Irrigation and Fertigation in Protected Structure

 

Irrigation System used in Greenhouse

The  precise  amount  of  irrigation  required  in  each  day  throughout  the  year  can  be  supplied efficiently by a well-designed irrigation system in greenhouse. The irrigation requirement  depends  on  cultivated  area,  type  of  crop,  weather  condition,  timing  of  cultivation,  capacity  of  ventilation  requirement,  etc.  Additionally,  the  type  of  soil  or  soil mix, type of container bed affects the irrigation requirement. The crop quality can be hampered by frequent application of water in greenhouse. Therefore, decision on irrigation scheduling should be taken by continuous inspection and understanding the requirement.  The  irrigation  systems  used  in  greenhouse  are  hand  watering,  perimeter  watering, overhead sprinklers, boom watering, and drip irrigation.

Hand watering:

Hand  watering  is  the  most  traditional  method  of  irrigation  and  is  uneconomical  in  today’s times. It is very tedious and takes considerable time to operate. The growers can  afford  hand  watering  for  high  density  crop,  irrigating  specific  pots  and  areas which dries earlier than others.

The disadvantages of hand watering are (i) operation cost  is  too  high  and  (ii)  higher  risk  is  associated  with  applying  too  smaller  amount  of water or waiting for long duration between two successive watering. Usually, it is done by unskilled workers, who might be tempted to speed up or later put the job off. For hand watering, a water breaker needs to be placed on the end of the hose. For a higher flow rate condition, the force of the water should not wash the root substrate out of the bench or pot.

Perimeter watering:

The perimeter watering system may be used on benches or beds for crop production. A  standard  device  consists  of  a  plastic  pipe  with  nozzles  spraying  water  over  the  surface  of  the  substrate  below  the  vegetation  across  the  circumference  of  a  bench.  It is possible to use either polythene or PVC piping. Being PVC pipe stationary, the polythene pipe tends to roll if it is not tightly fixed to the side of the bench. It makes nozzles to move from  the  correct  direction  to  the  surface  of  the  substrate.  Nozzles are constructed by using nylon or hard plastic material and a spray arc of 180°, 90° or 45° can be applied.  They are staggered  around  the  benches,  irrespective  of  the  types  of  nozzles  used,  so  that  each  nozzle  projects  between  two  other  nozzles  on  the opposite side. A 30.5 cm valve is required in 180° nozzles of Perimeter watering systems for benches.

Overhead sprinklers:

For disease control, the foliage on most crops should be kept dry though wet foliage is  tolerated  by  a  few  crops.  These  few  crops  can  be  irrigated  from  overhead  more  quickly  and  cheaply.  Bedding  plants,  azalea  liners  along  with  few  green  plants  are  typically  watered  from  overhead.  A  pipe  is  mounted  around  a  bed’s  middle.  Riser  pipes  are  mounted  frequently  to  a  height  just  above  the  crop  height.  The  height  of  0.6 m and 1.8 m is sufficient for flat bedding plants and fresh flowers, respectively. There is a nozzle installed at the top of each riser. Nozzles vary from those that cast a 360° pattern continuously to forms that spin around a 360° globe. Often, to capture water  that  may  fall  between  pots  and  waste  on  the  bottom,  trays  are  placed  under  pots. Every single tray is square, and the neighboring tray faces it. Almost all water loss is minimized in this manner. Each tray has a depression to match the pot and is then angled upward from the pot toward the tray perimeter. Drainage of excess water is  passed  through  drain  holes  of  the  trays  and  certain  quantity  of  water  is  stored  in  the tray, which is subsequently absorbed by the substratum.

Boom watering:

Boom watering, which is often used to produce seedlings grown in plug trays, can act either as an open system or as a closed system. Plug trays are constructed by plastic material  with  dimension  of  approximately  30  ×  61  cm  and  depth  of  13-38  mm,  and  contain around 100 to 800 cells. Each seedling is produced in its own individual cell. The  precision  of  watering  is  very  important  during  the  2  to  8  weeks  development  cycle  of  plug  seedlings.  A  boom  watering  system  generally  consists  of  a  water  pipe  boom that extends from one side of a greenhouse bay to the other. The pipe is fitted with nozzles that can either spray water or fertilizer solution down on the crop. The boom is connected to a carriage at its center stage, which travels along tracks, often suspended  above  the  center  of  the  greenhouse  bay  walk.  In  this  way,  the  boom  will  move  from  one  end  of  the  bay  to  the  other.  The  boom  is  propelled  by  an  electric  motor.  The  amount  of  water  delivered  per  area  of  the  plant  unit  is  adjusted  by  the  speed at which the boom is moving.

