How to manage fertigation well

While water scarcity is not yet a frequent, widespread issue in the UK, during summer and during droughts, irrigation is essential for most crops. Regular analysis of irrigation water and monitoring crop performance is important, and this section outlines some of the principles and technologies available to do this.

There is increasing societal pressure to use limited water resources efficiently. There is increasing competition from other sectors such as tourism, industry and for domestic use. Additionally, there is increasing pressure to maintain the ecosystem services value of water resources.

Optimising irrigation at farm or nursery level means providing the right amount of water at the right time to cover the needs of the crop at that moment. Those needs vary with crop development, weather conditions, soil type, and other site-specific factors. Poor irrigation management can result in less yield and quality. Options for managing irrigation fall into seven categories, and the ones you use will depend on your situation, crop and available capital:

• Weather measurement – weather sensors
• Estimation of irrigation volumes – calculations of daily variations in soil or growing media water content
• Irrigation strategies – manual, time-based or linked to monitoring
• Decision support systems/tools – weather forecast tools, simulation models adjusted to your conditions and crop growth rates, or remote sensing
• Monitoring the plant or crop – a range of non-destructive sensors that measure plant water status in different ways
• Monitoring the soil – a range of sensors that measure soil water status
• Monitoring substrate and other soilless systems – real-time monitoring of changes in the weight of slabs or containers with plants, or measurement of waste water volume (these tend to be used in semi-closed or closed systems under glass)

Nutrient levels that are too high or too low can result in crop losses or poor-quality plants. Accurate fertiliser applications will reduce your variable costs and contribute to your profitability. There are four principles behind good management of nutrients and salinity. For each one, there is a range of technologies available:

Testing soil/substrate

It is worth testing soil and unused substrate before establishing a crop, so you know what your starting point is. Samples can be sent off for laboratory analysis, or electrical conductivity (EC) can be measured in the soil/substrate directly (i.e. in the root zone), in the soil/substrate solution, or in the substrate wastewater.

Testing water to monitor water quality

This should be done frequently as water quality can vary throughout the year.

Formulating the correct feed recipe

Based on crop requirements, it is possible to calculate optimum levels of nutrients to be applied in stock solutions at different stages in crop growth during the season. RB209 can be used for fertiliser recommendations for crops grown in the UK. Decision support systems can help calculate the quantity and timing of fertiliser application, as well as nutrient uptake and leaching.

Monitoring crops

This should be ongoing throughout the season. Plant tissue and sap samples can be sent to laboratories, but there will be a time lag between the plants’ nutrient status, and when you receive the results. On-farm or on-nursery nutrient analysis is much quicker, using fluorescence sensors or chlorophyll meters. For best results, one person should do all the measurements (for consistency), and a baseline correlated with laboratory results should be established because readings can be variable between sites. Ion meters respond selectively to a particular ion present in the stock solution, giving a quick result, but readings can be affected by particles in the solution.

What is leaving or running off your site is just as important as what is being applied, both economically and environmentally. Capturing waste (or drain) water, testing it and reusing the nutrients in it, is a relatively new area, and much of the technology for nutrient recovery is still in the research phase. It is, therefore, expensive. However, with more variable rainfall, hotter summers, and the probable expansion of Nitrate Vulnerable Zones, recycling waste water and nutrients is likely to become more important.

There is a range of options for treating waste water and recapturing nutrients. Space on site, cost, amount of water to treat, and whether it is commercialised will affect which option you choose:

Duckweed (Lemna minor)

Water flows into a pond where duckweed is allowed to grow. It consumes any nitrogen and phosphorus as it grows, and can then be skimmed off or filtered out of the water. You cannot buy it commercially, but it colonises and grows freely on bodies of water.

Moving bed biofilm reactor

Small plastic rounds (or carriers) are put in a tank and are moved around by stirrers or blasts of air. Each plastic carrier has a biofilm growing on it, which breaks down plant protection products and absorbs nutrients from the water.

Adsorption media for phosphorus

Iron-coated sand (a waste product from water treatment facilities) is placed in a filter, and water passes through it. The phosphorus binds to the sand. Methods of retrieving the phosphorus to use again are currently being tested.

Electrochemical phosphorus precipitation

A cathode and anode made of magnesium are placed in a tank. The waste water being treated flows through the tank between the cathode and anode, and an electric current is applied. This causes the magnesium anode to oxidise and break down, bonding with the phosphate to form P-salts. These then separate out from the water and sink to the bottom of the tank.

Constructed wetlands

These can be very large, but recent research has tested smaller, more efficient constructed wetlands. Waste water flows into a series of pools or tanks containing plants, which remove the nutrients as they settle out from the slow-moving water. Smaller reed or iris beds or channels are already in use at some nurseries and are efficient at extracting nitrogen and phosphate from the water.