixed with irrigation water, using a bypass fertilizer tank, according to the plant needs at different growth stages. This method
is widely used in fruit crops and medium to heavy soils, which have a considerable ability to store water and nutrients applied. The factor controlled by the grower is the total amount of fertilizer, rather than the exact concentration. Fertilizer concentration in water decreases gradually during the irrigation session. Therefore the application is expressed in terms of kg/ha/day (or week). This method requires the use of fertilizer tank.

Quantitative fertigation by fertilizer tank:
This method is cheap and simple and involves low maintenance costs. Both dry (water
soluble) and liquid fertilizers can be used. It enables high discharge rate. However, the fertilization through bypass tank is not proportional and ability to automatically control the discharge rate is rather limited. This system works on existing system pressure only, no additional power is required for running the fertigation tank. The pre-determined water soluble fertilizer is placed in the tank, throttle valve is opened sufficiently and water flow passing through the tank creates turbulent motion inside the fertilizer tank. Allow fertigation for 2/3 of the irrigation period, this will leave the system clear of fertilizer and prevents clogging problem if any.

Proportional fertigation:
The most widely used equipment for proportional fertigation is the fertilizer injector.

Proportional fertigation by fertilizer injector
As the water flows through the tapered venturi orifice, the increases velocity causes lower pressure (partial vacuum), which draws fertilizer stock solution into the system. The main advantage of this method is that the actual fertilizer concentration in the irrigation water can be set to the optimum level. This is a special virtue for sandy and light soils and for soil less media. The main disadvantages are very high head (pressure) loss and relatively low discharge rate. The irrigation system should be operating at full capacity prior to injecting the fertilizer solution.

G. Levy / Banana Manual / Feb 2006.©