Adding fertilisers into the irrigation system

How you add fertilisers into your irrigation system depends on the type of system you have, and the amount of money you invest. This page will outline the systems used, and some of the equipment available to make it a fertigation system.

The type of cropping system used largely depends on the crop(s) being grown, and the intended market. For example, many crops, such as tomatoes, cucumbers and ornamental pot plants, need to be grown under cover in substrate-based systems to achieve the product, consistency or quality required by the market, and to avoid many soilborne diseases.

The selection of a soil-based or soilless system will influence which technology you choose to add fertilisers to the irrigation system.

In soil-based agriculture and horticulture, different types of soils are used to cultivate different crops. For example, root crops grow better in fine or sandy soils because they allow for better root growth. While most soils have a greater buffering capacity (the ability of the soil to resist pH change) and cation exchange capacity (the ability to hold onto essential nutrients) than many substrates, it is still important to accurately inject fertilisers into the irrigation system if using fertigation. An unsuitable nutrient solution coupled with excessive irrigation, can lead to leaching of nutrients into the environment, and pollution.

Soilless cultivation systems are cropping systems that grow plants in a variety of organic and inorganic substrates or in nutrient solutions, allowing better control of soilborne diseases and accurate supply of water and nutrients to the crop.

These can be divided into:

• Open systems, where water and nutrient solutions are not captured and reused
• Semi-closed systems, which recirculate some of the water and nutrient solution, cleaning and disinfecting it before reuse
• Closed systems that recirculate all of the water and nutrient solution. Closed systems require more precise control of the nutrient solution than other systems

The aim of the closed and semi-closed systems is to substantially reduce pollution of water resources by nitrates and phosphates from fertigation waste water.

Soilless systems can:

• Use different substrates (e.g. rockwool, coir, wood fibre, perlite, peat)

Use systems where the nutrient solution is delivered to plant roots using different water volumes (e.g. hydroponics, nutrient film technique, deep flow technique or aeroponics)

Nutrient solutions can be made up by either diluting proprietary soluble liquid fertilisers, or by dissolving solid fertilisers. The solid fertilisers are most often sold as crystals that need to be dissolved in water to form a concentrate solution, before they are injected into the fertigation system. This is a crucial step, as partially dissolved fertilisers can end up clogging equipment and mean that you cannot be sure of the amount of fertiliser actually getting to your crop.

• Compound fertilisers are easier to use
• Straight fertilisers are cheaper
• Straight fertilisers allow modification of feed during the growing season according to the need of the crop

• Use monitoring technology and good agronomic knowledge
• Take into account any additional nitrogen or phosphorus from acid dosing

• Fertilisers are dissolved in separate tanks before mixing, to prevent precipitation of phosphates or sulphates due to calcium
• Automated injection equipment in A/B tanks enables more precise control of the doses of nutrient solution going into the irrigation system

• Often applied as chelates to keep them in an available form
• The metal nutrient ion (e.g. iron, manganese, zinc, or copper) is surrounded by a larger organic molecule
• Chelated micronutrients are protected from oxidation, precipitation, and immobilisation, meaning it is more likely to be available for your crop to take up

Liquid fertilisers need to be applied more frequently than other application methods, which requires more knowledge and labour to manage, and more frequent monitoring. However, it also means that you have more control over your feeding regime and can rectify issues with crop nutrition more quickly.

Despite the benefits of having access to a particular market (the organic market), it is rarely worth applying organic fertilisers via fertigation, as they tend to clog drippers – adequate drippers and filtration systems must be used, as well as frequently flushing the irrigation pipes. High-quality fertilisers are necessary if they are to be used in fertigation systems. Organic fertilisers suitable for fertigation also vary in cost.

It is important to maximise water and nutrient use efficiency in your system, while minimising impacts on the environment. The use of combined drip irrigation and fertigation systems can reduce fertiliser applications, which will reduce your variable costs and contribute to the profitability of your business.

