Fertigation and chemical injection

"Fertigation" is die injection of fertilisers into the irrigation water. Although this is possible with other methods of irriga­tion, it is a common and easily achieved practice with microirrigation systems. Being applied with irrigation, the fertiliser is easy to apply, with little extra labour and no extra machinery. Provided the hydraulic design is adequate, the fer­tiliser is applied uniformly to each plant, targeted at the root zone. Fertiliser can be applied at frequent intervals and at any required concentration, to suit plant requirements.

Figure 7.10 One type of fertiliser injection unit

Nitrogen, phosphorous and potassium fertilisers are avail­able for use by fertigation. Decisions need to be made regard­ing the type of fertiliser to use, the quantity to be applied, the effects it might have on equipment and soil, and its compat­ibility with other dissolved or suspended materials in the irri­gation water. Soil and water testing should be thorough to minimise potential problems. Equipment and pipelines should be flushed with clear water at the end of fertilising, before the end of the irrigation cycle.

Fertiliser concentrate is mixed in a separate storage tank. Thorough mixing is required. For small projects, this may be just a large drum, with initial mixing by hand. Larger projects, where larger quantities and volumes are involved, will re­quire larger storage tanks, and preferably continuous agita­tion by recirculation.

Various types of injection equipment are used to deliver fertiliser concentrate into the irrigation mainline. The most common use:

• Differential pressure across a valve to divert some water into a pressurised concentrate tank, then deliver it into the mainline.

• Differential pressure across a valve to divert water into a venturi suction device, which then draws concentrate from the storage tank before delivery to the mainline.

• Positive displacement (diaphragm or piston) injection pump with electric or hydraulic drive. Sometimes the speed can be varied.

The latter is the most expensive, but gives greatest con­trol over fertiliser application and therefore more accurate and flexible management. Other factors in the choice of sys­tem include suitability to automation, effect on pressure in the mainline, resistance to corrosion, maintenance require­ments, and whether fertiliser is to be injected in proportion to water flow through the main on a continuous basis, or in separate, defined doses (so that injection rate is independent of irrigation rate).

Chlorine may need to be added to the system, to control algae and bacteria, and acid injection may be needed to con­trol calcium precipitates. Because of the risks associated with handling such chemicals, expert advice should be sought to determine the best form of chemical, the required dose rate and regime (continuous, intermittent or slug), and the equip­ment and technique to do the job. The need for this should be considered in the initial design.

Thorough analysis of water quality will help predict the need for these treatments. Apply treatment as a preventative measure before problems escalate and risk emitter blockage. Treatment can restore blocked emitters, but this requires high concentrations of chemicals to remain in the pipeline to enable cleaning to occur. Thorough flushing must follow.

Figure 7.11 The simplest form of controller instructs the values to open and programmed schedule

Automation.The cost and level of management required for a microirriga­tion system warrant the use of an irrigation control system. Even for systems of only a few blocks, relatively inexpensive controllers add great flexibility and automation capacity. Highly sophisticated controllers are available for larger in­stallations which can also log and analyze data collected dur­ing irrigation. Reports from such systems act as a monitor over system performance.

Laterals are grouped into blocks, each supplied by a sub-main. The block size is determined as much by flow rate and uniformity criteria as by planting area and soil type, although it is convenient if they are the same. Flow into each block is controlled by a solenoid valve, operated by low voltage electricity. A controller sends signals to the valve to open and close according to a program keyed into the controller by die irrigation manager. The programme would normally nominate the time and duration die valve opens, and the sequence required to irrigate multiple blocks. The program can be integrated with calculations of evapotranspiration from weather station data.

Signals are usually transmitted to the solenoid by wire cable(s) buried with the pipelines, although radio controlled equipment is available. Lightning and wiring defects remain die biggest technical problems. Some systems use multiple wires (one to each valve plus one common to all of them) and others use a communication cable of only two wires for all me valves.

Summary of design procedures.Microimgation is typically applied to intensively managed high value enterprises, so a high degree of planning is justi­fied. Also die system is able to accurately control water appli­cations to individual zones or parts within die field. A detailed soil assessment is conducted at die planning stage, by die use of a large number of soil pits throughout die proposed area. The readily available water at each pit is determined along with other soil characteristics. This will allow accurate match­ing of plant growth characteristics and water requirements to soil variation throughout the area, and the detection of sub­soil problems. Climatic information is assessed to determine the peak irrigation requirement.

A detailed contour map of the site is constructed, includ­ing location and elevation of pump stations, and preferred planting sites noted.

The optimum layout of the irrigation system can then be drafted, largely based on the size and layout of crop units that need to be watered as an individual block. If the blocks are large, they may need to be divided into more than one irrigation station so that uniform water application can be achieved. The location of crop rows dictates die alignment of laterals, but die supply to them from sub-mains and mainlines can be varied to suit hydraulic and economic criteria.

Pipe diameters are selected based on die peak flow rate to die various irrigation stations, die allowable flow or pressure variation permitted within each block, and costs. Irrigation designers use computer software to determine die optimum design. Pump specifications are then determined. Fertiliser injection, chemical injection, filtration, controller, flow meters, air valves and scour valves are added to suit die situation.

Maintenance.A high level of maintenance is required for microirrigation systems. The need to keep laterals and emitters free from blockage and me need for filtration has already been discussed, as has chemical treatment and flushing. This may be required on a regular basis throughout die irrigation season to prevent major problems occurring.

A complete pre-season check is necessary. The operation of mechanical, electrical and hydraulic equipment should be checked, and die system flushed and pressurised to check for leaks. Performance and condition of filters and check niters should be checked.

During die season, frequent pressure checks should be made in die field, to ensure water output is as it should be. Sub-main valve stations should be equipped to enable use of a portable pressure gauge to check pressures on die spot. Check dripper output occasionally by measuring die volume over a one hour period, and compare to specifications. During routine operations, check for leaks, noisy valves and pumps, and die operation of drippers. For example, if die dripper starts to squirt instead of drip, it is possible that partial blockage could be commencing. A noisy pump could be a mechanical problem, but it could also be cavitation, a condition created by excessive suction lift or restriction to water flow entering the pump.

As part of a preventative maintenance schedule, inspect filters and flush die system on a regular basis.

Most major components (filters, pumps, valves and so on) will require an annual service at the end of die season. Details should be supplied by die equipment manufacturer.

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