Drip Irrigation:

Drip  irrigation,  also  known  as  trickle  irrigation,  consists  of  the  lying  on  the  surface  or subsurface of the field or greenhouse of small-diameter plastic tubes beside or below  the  plants.  Water  is  supplied  at  regular  intervals  to  the  plants  through  small  holes or emitters located along the tube. Drip irrigation systems are commonly used in conjunction with protected agriculture, as an important and significant part of the comprehensive design. Drip irrigation is the only way to add standardized water and fertilizer to the plants without using plastic mulches, row covers, or greenhouses. Drip irrigation provides complete control over environmental variability; retains optimum production  with  minimal  use  of  water  while  retaining  soil  and  fertilizer  nutrients;  and  controls  the  cost  of  water,  fertilizer,  labor  and  machinery.  Drip  irrigation  is  the  safest method of water management. Depending on soil type, field level and how water is applied to the furrows, the application efficiency is usually 90 to 95 percent, compared  with  70  percent  sprinkler  and  60  to  80  percent  furrow  irrigation.  Table  1  indicates  improvements  in  yield  and  savings  in  water  under  drip  irrigation.  It  is  not only recommended for protected agriculture, but also for the production of open field crops , especially in arid and semi-arid regions of the world. However, these  costs need to be calculated by comparing them with the cost of land preparation and maintenance  that  is  also  required  for  surface  irrigation.  Basic  irrigation  equipment  consists  of  a  pump’s  laterals  or  emitters,  main  line,  distribution  pipes,  laterals  of  manifold,  and  drip  tape.  The  head  is  typically  made  up  of  control  levers,  couplings,  filters,  time  clocks,  fertilizer  injectors,  pressure  controls,  flow  meters,  and  gauges between  the  pump  and  the  pipeline  network.  Because  water  passes  through  very  small  emitter  outlets,  it  is  absolutely  necessary  to  screen,  filter,  or  both  before  it is distributed in the tube system. The initial field positioning and layout of a drip system is affected by the land topography and the expense of different configurations of the system.

Drip Irrigation:

Drip irrigation is the slow, nearly continuous application of water as discrete drops. Water can be applied at single point on the land surfaces through devices called as emitter or as a line source from either closely spaced emitters or tubes with continuous or equally spaced openings that discharge water drop at a time.

Advantages of Drip irrigation:

•  Water is directly applied to the root zone of the crop and hence deep percolation and surface runoff are reduced. This increases the application efficiency.
• There is elimination of land leveling as required by the surface irrigation methods and hence cost of irrigation system is less.
•  Irrigation can be done in any type of soil, topography and even steep slopes.
• Irrigation can be done with poor quality water including saline water.
• Irrigation is frequent and low and maintains favourable moisture range always in the root zone. This saves a lot of irrigation water and increase water-use-efficiency.
• Weed growth is less.
• Along with irrigation water, fertilizer, herbicides and pesticides can also be applied to the crops. This increases the input use efficiency and reduces the cost of cultivation.
• Intercultural operation can be done easily without any interruption, Need for construction of irrigation and drainage channels as is required in surface irrigation is eliminated. This saves the land which otherwise would have been lost for construction of channels and this saved area can be used for cultivation.
•  Quality of crops including fruits and vegetables are good.
• Requires less operating pressure and so less energy for operation of the irrigation system as compared to sprinkler irrigation,
• For widely spaced crops like fruit trees, the system is less costly than sprinkler irrigation.
• It is possible to vary the supply as per the need of water by the plants at different growth stages.

Disadvantages of Drip Irrigation:

•  Initial cost is very high which limits for its large scale operation.
• Requires high technical know-how than any other irrigation methods.
• There is chance of clogging of emitters/drippers especially by chemical and biological materials, Clogging cause poor water distribution and if it continues for a long period, then crop may be damaged.
• Maintenance cost is also high.
• A salt accumulation problem may occur in this system when only a portion of the root zone is wet and saline water is being used for irrigation. This may damage the crop.
• Possibility of water leakage from pipes exists.

 

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

Water out put in drippers

Ø 16mm dripper at 2.8kg/cm2 pressure gives 2.65 litres/hour ( LPH).

