Potato growers could save costs by better targeting water use, as Olivia Cooper reports

The drought is now a distant memory with reservoir levels looking much healthier than back in March, but it just takes a few dry days on lighter land before the rain gun is back out.

And every day the rain gun is on is adding to the season’s costs, as water is an expensive resource.

Yet about half of British potato growers do not calculate crop need. Many still irrigate a fixed quantity, regardless of rainfall and evaporation rates, leading to either wasteful or insufficient application of water.

By calculating when and how much water the plant needs, growers can ensure they are meeting their crops’ needs without wasting valuable water supplies by over-irrigating, says Melvyn Kay, executive secretary at the UK Irrigation Association.

“Irrigation scheduling relates to the crop and weather conditions – there are times when water supply is more critical than others, for example during flowering. If you miss irrigating at that time then yields will suffer, whereas irrigating at other times may just be wasteful.”

What is irrigation scheduling?

The objective of irrigation scheduling is to maintain optimum soil water conditions to meet crop yield and quality targets with minimum water wastage.

“Under-irrigation is not desirable – it can cause plant stress resulting in reduced yields,” says Mr Kay. “Poor timing of irrigation can also affect crop quality, such as scab.

Some growers over-irrigate to make sure their crops have enough water, but this is not only a waste of water, it is a drain on labour and electricity and can cause fertiliser leaching and soil erosion.

What should I consider?

Different soils hold different amounts of water, but not all of it is available to the plants. “You need to be aware of four levels in your soil reservoir: saturation, field capacity, critical soil water deficit, and permanent wilting point.”

When the soil reservoir is full the soil is saturated, but natural drainage soon empties the larger pores and reduces the soil water level to field capacity – a much more favourable condition when plant roots can easily take water from the soil.

During dry spells, plant roots find it increasingly difficult to extract water from the smaller soil pores. Eventually a critical soil water deficit is reached when the roots cannot extract water fast enough; the plants begin to stress, and crop yield and quality are affected.

“At this point the soil water reservoir needs irrigation or rainfall to fill it back up to field capacity.” If water is not added, and the soil reservoir empties further, plants will wilt and are unable to recover, even when more water is added. This is the permanent wilting point.

“The critical soil water deficit is different for different crops and growth stages,” says Mr Kay. For example, early in the season small, frequent irrigations are necessary to prevent scab.

“It is important to know what the critical soil deficits are for the crops you grow on your local soil types. Good irrigation management ensures that the critical soil water deficit is not exceeded at all the different growth stages.”

Can’t I just walk the crop?

Feeling the soil and walking the crop are essential for good irrigation management, but this should be combined with more objective methods.

“It’s best to have a rain gauge in every field, as you can get localised rainfall, and that is the biggest variable. Evaporation rates are important, but don’t vary so much and can be averaged across the farm.”

There are many different ways to schedule irrigation – the key is to find one that is most appropriate for your farm, and start planning.

“At the start of each season you need to think about your irrigation strategy – what critical soil water deficits will you allow and how much water will you apply each time you irrigate,” says Mr Kay. “Will you bring your soil back to field capacity each time or will you leave a deficit to take advantage of any rainfall?

“Your strategy will vary through the season as you take account of crop sensitivity to water stress at different growth stages.”

What are my options?

The most basic approach is a water balance calculation. Using a spreadsheet growers can input rainfall and irrigation rates, weighing them against crop evapotranspiration (ET) rates. These can be obtained from the Met Office or farmers can use automatic weather stations or atmometers to measure ET rates themselves. “The soil water deficit gradually builds up and reaches a critical point to signal the start of the next irrigation.”

However, the price of simplicity is often a loss of accuracy. Growers can use specialist bureau services to analyse the data and offer irrigation advice, but it is only ever as good as the input data, requiring growers to regularly measure rainfall and irrigation and provide agronomic information like planting dates, emergence and ground cover.

Also the amount of water applied to the soil isn’t always the same as that taken into the soil, warns Simon Turner, managing director of Agri-tech Services UK. “Often you’re applying water on to warm, bare soil and it’s not uncommon to lose 40-50% through evaporation. That is where measuring in a more accurate fashion comes into its own.”

Soil water measurement

Measuring water levels in the soil is growing in popularity and there are numerous ways of doing it, says Mr Turner.

Neutron probes are most accurate for deep-rooted crops, but can only be used by commercial scheduling services due to their radioactive source. It takes a weekly reading at different soil depths.

Those seeking a more regular update may choose a capacitance probe, which measures soil moisture in real time and links to a live website. “If you have overhead irrigation you may only irrigate once a week anyway, but if you have drip irrigation you can react to the live data very quickly.”

What does it cost?

A neutron probe service costs about £500-950 a field a year, with the equipment remaining property of the operator. In contrast, capacitance probes must be bought for permanent installation, at a cost of £750 each, with an annual charge of £350-700 a field, depending on the crop.

Can’t I do it myself?

Tensiometers are a relatively inexpensive option, at £80-90 apiece. Easy to read, and well-suited to sandy soils, they can give a useful guide, but they do require careful calibration and must be installed correctly.

Portable capacitance probes could also be used, but farmers must allow for the time taken to read and analyse the data correctly, says Mr Turner.

Is it worthwhile?

Irrigation scheduling can be time consuming and expensive, but with water costing about £40/acre-inch plus labour and electricity, the benefits increasingly outweigh the costs.

“A recent water audit on four farms growing potatoes in Norfolk showed that on average they tended to over-irrigate by some 12% – even though the farmers thought they were applying the right amount of water,” says Mr Kay.