Less than half of the UK’s irrigated area has an irrigation schedule and growers must do more to maximise water use efficiency.

That was the message from Cambridge University Farms’ Mark Stalham, speaking at the Scottish Agricultural College’s annual potato producers’ conference near Perth (25 Jan).

“Things haven’t really changed much since the 1970s & 80s and it [not scheduling irrigation] will not be allowed under the Water Framework Directive.

We have to better understand crop demand for water and maximise the quantity and rate of water extraction from the soil.”

The timing and amount of irrigation fundamentally depends on the soil moisture deficit (SMD) – the balance between water loss (via evapotranspiration, drainage and run-off) and water inputs (rain, irrigation, run-on and capillary rise), he explained.

Ideally, the deficit should be minimised to allow adequate water availability, even on hot days.

But, scheduling to keep soils close to field capacity is “very dangerous” and increases the risk of anaerobic conditions and powdery scab developing, he warned.

This may be a particular issue for growers in northern England and Scotland, where summer rainfall is more common.

Trials near Dundee found that the typical water requirement for a potato crop with full ground cover in July or August is around 25mm/week, however this can vary from 12-40mm, depending on the weather, he says.

“You can’t just go on irrigating 25mm/week without regularly accounting for rainfall.

You need to use real-time data for scheduling and ideally look at rainfall variation between fields.”

Optimising soil moisture benefits tuber development and quality by helping to control key skin diseases such as common and powdery scab – increasingly important for crop marketability, he added.

Greenvale AP’s senior agronomist, Denis Walsh agreed, highlighting scab as one of the five main problems that account for over 70% of defects in most supplies.

“It’s all about skin finish – the fewer defects, the better.

There are a lot of other products [on supermarket shelves] that now compete with potatoes, so they’ve got to look right.”

He believed that the requirement for growers to carry out on-farm quality assessments will become an increasingly important part of supply agreements.

Preventing problems such as scab, greening, mechanical damage, bruising and black dot will reduce the risk of deductions, he said.

For effective common scab control, irrigation should start at tuber initiation if the soil is dry, with even wetting throughout that zone, Dr Stalham said.

“It’s very difficult to get even wetting in cloddy soil and if it’s too fine, this can lead to siltation of fine particles to the base of the ridge and cause anaerobic conditions and powdery scab.”

Frequent, small doses of water, over longer periods (eg 5mm at a SMD of 10mm) are generally more effective than applying large, infrequent amounts, he explained.

Generally irrigation should be applied for 25-48 days after tuber initiation, although this will vary depending on variety and duration of emergence.

paul.spackman@rbi.co.uk