Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
Reviewed May 07
Selection of fertigation equipment Farmnote 35/2001 By Tim Calder, Technical Officer, Research and Development Services, South Perth and John Burt, Research and Development Officer, South Perth Fertigation is the technique of applying soluble fertilisers to crops through sprinklers or trickle irrigation systems. It is the preferred system of applying top-dressings of nutrients to crops, especially after crops have . closed over. across the rows. This system is fully described in the 2001 Bulletin on 'Use of Fertigation In Vegetables in Western Australia'. There are two types of fertigation: â—? Proportional fertigation delivers a constant ratio of nutrients according to the flow rate â—? Quantitative fertigation is the application of the plant nutrients at a set rate to the irrigation system. The type of fertigation that is chosen depends on the crop grown and the farm management system. Not all injection systems can be applied to both types of fertigation.
Fertiliser injection methods It is important to select a fertiliser injection method that best suits your irrigation system and the crop to be grown. Each fertiliser injector is designed for a specified pressure and flow range. Care must be taken in selecting a fertigation system that suits your requirements.
Figure 1a. A metering pump injection system based on an electric pump. It is important to decide on the required fertiliser injection rate, flow rate, pressure and whether a
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
pressure loss in the system is acceptable. The five main fertigation systems (see Table 2) are: ● Metering pumps ● Pressure differential ● Venturi (vacuum) ● Suction injection. ● Boom spraying
Metering Pumps The metering pump method uses a pump to inject the fertiliser solution from a supply tank into the mainline. The metering pump may be driven by an electric or diesel motor (see Figure 1a) or by water pressure (see Figure 1b). A metering pump injection system based on a hydralic pump.
● ● ● ● ●
Figure 1b. A metering pump injection system based on a hydralic pump. Simple to install and operate Suitable for proportional or quantitative fertigation Easily adjustable injection rate No pressure loss in the mainline Suitable for automation.
Disadvantages ● ●
Pumps and piping must withstand irrigation mainline pressure Needs electricity or engine if not hydraulically driven.
A metering pump is often the most trouble free choice. The metering pump system is flexible and simple to install and operate.
Pressure Differential The pressure differential method (see Figure 2) is of the quantitative type and creates a pressure difference through the use of a regulating valve between the tank inlet and outlet. The difference in pressure is sufficient to cause water to flow through the tank.
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
Figure 2. The pressure differential method creates a difference in pressure across the fertiliser tank. There are two systems that use a pressure differential method. In one, a nutrient bag containing the fertiliser solution is placed in the tank, allowing the water pressure to force the solution out and into the system. In the other system, water enters the tank and mixes with the fertiliser solution before flowing into the system.
Advantages ● ● ● ● ●
Simple and inexpensive to operate Easy to maintain Easy to alter the type of fertiliser Suitable for powder formulations, if used without a bag No electricity or fuel needed.
Disadvantages ● ●
Requires pressure loss in the mainline, or a booster pump If used without a bag, the concentration of the fertiliser solution decreases over time.
A pressure differential tank is best when fertigation is done on an irregular basis, and where stock solutions are not proportional.
Venturi (vacuum) The venturi (vacuum) injection method (see Figure 3) uses a venturi device to create a reduced pressure (vacuum) that sucks the fertiliser solution into the line. It is more suitable for the proportional type of fertigation and can use minimal equipment, provided the irrigation system is hydraulically sound. A metering pump may be a better option if the irrigation system is unsuitable.
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
Figure 3. This is one of several arrangements that can be installed using a venturi (vacuum) injector.
Advantages ● ● ● ●
Easy to maintain Easily adjustable injection rate Suitable for low injection rates Injection rates can be controlled with a metering valve.
Disadvantages ●
Requires pressure loss in the mainline, or a booster pump.
As a venturi system works on velocity to force the solution into the line, it depends upon a sound irrigation system which can deliver a constant flow, in order to maintain a constant supply of plant nutrient.
Figure 4. The suction injection method is simple but limited. All connections to the suction pipe must be tight.
