Drip Irrigation And Fertigation Systems In Greenhouse Cultivation

By TamilNadu Agricultural University on 05 May 2016 | read

The plant is required to take up very large amounts of water and nutrients, with a relatively small root system, and manufacture photosynthates for a large amount of flower per unit area with a foliar system relatively small in relation to required production.

Watering system
Micro irrigation system is the best for watering plants in a greenhouse. Micro sprinklers or drip irrigation equipments can be used. Basically the watering system should ensure that water does not fall on the leaves or flowers as it leads to disease and scorching problems. In micro sprinkler system, water under high pressure is forced through nozzles arranged on a supporting stand at about 1 feet height. This facilitates watering at the base level of the plants.

Equipments required for drip irrigation system include
i) A pump unit to generate 2.8kg/cm2 pressure
ii) Water filtration system – sand/silica/screen filters
iii) PVC tubing with dripper or emitters

Drippers of different types are available
i)  Labyrinth drippers
ii) Turbo drippers
iii) Pressure compensating drippers – contain silicon membrane which assures uniform flow rate for years
iv)  Button drippers- easy and simple to clean. These are good for pots, orchards and are available with side outlet/top outlet or micro tube out let
v) Pot drippers – cones with long tube

Water out put in drippers
 a. 16mm dripper at 2.8kg/cm2 pressure gives 2.65 litres/hour ( LPH).
 b. 15mm dripper at 1 kg/cm2 pressure gives 1 to 4 litres per hour

Filters: Depending upon the type of water, different kinds of filters can be used.

Gravel filter: Used for filtration of water obtained for open canals and reservoirs that are contaminated by organic impurities, algae etc. The filtering is done by beds of basalt or quartz.

Hydrocyclone: Used to filter well or river water that carries sand particles.

Disc flitersL: Used to remove fine particles suspended in water

Screen filters: Stainless steel screen of 120 mesh ( 0.13mm) size. This is used for second stage filtration of irrigation water.

Fertigation system
In fertigation system an automatic mixing and dispensing unit is installed which consists of three systems pump and a supplying device. The fertilizers are dissolved separately in tanks and are mixed in a given ratio and supplied to the plants through drippers.

Fertilizer dosage has to be dependent on growing media. Soilless mixes have lower nutrient holding capacity and therefore require more frequent fertilizer application. Essential elements are at their maximum availability in the pH range of 5.5 to 6.5. In general Micro elements are more readily available at lower pH ranges, while macro elements are more readily available at pH 6 and higher.

Forms of inorganic fertilizers
Dry fertilizers, slow release fertilizer and liquid fertilizer are commonly used in green houses.

Slow release fertilizer
They release the nutrient into the medium over a period of several months. These fertilizer granules are coated with porous plastic. When the granules become moistened the fertilizer inside is released slowly into the root medium. An important thing to be kept in mind regarding these fertilizers is that, they should never be added to the soil media before steaming or heating of media. Heating melts the plastic coating and releases all the fertilizer into the root medium at once. The high acidity would burn the root zone.

Liquid fertilizer
These are 100 per cent water soluble. These comes in powdered form. This can be either single nutrient or complete fertilizer. They have to be dissolved in warm water.

Fertilizer Application Methods
1. Constant feed
Low concentration at every irrigation are much better. This provides continuous supply of nutrient to plant growth and results in steady growth of the plant. Fertilization with each watering is referred as fertigation.

2. Intermittent application
Liquid fertilizer is applied in regular intervals of weekly, biweekly or even monthly. The problem with this is wide variability in the availability of fertilizer in the root zone. At the time of application, high concentration of fertilizer will be available in the root zone and the plant immediately starts absorbing it. By the time next application is made there will be low or non existent. This fluctuation results in uneven plant growth rates, even stress and poor quality crop.

Fertilizer injectors
 This device inject small amount of concentrated liquid fertilizer directly into the water lines so that green house crops are fertilized with every watering.

Multiple injectors
 Multiple injectors are necessary when incompatible fertilizers are to be used for fertigation. Incompatible fertilizers when mixed together as concentrates form solid precipitates. This would change nutrient content of the stock solution and also would clog the siphon tube and injector. Multiple injectors would avoid this problem. These injectors can be of computer controlled H.E. ANDERSON is one of the popular multiple injector.

Fertilizer Injectors
Fertilizer injectors are of two basic types: Those that inject concentrated fertilizer into water lines on the basis of the venturi principle and those that inject using positive displacement
A. Venturi Principle Injectors
1.Basically these injectors work by means of a pressure difference between the    irrigation line and the fertilizer stock tank.
a) The most common example of this is the HOZON proportioner.
b) Low pressure, or a suction, is created at the faucet connection of the Hozon at the suction tube opening. This draws up the fertilizer from the stock tank and is blended in to the irrigation water flowing through the Hozon faucet connection.
c) The average ratio of Hozon proportioners is 1:16. However, Hozon proportioners are not very precise as the ratio can vary widely depending on the water pressure.
d) These injectors are inexpensive and are suitable for small areas. Large amounts of fertilizer application would require huge stock tanks due to its narrow ratio.

B. Positive displacement injectors.
1. These injectors are more expensive than Hozon types, but are very accurate in proportioning fertilizer into irrigation lines regardless of water pressure.
2. These injectors also have a much broader ratio with 1:100 and 1:200 ratio being the most common. Thus, stock tanks for large applications areas are of manageable size and these injectors have much larger flow rates.
3. Injection by these proportioners is controlled either by a water pump or an electrical pump.
4. Anderson injectors are very popular in the greenhouse industry with single and multiple head models.
            a.Ratios vary from 1:100 to 1:1000 by means of a dial on the pump head for feeding flexibility.
            b.Multihead installations permit feeding several fertilizers simultaneously  without mixing. This is especially significant for fertilizers that are incompatible (forming precipitates, etc.) when mixed together in        concentrated    form.
5. Dosatron feature variable ratios (1:50 to 1:500) and a plain water bypass.
6. Plus injectors also feature variable ratios (1:50 to 1:1000) and operates on water pressure as low as 7 GPM.
7. Gewa injectors actually inject fertilizer into the irrigation lines by pressure.
            a. The fertilizer is contained in a rubber bag inside the metal tank.     
                Water pressure forces the fertilizer out of the bag into the water supply.
            b. Care must be taken when filling the bags as they can tear.
            c. Ratios are variable from 1:15 to 1:300.
8. If your injector is installed directly in a water line, be sure to install a bypass
    around the injector so irrigations of plain water can be accomplished.

General problems of fertigation
Nitrogen tends to accumulate at the peripherous of wetted soil volume. Hence, only roots at the periphery of the wetted zone alone will have enough access to Nitrogen. Nitrogen is lost by leaching and denitrification. Since downward movement results in permanent loss of NO3 –N, increased discharge rate results in lateral movement of N and reduces loss by leaching.

It accumulates near emitter and P fixing capacity decides its efficiency. Low pH near the emitter results in high fixation.

It moves both laterally and downward and does not accumulate near emitter. Its distribution is more uniform than N&P.

Excepting boron, all micronutrients accumulates near the emitter if supplied by fertigation. Boron is lost by leaching in a sandy soil low in organic matter. But chelated micronutrients of Fe, Zn can move away from the emitter but not far away from the rooting zone.