Ø 5mm dripper at 1 kg/cm2 pressure gives 1 to 4 litres per hour

Layout of drip irrigation

Fig. 2 represents typical drip irrigation system for a greenhouse. Mainlines, submains, and laterals provide water into the fields from the control head. They are typically made of PVC or polyethylene hose and should be buried under the ground because, when exposed to direct solar radiation, they degrade quickly. Lateral pipes normally have a diameter of 13-32 mm. Emitters or drippers are devices that are used to monitor water discharge from the side of the plant. For one or more emitters used for a single plant, such as a tree, they are normally spaced more than 1 meter apart. Spaced emitters may be used for wetting a strip of soil more tightly for row crops. In recent years, several different emitter designs have been manufactured. The basis of the design is to generate an emitter that will provide a specified constant discharge that does not differ much with changes in pressure and does not easily block. Some types are short path, long path, short orifice, compensating strain, self-flushing and emitters of porous tubing. Two types, point source and line source, can be grouped into these  designs. Point source systems discharge water at least 1 m apart from individual or multiple outlets that are spaced. In the irrigation tubes, line source systems have perforations, holes or porous walls that discharge water at near intervals or even continuously along a lateral line. For widely spaced crops and line source systems for close-growing crops, point source systems are used.

Advanced Micro-irrigation Systems for Greenhouse

Micro-irrigation systems for greenhouse can be broadly categorized into aerial, surface, and subsurface passing of drip lines (Fig. 1). The surface micro irrigation methods are widely adopted due to their less maintenance cost. There are two types of lateral mains can be adopted that differ depending on how the emitters are installed, that is (1) in-line emitters or driplines and (2) online emitters. Flow rate and working pressures of all the different micro irrigation systems given in the above flow chart.

Fertigation

Fertigation is a method of fertilizer application in which fertilizer is incorporated within the irrigation water by the drip system. In this system fertilizer solution is distributed evenly in irrigation. The availability of nutrients is very high therefore the efficiency is more. In this method liquid fertilizer as well as water soluble fertilizers are used. By this method, fertilizer use efficiency is increased from 80 to 90 per cent.

Water and fertilizers are the two most important inputs in the crop production. Due to increasing pressure on both these resources, there is immediate need to increase the efficiency of these inputs. Fortunately, the technology of drip irrigation can be used effectively to enhance the efficiency of these resources. Application of fertilizers through drip/sprinkler is termed as "fertigation'. Though fertigation technology is a boon to increase the fertilizer use efficiency, there is a great deal of science to be understood before it becomes successful on the farmers' fields. Following sections describe this science in a nutshell.

Keywords in fertigation are as under:

• Chemigation is an inclusive term referring to the application of a chemical into or through an irrigation system. It includes the application of fertilizers, acids, chlorine and pesticides.
• Fertigation is specifically the application of fertilizer (plant nutrients) through an irrigation system.
• Acidification is the introduction of an acid, such as phosphoric, sulfuric or hydrochloric acid into an irrigation system.
• Chlorination is the introduction of chlorine, such as liquid sodium hypochlorite (household bleach) or chlorine gas into an irrigation system

Advantages of fertigation

• Eliminates manual application: Manual operation requires labour and time and is less efficient compared to the advanced fertigation methods. The fertigation eliminates this manual operation.
•   Quick and convenient: Fertigation is done through either injector pump venturi or fertilizer tank. The operation of injecting fertilizers with these methods is quite quick and convenient.
• Uniformity in application: Since the drip or microsprinkler irrigates a limited area of active root zone and fertilizers are placed directly in this active root zone, there is very high uniformity in the application of fertilizers.
• High efficiency and saving of fertilizer: In fertigation, fertilizers are applied in small quantities and more frequently. Fertilizers are directly placed in the active root zone. Hence, there is no loss of fertilizers through runoff, leaching or volatilization. It results into high efficiency of fertilizer application and saving of fertilizers to the tune of 30 to 40%.
• Less fertilizer leaching and groundwater pollution: As controlled volume of water and fertilizer is applied in fertigation there is less (or negligible) leaching of fertilizers which results in less groundwater pollution.
• Better penetration of P and K in the layers: Phosphatic fertilizers get fixed when applied in the soil. But in fertigation they are placed in the soluble form and hence can be placed in different layers where water percolates.
•   Improvement in nutrients availability and their uptake: In fertigation fertilizers can be given every day and the quantity given is based on the crop nutrient requirement. Hence, nutrients are available as and when the crop requires it.
•   Nutrition requirements tailored with crop stage or development:
• Application of herbicides, pesticides, acid etc: The same system can be utilized for the application of herbicides, pesticides, acids etc.