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
Suction Injection Suction injection (see Figure 4) is a method widely, but inefficiently, used. It is used for the quantitative system. The fertiliser tank is connected by pipe to the suction line of a centrifugal pump. The level of fertiliser in the tank must not go below the outlet.
Advantages ● ●
Simple and inexpensive to operate. Easy to maintain.
Disadvantages ● ● ● ● ●
With a refill system, the concentration of fertiliser solution decreases over the injection period. Air may enter the pumping system. Pump bowls and fittings may corrode. Less suitable for automation. Water supply may be contaminated if back-flow occurs. A non-return valve, at the very least, is necessary.
Suction injection may be suitable for a small operation. For larger blocks, or where greater accuracy is needed, other systems are more appropriate. When preparing fertiliser solutions for injection units, care must be taken with chemicals. Fertilisers that must not be mixed together are outlined in the separate Bulletin on fertigation. Most materials used in fertigation are corrosive. The injector is also often used to inject acid into the irrigation system for regular maintenance. Therefore, all injector parts should be made of corrosion resistant material.
Table 1. Common fertiliser injectors on the market Manufacturer Venturi
Type of equipment Pressure Differential
Metering
Elecrically driven Acromet ADD-IT (TFS Systems)
X X
Possible
Engine driven
Injection
Hydraulic driven
Rate (L/hr)
2-50 0.3-9 L capacity
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
AMIAD* Injector Unit (duplex unit)
X
Dosatron
X
Up to 320 (up to 640)
16-160
Mazzei
X
0-1930
Mini-Fert
X
X
9-50
>
X
X
Up to 230
X
15-220 L capacity
Netafim** ODIS Fertiliser tanks Prominent
X
UN
X
X
TMB â—?
18-72 X
3-1200
Can have electronic controller and pulse transmitter. ** Can have a booster pump.
Mention of trade names does not imply endorsement or preference of any company. s product by Agriculture Western Australia and any omission of a trade name is unintentional. Recommendations are current at the time of printing.
Size of tank To work out the size of tank for fertigation, you need to work out the amount of each fertiliser that will be applied for each area. The size of the tank is determined by the solubility of the least soluble fertiliser. This can be seen in the following example for one hectare, where urea is the most soluble fertiliser and potassium sulphate is the least soluble fertiliser. In this example, the use of potassium sulphate determines that the size of the tank should be 730 litres, or 1000 litres to allow for lower solubility in cold weather. Table 2. Size of tank for fertigation Fertiliser
Solubility kilograms per 100 litres at 20 °
Amount of required fertiliser (kilograms per hectare)
Water needed to dissolve that amount of fertiliser (litres)
Farmnote 35/2001 : Selection of fertigation equipment [Western Australia]
Potassium nitrate
32.0
80
250
Urea
105.0
30
29
Potassium sulphate
11.0
80
730
Use of boomspray The use of fertigation is less suitable where the lay-out of the sprinklers gives a poor uniformity of water. In this situation, the use of a boom-spray can apply soluble fertilisers more efficiently. Apply sprinklers for 2 to 5 minutes after boom spraying to wash the fertilisers from the leaves and into the soil.
Acknowledgements: Scott Moorhead wrote the original leaflet. Disclaimer: In relying on or using this document or any advice or information expressly or impliedly contained within it, you accept all risks and responsibility for loss, injury, damages, costs and other consequences of any kind whatsoever resulting directly or indirectly to you or any other person from your doing so. It is for you to obtain your own advice and conduct your own investigations and assessments of any proposals that you may be considering in light of your own circumstances. Further, the State of Western Australia, the Chief Executive Officer of the Department of Agriculture, the Agriculture Protection Board, the authors, the publisher and their officers, employees and agents do not warrant the accuracy, currency, reliability or correctness of this document or any advice or information expressly or impliedly contained within it and exclude all liability of any kind whatsoever to any person arising directly or indirectly from reliance on or the use of this document or any advice or information expressly or impliedly contained within it by you or any other person. This file: F03501.pdf Date converted: 6 September 2001 Š Copyright Chief Executive Officer, Department of Agriculture, Western Australia, 2001