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Soil type an important factor in tuber greening

Publication Date: 
14 May 2018
Author/Contact :
Author/Contact: 
Researcher: Dr Simon Smart, NIAB CUF

Planting depth and soil type are two of the most significant factors when it comes to in-field tuber greening. These were two of the main findings from a research project completed by Dr Simon Smart at NIAB CUF, and funded by AHDB Potatoes.

Tuber greening is a substantial cause of waste in the potato industry, costing millions of pounds each year.  On average, 5 % of yield is affected, but in the most severe cases, 20 % of yield is affected.  Tubers turn green when they are exposed to light, causing the production of chlorophyll and glycoalkaloids.  While chlorophyll is tasteless and harmless, glycoalkaloids are bitter-tasting and can occur at potentially toxic concentrations in green tubers.  Consumers are aware that green tubers may be dangerous and do not purchase or consume them.  Consequently potato packers have very low thresholds for tuber greening, in contrast to tuber blemishing diseases, where up to 5 % surface area can be acceptable.

The project:

The research project, carried out at NIAB CUF, looked at various factors including tuber formation, stolon architecture and nitrogen rates, but concluded that none of these was as important as the amount and type of soil above the potatoes as they grow.

Dr Simon Smart, Research Associate at NIAB CUF said: “Factors known to influence tuber greening include row width, planting depth, ridge shape, soil cracking and variety.  Differences between varieties are considered anecdotally to be caused by differences in stolon length and stolon depth between varieties - if tubers develop on stolons close to the surface or on long stolons they should be more likely to turn green.”

Stolon length is known to vary between varieties but the influence of this on the position of tubers in the ridge and subsequently on their susceptibility to tuber greening had not been investigated.  A fellowship from AHDB and additional support from Cambridge University Potato Growers Research Association (CUPGRA) provided an opportunity to investigate the physiological and agronomic causes of tuber greening over 3 years.

Seven varieties (Estima, Jelly, King Edward, Marfona, Maris Piper, Markies and Melody) were grown in a replicated experiment in Cambridge in order to investigate the relationship between stolon architecture and tuber greening.  Plots were sampled around the time of tuber initiation and the length and depth of every stolon was measured.  In the middle of the season, stolon length and depth, and the position of each tuber were measured, by painstakingly removing soil from around each plant (Figure 1).  Samples were taken after desiccation to assess for tuber greening. 

In addition, 36 commercial crops were surveyed to quantify stolon architecture and to relate the position of tubers in the ridge to tuber greening. One such crop was at The Strategic Potato Farm West of the time, W B Daw & Son, Staffordshire.

Results and recommendations:

Both within and between years, differences in stolon architecture between varieties did not account for differences in tuber greening (Table 1). 

Dr Smart said: “This was probably due to tuber depth not being directly related to stolon depth, because of differences in tuber size. In commercial crops, planting depth varied widely, both between and within crops.”

​Variety Stolon length (cm) Relative stolon depth (%)* Tuber greening (% yield)
Table 1. Average stolon length and depth and tuber greening over 3 years of the variety experiment. *Mean stolon depth as a percentage of planting depth
Estima 5.1 76.1 19.5
Jelly 7.5 81.1 15.1
King Edward 4.8 76.4 18.8
Marfona 5.8 82.5 20.1
Maris Piper 3.9 78.1 9.5
Markies 6.1 77.1 11.1
Melody 5.9 76.0 7.1
S.E. (54 D.F.) .20 1.27 1.94

The work showed that planting depth and soil type are two of the most significant factors when it comes to in-field tuber greening. Soil cracking was found to be a significant factor, so clay-based soils require more soil above the tubers to prevent greening.

Crops grown on soils with a high clay content and planted shallowly, are particularly at risk from tuber greening, especially when yields are high and tubers are large.  The optimum planting depth to limit tuber greening may be deeper on soils with a higher clay content, but may reduce overall yield.

Dr Smart said: “On sandy and peat soils, few green tubers were found with more than 2.5 cm soil coverage, but on clayey soils, green tubers were found with more than 5 cm of soil coverage.  Tuber greening was most severe on sites where more tubers were close to the soil surface and where the clay content of soil was higher.

“Unfortunately, planting deeper is known to delay emergence and therefore reduce yield, so growers must find the balance between achieving a high yield while also minimising tuber greening.”

Other relevant work:

Claire Hodge, Knowledge Exchange Manager Scotland for AHDB Potatoes, said there were correlations between planting depth and cultivation depth, after demonstration work at SPot Farm Scotland at Blairgowie in 2017. The results of the demonstrations showed increased yield when the new shallower cultivations were compared to standard farm practice.

Claire said: “Growers will look to strike the balance to make sure cultivation depth allows enough soil in the bed to get the planting depth to an optimum. They should be able to achieve reduced cultivation and maintain planting depths if needed.

“Simon’s research reminds us that we may have to consider planting deeper in soils with higher clay content or if we are growing varieties that are more susceptible to greening, but even at the shallowest cultivation treatments, there should still be sufficient soil to plant into.”

Conclusions and further reading:

Dr Smart concluded: “Where tuber greening is substantial, growers should investigate where the green tubers are in the ridge, e.g. exposed at the surface, growing out of the flanks or unexposed and adjust their planting depth and ridge geometry accordingly.”

The full project report, titled Understanding tuber formation: maintaining the capability to improve tuber quality attributes including greening, is available here: https://potatoes.ahdb.org.uk/publications/11140001-agronomy-fellowship

SPot Scotland results - Cultivation depth: https://potatoes.ahdb.org.uk/publications/spot-scotland-results-day-presentations​

SPot West 2015 - in-field greening results: https://potatoes.ahdb.org.uk/knowledge-hub/media/presentations?keys=&page=1​

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