Limitations of fertigation

• Possibility of clogging of emitters if the pH of irrigation water and fertilizer sources is not managed carefully.
•  May result in possible contamination of the drinking water supply if devices are not used to prevent backflow of nutrients into the well or other water sources.
• Some of the chemicals are quite corrosive to metal and also cause skin burning if safety devices are not provided to protect the workers against unexpected discharge or spilling of chemicals.
•  Insoluble fertilizers are not suitable (e.g. super phosphate)
• Phosphate may get precipitated in the pipe line and dripper due to pH reaction.  
• Fertilizers are chemical substances and when they come in contact with other materials (e.g. water, iron or other chemicals) may react chemically and many times the results are unfavorable. Hence, there are certain criteria to be met by the fertilizers when they are to be used for fertigation.

Fertigation in Protected Cultivation

In modern irrigated farming, the application of chemicals through the irrigation system has become a common practice. With fertigation, the nutrient is applied with irrigation water in the form of soluble fertilizer expected to meet most crop needs according to their stage of development. The success of fertigation depends primarily on the characteristics of the fertilizers used. Those are:

• ™™ Must be completely soluble in water (< 0.02% insoluble in water) and have quick dissolution in water with minimum content of conditioners.
• ™™ Must not react with dissolved elements in water especially calcium and magnesium salts.
• ™™ High nutrient content in the saturated solution must not get leached down easily from the soil.
• ™™ Should not change the pH of water leading to precipitation and clogging
• ™™ Should avoid corrosion of the system.
• ™™ Should be safer for field use and for mixing with other chemicals.

Common fertilizers suitable for fertigation in protected cultivation

For proper growth, plants need to be supplied with following nutrients either alone or combination at appropriate concentrations viz., nitrogen (150-200 ppm), Phosphorus (50 ppm), Potassium (200-400 ppm), Calcium (150-200 ppm), magnesium (50 ppm), boron (0.2 ppm), zinc (0.1 ppm) Copper (0.1 ppm), Manganese (1 ppm), iron (5 ppm) and molybdenum (0.05 ppm). Fertilizers available in market best fit for fertigation are given in Table 3.

Chloride free fertilizers

These fertilizers are produced by using Urea, ammonium nitrate phosphate and potassium nitrate as basic ingredients and are useful for high value crops and crops which are more sensitive to the chloride injury. Ex: Tobacco, grapes, citrus, arecanut and vegetables.

Liquid fertilizers

Bulk fertilizers such as ammonium sulphate, ammonium nitrate, urea, ammonium phosphate, phosphoric acid, potassium nitrate, potassium chloride, potassium sulphate, etc. are primarily the raw materials used in the manufacture of liquid fertilizers. The liquid fertilizers do not precipitate and are pure. The liquid fertilizers are typically acidic (pH 5.5-6.5) and help to some degree to correct the soil pH and also help avoid the clogging of emitters. Liquid fertilizer with a neutral pH or even higher pH may be used for acidic soils.

Normal fertilizers

These are produced by using ammonium nitrate, urea, ammonium phosphate, ammonium sulphate, phosphoric acid, potassium chloride etc.

Micro nutrients

Micronutrients are generally applied separately to plants in most soils as their application through fertigation would react with salts in the irrigation water and cause precipitation and clogging. Chelated micronutrients are highly water soluble and can be applied through fertigation since they cause very little clogging or precipitation.

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.

Precautions to be taken during fertigation

• ™™ The same volume of water has to be supplied by each emitting point.
• ™™ Deposits or residues free materials should be used and it must be non-corrosive to the system.
• ™™ Constant operating pressure should be maintained to facilitate uniform mixing of water with fertilizers.
• ™™ Most appropriate fertilizer, injection system and crops for fertigation should be selected.
• ™™ The injection of fertilizer should not start until all lines are filled with water and the emitters are running.
• ™™ Prior to fertilizer injection, the drip irrigation system should be allowed to run at its working pressure.
• ™™ At the same time, fertilizers, pesticides and chlorine should not be injected.

Timing of fertilizer application

Since plant nutrients are the source of food for growth, development and yield of crops, their application must be ensured as and when they require to attain the potential yield of crops envisaged. The frequency of fertigation is regulated by the nature of the crop, the duration, the habit of growth and the ability to yield. Fertigation is typically conducted on a regular, weekly, fortnightly basis and can be considered depending on the response of the crop.

