Making efficient use of water this season is likely to become even more crucial for potato growers, with some having already used precious resources irrigating other crops.

Water supplies are likely to come under pressure, with many growers resorting to irrigating cereals to save crops and maintain yields, says Mark Stalham, Cambridge University Farm’s irrigation expert.

But growers can make best use of water by following some key advice. The first is to apply water only when the crop needs it, says Dr Stalham.

“There is no point in holding back water in reserve. As long as it is needed when you apply it, you’re probably going to use it most efficiently.”

Withholding it for later months could mean having water available at the same time as when it is raining, he points out.

But for general crop growth, it could be worth running soils slightly drier than normal. In experiments there is little difference in yield between fields run at field capacity and 25mm soil moisture deficits, he says.

Having said that, running deficits close to field capacity risks wasting water, as large rainfall events risk topping up soils beyond field capacity, resulting in drainage. There is also a risk of damaging tuber quality from applying too much water.

“Where growers risk running out of water this season, for most varieties, it is better to spread the water more thinly. For example, if you were planning on irrigating four fields, and crop demand exceeds supply capacity, it is better to continue to irrigate all four less frequently.

“That will give a bigger overall yield response than satisfying water requirement fully on three fields and letting the other one exist on rainfall.”

The same does not apply for common scab control, where soil moisture deficits of more than 15mm on the most susceptible variety, Maris Piper, can result in scab severe enough for rejections for packing.

The potential problem when irrigating for scab control this season, with the earlier planting season bringing forward emergence and subsequent tuber initiation, is not one of water shortage, but in getting around crops, Dr Stalham suggests.

Irrigators have been under pressure, not only because of the dry weather, but also because high winds have increased evapo-transpiration rates and shortened the intervals between irrigation events, as well as making it impossible to irrigate with an acceptable degree of uniformity.

“So you might have a situation where you’re on a five-day interval and it is taking eight days to get round,” he suggests. “Clearly that is a case for restricting area so you can get round in time.”

Deciding which area or field to drop should be based on the risk of scab infection. “So that could be a variety with more resistance to scab, or a soil type able to hold water better, or a field with less history of scab.”

Another factor could be the stage of tuber development. Some new Potato Council-sponsored research has suggested that the critical period for scab control is the second and third weeks after tuber initiation, so if the field is past that period, irrigation could be stopped, suggests Dr Stalham.

“The important thing for scab control is to perform the regime properly on a reduced area, if needs be, but then use the remaining water as needed.”

Water for scab control

Water use for scab control could be reduced in the longer term, Dr Stalham believes. Based on average weather and emergence dates, around 30% (35mm) of the average seasonal irrigation total of 118mm should be used during the four-week scab control programme, but growers apply nearer 48% (61mm), according to a survey of growers.

“It is an area that we are targeting to improve efficiency. Growers are frequently applying larger doses (eg 25mm) of irrigation for longer than we would recommend for scab control.”

That information has resulted in a Potato Council project researching different soil types and varietal interactions with irrigation for common scab control.

“Most growers who schedule irrigation for scab control do so based on the most susceptible variety, Maris Piper, where the risk of scab is high, and where keeping soils wet will reduce scab to minimum levels.”

However, recent work has shown that some varieties can survive drier soils. A good example is Vales Sovereign, where soil moisture deficits up to 30mm during scab control have not led to rejections from scab.

“So if we can adopt that approach more widely, we can immediately save water during the scab control period, when the risk of losing water from the system by drainage is greatest. The largest inroads into improving efficiency are to be made in scheduling scab better on different soils.”

Extending that information to other varieties is part of an ongoing project, which will attempt to group varieties into similar scab susceptibilities and their tolerance to drier soils.

There are also differences that could be exploited between varieties in their responsiveness to irrigation for yield. Although all varieties will yield highest when the soils are kept wet, later in the season when soil moisture deficits are run higher, it is important to know at what point varieties start to lose yield and respond best to irrigation, he says.

“We have identified some varieties that grow adequately at a soil moisture deficit of, say, 50mm on sandy loam soils, and others where growth rates are slowed at 40mm.”

Another area where efficiency could be improved is during late bulking. “Many growers stop irrigating too early in August under the belief that soils will subsequently be dry at harvest.

“But even with maintaining soil moisture deficits at 40-50mm in August, the rainfall in September and early October can frequently be double that, so it doesn’t really matter if you’re maintaining a deficit of 25mm or 65mm, as the rain can undo all the deficit and more.

“There are many crops that suffer premature senescence and also a slow rate of bulking even with a full canopy because they run out of water in August.”

Maintaining moderate soil moisture deficits also helps guard against bruising, he points out. “If the crop senesces prematurely through a lack of water, then bruising tends to increase at harvest.”

The key is to continue monitoring soil moisture deficits throughout August, he says. “By ceasing irrigation too early, growers could be losing 5t/ha in some dry seasons.”

That scenario doesn’t necessarily mean applying more water, he stresses. “From our survey, it shows growers are applying too much water for too long during scab control, and not enough during late bulking, so it could be a case of shifting the balances around a little.”