Many substrate-grown and open field crops are irrigated via drip irrigation with the expectation of higher yields and reduced water use compared with other irrigation methods. A relatively even distribution of irrigation within a crop is essential. The characteristics of the drip irrigation system selected depend on:

• Type of cropping system (protected vs. open field; soil vs. substrate-grown or hydroponics)
• Crop type (horticultural, arable)
• Crop species
• Water source

Using more sophisticated irrigation methods, such as subsurface irrigation (below the soil surface), requires more skilled workers, a more careful system design, and good management of irrigation and fertilisation are required to maximise efficiency and to avoid emitter clogging.

Irrigation technology can be divided into the following categories:

Irrigation equipment: Systems

• E.g. Subsurface drip irrigation

Irrigation equipment: Design and management

• Installation of drip irrigation systems on sloping fields

• Adaptation of drip irrigation systems to water with high biological loads

Irrigation equipment: Materials

• Irrigation pipes

• Drip emitters and drip lines

• Thin-walled dripper lines (irrigation tape)

• Drip pipes and drippers with anti-microbial substances, and substances to prevent roots growing

Once you know the type of fertiliser(s) appropriate for your crop and production system, the next thing to consider is what equipment to use to add fertiliser into the irrigation system. This depends on factors such as the crop requirement, the cost of equipment, and maintenance costs (including how much labour is required to run it).

Fertilisers need to be dissolved in one or several concentrated solutions. These nutrient (or stock) solutions are then injected into the irrigation system using one of the different fertigation systems available. In simpler systems, absolute quantities of nutrients are supplied to the crop. However, many modern fertigation systems are based on electrical conductivity (EC) and pH measurements, and can supply a balanced nutrient solution that changes depending on those measurements. The idea is to maintain adequate nutrient concentrations at the root level.

For smaller farms and nurseries, the cost of the best technology can be a barrier. Although simpler and cheaper technologies like Venturi injectors can have lower accuracy, they are often considered to be more reliable as they have fewer issues with blocking or failure of injection pumps.

Stock solutions are the nutrient solutions made by dissolving different soluble fertilisers. These can then be diluted further, depending on the nutrient requirement of your crop:

• Only 100% water soluble fertilisers must be used in fertigation. Good knowledge is required for dissolving compatible fertilisers in different tanks
• Preparing concentrated solutions takes time, requires space and electricity to run the mixer. The cost of creating your own stock solutions tends to be lower than buying pre-prepared fertilisers

The technologies below are listed in order of cost (cheapest option first) and level of complexity (easiest to implement first). For all these technologies, it is important to clean out filters and check the injection flow rate regularly.

Simple fertiliser tanks

These need to be filled with dissolved fertiliser before each irrigation event, and a pump or valve diverting water combines the stock solution with the irrigation water. There is no control of fertiliser injection, so this system only supplies absolute quantities of nutrients, usually to soil-grown crops. The simplest version of this technology is the positive displacement method, which does not require any electricity or moving parts. A flexible bag containing liquid feed is held within a barrel. Water slowly enters the barrel, and the positive pressure created by the water drives the feed into the irrigation system.

Venturi injectors

These inject stock solution into a pressurised irrigation system. They do this by causing a pressure decrease, sucking stock solution into the injector and mixing it with irrigation water. They can be installed in line with the irrigation system (low water capacity systems), or as a by-pass from the main irrigation line.

Injector pumps

The stock solution is pumped into the irrigation system in small, accurate doses. The control of the injection is performed with small valves that partially open at each injector pulse time, keeping a constant nutrient equilibrium. A water-powered proportional dilutor is an example of an injector pump.

• In automatic systems, multiple fertiliser tanks can each be linked to a venturi injector, releasing doses of stock solution into the water. The amount released each time is controlled by solenoid valves and is based on a programme designed by you from EC and pH readings
• Magnetic drive pumps can cope with high water flow rates, while using very little energy. They run continuously, sucking solution from the tanks and releasing it through solenoid valves. Target EC and pH affect how frequently the valves open. Flow meters can be installed for each pump, measuring and regulating the amount of solution injected, improving accuracy
• Automatic injection equipment based on quantitative addition is where the injection of fertilisers is proportional to the irrigation flow, rather than being based on EC. The volume of injected stock solution is tracked, as well as the irrigation flow rate. This means the ratio between them can be calculated and the amount of stock solution adjusted to achieve desired levels. This is the most accurate, and most expensive, technology