Fertilizer efficiencies of various application methods

Water saving, yield and profit under drip and drip fertigation systems

 Fertilizer used in fertigation

• Urea, potash and highly water soluble fertilizers are available for applying through fertigation.
• Application of super phosphorus through fertigation must be avoided as it makes precipitation of phosphate salts. Thus phosphoric acid is more suitable for fertigation as it is available in liquid form.
• Special fertilisers like mono ammonium phosphate (Nitrogen and Phosphorus), poly feed (Nitrogen, Phosphorus and Potassium), Multi K (Nitrogen and Potassium), Potassium sulphate (Potassium and Sulphur) are highly suitable for fertigation0 as they are highly soluble in water.   Fe, Mn, Zn, Cu, B, Mo are also supplied along with special fertilisers.

Fertilizers commonly used in fertigation

N fertigation

    Urea is well suited for injection in micro irrigation system. It is highly soluble and dissolves in non-ionic form, so that it does not react with other substances in the water. Also urea does not cause precipitation problems. Urea, ammonium nitrate, ammonium sulphate, calcium ammonium sulphate, calcium ammonium nitrate are used as nitrogenous fertilizers in drip fertigation. 

 P fertigation

    Application of phosphorus to irrigation water may cause precipitation of phosphate salts.  Phosphoric acid and mono ammonium phosphate appears to be more suitable for fertigation.

K fertigation

    Application of K fertilizer does not cause any precipitation of salts. Potassium nitrate, Potassium chloride, Potassium sulphate and mono potassium phosphate are used in drip fertigation.

Micro nutrients

    Fe, Mn, Zn, Cu, B, Mo could be used as micro nutrients in drip fertigation.

Fertigation equipments

    Three main groups of equipments used in drip system are :

• Ventury
• Fertilizer tank
• Fertilizer pump

Ventury

Construction in the main water flow pipe causes a pressure difference (Vaccum) which is sufficient to suck fertilizer solution from an open container into the water flow. It is very easy to handle and it is affordable even by small farmers. This equipment is most suitable for smaller area.

Fertilizer tank

A tank containing fertilizer solution is connected to the irrigation pipe at the supply point.  Part of the irrigation water is diverted through the tank diluting the nutrient solution and returning to the main supply pipe.  The concentration of fertilizer in the tank thus becomes gradually reduced.

Fertilizer pump

The fertilizer pump is a standard component of the control head. The fertilizer solution is held in non-pressurised tank and it can be injected into the irrigation water at any desired ratio. Therefore the fertilizer availability to each plants is maintained properly.

Fertigation equipment

Pressure differential method or By-pass tank system

In this system, water-soluble solid fertilizers and liquid fertilizers can be pumped into the system (Fig. 4). At the supply point, a tank containing fertilizer solution is linked to the irrigation pipe. The nutrient solution is filtered and returned to the main supply pipe by diverting a small part of the irrigation water into the tank. Over time, the fertilizer content in the tank is steadily reduced.

Venturi or vacuum pump

This system is ideal for liquid fertilizers. It has low discharge rate. The nutrient concentration and quantity controls are medium. It delivers fertilizer at constant concentration which depends on the water flow (Fig. 5). It should be connected in parallel with the pipe line. It is a cheap system but there will be pressure drop in the system.

Displacement pump

This system is also for liquid fertilizers. The concentration of nutrient and quantity control is good. The automatic control is of higher order. It is more accurate but the cost is high. Fertilizer solution is prepared in a tank from which it is pumped and injected into the irrigation system. The fertilizer solution is delivered at a constant concentration. The impeller and casing of such a pump must be made of noncorrosive material such as nylon or polycarbonates. It needs separate power source.

Hydraulic dosing pumps or non-electric proportional injectors

The injection mechanism is driven by the water supply under pressure. The injection rate is factory pre-set or may be chosen by an adjustable setting on injector body. The injection rate is proportional to the flow of water passing through. The resulting solution strength is constant even though the water flow varies.

Disadvantages of fertigation

• ™™ If the irrigation system is defective, unequal nutrient distribution occurs. When excess water is applied to crops, it results in overfertilization or nutrient leaching.
• ™™ Chemical reactions of calcium and magnesium fertilizers, water bicarbonates, which can cause chemical clogging.
• ™™ Suitable for fertilizers that are easily soluble or liquid. In the micro irrigation system, phosphatic fertilizer and some micronutrients can precipitate.
• ™™ Fertigation equipment resistant to corrosion is required.
• ™™ Potential chemical backflow into the source of water sources

General problems of fertigation:

Nitrogen

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.

Phosphorous

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

Potassium

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

Micronutrients

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.