Tuesday, 18 July 2017

Improving the eating quality of Potatoes

Use of nitrogen and the effect on dry matter

By Knowledge Exchange Manager, Anne Stone 

At the first field walk of SPot SW on July 4th, sixteen visitors looked at the Electra crop in Joe-S field, together with Ollie Blackburn, farm manager of Dillington Estates, Ben Mitchell and Merren Lewenden of Branston Ltd and Marc Allison of NIAB CUF.  
The topic for the demonstration arose from the experience of growers of Electra for Branston last year. As Ben Mitchell explained, 18 growers on 350ha achieved 60t/ha on average. The crop ranged from 15-22% in dry matter, and at the lower end of this range, the eating quality was poor.  Data collected by Merren Lewenden in the graph below showed how dry matter development over the season was affected by the rate of nitrogen fertiliser.

It appeared that excessive nitrogen resulted in too low dry matter percentage. Not enough nitrogen can also be damaging, as in one case on a sandy, gravelly soil where70kg/ha N was applied. The resultant tubers had 22% DM and dehydrated in store. About 19% DM is ideal and, with that target, Branston Ltd recommended 100kg/ha N for the 2017 season.

% dry matter / no. days after planting & harvest / nitrogen rates

The rebels at Dillington

In spite of that recommendation, Ollie Blackburn revealed that the Dillington Electra has received 120kg/ha this year. Nobody commented, but I’m sure we all like it when  people don’t give the ‘right’ answer.

Not so simple.

Relationship of N with dry matter is affected by moisture in the soil and varies from year to year. As Marc Allison said, in NIAB CUF research on 25 crops around the country with differing nitrogen rates, reduction of 30kg N only increased dry matter by 0.3%.  Knowing that a tidy correlation can’t be relied on each year led Dillington Estates and NIAB CUF to look at both irrigations and fertiliser as in the plan shown below. A base rate of 90 kg/ha N was applied to the whole field, with top dressing sprayed to achieve 120kg and 150kg/ha. Irrigation was scheduled for July 6th, and one side of the field will always receive less than the other.


There was a small snag setting up the demonstration. Practical problems led to spraying the top dressing later than on the rest of the farm, on June 24th.  Due to risk of scorch, Ollie chose an Omex product at 18% N, rather than the 26% N product originally planned. Still there was some scorch, especially where 60kg/ha had been applied, as the photo (right) shows.

Cheapskate Marc

In response to the question of whether sulphate of potash would help raise dry matters Marc replied that in NIAF CUF experiments there had been little difference between the results of using the sulphate or the muriate form of potash. There is a big difference in price between the two forms, making potash about twice as expensive if applied as sulphate.

 It is sometimes thought that keeping the crop green until burndown, with more fertiliser than required for yield, can protect against black dot. Marc Allison asked if those present had found that N fertiliser rates which led to yellowing of leaves from early August was associated with black dot or any other disease. No one had that experience, which encouraged Marc in his view that the low N strategy was sound.

Matt Hallett and Ben Mitchell
Whether to apply foliar nutrition was another query put to our visiting scientist. Marc thinks that although petiole tests for N are commonly used in America there is a lack of understanding of how the results relate to any benefit of nitrogen subsequently applied. He suggested that although growers are seduced by the low cost of adding foliar feed to a blight spray, these small costs add up. Similarly with trace elements, there is rarely an economic response. So his advice if you feel you really have to use a foliar feed is to leave an unsprayed strip to convince yourself over time that it doesn’t work.

With every question, Marc favoured the cheaper option. If he applies the same approach to family holidays the Allisons must have a nice time paddling at Felixstowe and Clacton on Sea.


Much of the discussion was about the end of crop life and whether there would be an effect on dry matter content of tubers if diquat is lost. A range of approaches were suggested by different visitors:

·         Flail first, one day, diquat (Reglone)at 1l/ha, followed by carfentrazone-ethyl (Spotlight) at 1l/ha

·         If lush crop: diquat at 0.5l/ha, 7 days, diquat at 1.0l/ha, 5 days, flail

·         If a senescing crop: Flail, 2 days, diquat at 2l/ha, 7 days, carfentrazone-ethyl (Spotlight) 1l/ha

·         cyazofamid (Ranman) and diquat together

·         If blight or blackleg; diquat before flailing

Delayed skin set can be a problem in Electra and two of those present mentioned times when it had taken 7-8 weeks after burn down. This is another reason for not using too much nitrogen.

When can we get an idea of the results?

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