Get to the root of the issue – British Potato Review

Based in Herefordshire, Agronomist and Potato Soecialist Andrew Goodinson has been working for Hutchinsons for 17 years and looks after nearly 8000 ha of farmland, including the Welsh borders, south Shropshire and Worcester. Most of the potato crops he looks after are destined for the crisping or processing markets. This month he looks at ways to help this year’s crop get the best start possible.

Creating the best conditions for getting the crop established and the canopy to start photosynthesising as soon as possible is the most important aim when it comes to planting, says Andrew.

Ideally, he likes to see a strong canopy that closes quickly but says yield is not only driven by the canopy’s ability to intercept light for photosynthesis. The root structure and architecture play a crucial role in facilitating water and nutrition uptake.

Any imbalance caused by excess of a nutrient can be detrimental, he warns. For example, if a crop has too much nitrogen, it not only leads to soft growth that is more susceptible to pests and diseases such as late blight, it also creates more green mass to defoliate.

As with all crops, root development and structure is crucial, and Andrew points out that, unlike combinable crops, the potato is not good at producing roots. For example, in perfect conditions, a potato plant can grow a root measuring 1m, but roots 60cm are more typically seen.

“If we look at this another way, in a potato field there are 3.8km/m2 of roots in the topsoil, which sounds a lot until you realise that cereals produce 24km/m2 and oilseed rape 19km/ha.”

He recommends using a starter fertiliser in addition to applied organic manures at the time of planting, adding that base fertiliser can help the crop establish and provide a yield boost of between 5% and 15%.

Products such as Primary P, which contain 10% N, 40%P 2% Mg and 2% zinc, can be applied in a granular or liquid form, and cost between £50-£70/ha.

As for most crops, the main nutrients are nitrogen (N), potash (K) and phosphate(P), but micronutrients such as zinc (Zn) and calcium (Ca) are often underestimated, as are the interactions between the different elements.

“All nutrients, whether major or secondary, play an important role in growing good quality crops that yield well and, as found by German chemist Justus von Liebig, one of the founders of agricultural chemistry, there is a Law of Minimum which states that deficiency in just one of these can impact on yield and quality, said Andrew. “For example, zinc is often considered as a secondary nutrient, but it plays an important role in the plant’s uptake of phosphate, N metabolism, auxin formation, cell division, and elongation.

“This is critical because elongation plays a role in tuber growth, but also because it has an effect on skin quality and helps the development of skin elasticity.”

However, it should not be applied at tuber initiation (TI) as it can affect tuber numbers and growth, he said.

Zinc deficiency can occur with high phosphate levels and although symptoms can include interveinal chlorosis and small leaves in the canopy, symptoms not necessarily visible in the field.

“If your soil is very high in magnesium, there can be imbalances in potash uptake. To ameliorate this risk, it is often a good idea to apply foliar Mg during the season.”

Soils high in calcium can also result in phosphate being locked up. This can happen in the southwest. The phosphate is locked up because of high pH (a measure of hydrogen ions, the higher the hydrogen ions, the lower the pH). As a result, the higher the ratio of calcium to phosphate, the more locked up the phosphate is.

Potatoes do not need much N for the first four to five weeks after planting, with 60-70%being taken at tuber initiation (TI) and bulking.

While there is no benefit to applying more N either by foliar or top-dressed after bulking in periods of stress, high temperatures or drought, an application of seaweed can help reduce stress symptoms right up to the final stages of the crop’s development. Some bio-stimulants applied just prior to a stress event can also help maintain the rate of growth.

The critical time for P uptake is around 50days after planting, which is around TI, and also the second half of bulking.

“While phosphate makes roots, potash makes shoots,” he says, noting that the plant needs K at the same time as N. “On its own, K affects dry matter and bruising susceptibility, while when it is together with N, it contributes to haulm longevity and therefore impact on yield.”

Nutrition starts with the soil, so the place to start is to invest in a good analysis to assess soil health and what nutrients will be available at the right time, he emphasises.

“As the crop needs a balanced supply of nutrients, it is not just about having the nutrients in the soil, the many interactions between these nutrients needs careful analysis as these can interfere with the uptake of other nutrients.”

P not only helps increase ground cover, which impacts on tuber numbers and marketable yield, but also when crops grow quickly, they tend to have fewer symptoms of stem canker and FLN damage.

Knowing the availability of P in the soil remains crucial, and there are alternative ways of measuring than Olsen P, as it only measures available P and does not give the full picture of any legacy in the soil.

“Potatoes only remove a small amount of phosphate, leaving it behind in the soil” he said, adding that P is negatively charged and sticks to positively-charged soil particles and organic matter, making it less prone to leaching than positively-charged nutrients.

“For example, if P is Index 3, it only needs 50kg of fresh P to grow. What is important is that potatoes need access to the nutrient to be within 1mm of the roots because phosphate is so immobile in the soil.”

However if granular fertiliser is placed too close to the seed tuber, it can cause scorching of the developing roots owing to chloride in muriate of potash (MOP).

“If you are replacing your planter, it is worthwhile having a liquid/solid fertiliser applicator fitted on the new one.

Calcium (Ca) is another crucial nutrient for potatoes, says Andrew, noting hat it reduces incidence of rust spot and other disorders as it is only taken up by the roots, and trying to remedy a shortfall by the application of foliar Ca will only lead to inconsistent results at reducing deficiency symptoms. If your soils are low in Ca, an application of gypsum is useful because it contains Calcium and sulphur but does not increase pH levels.”

Some soils, including many of those in Herefordshire, have a high magnesium to calcium ratio, which affects the workability and friability and Andrew finds that gypsum helps redress the balance.

“Sulphur affects chlorophyll production, and deficiency results in upward leaf curving and light green or yellowish leaves, which affects photosynthetic efficiency.”

Another benefit of gypsum is that it also contains sulphur, which is sometimes considered as oil for the soil’, observes Andrew, adding that the combination of sulphur and calcium is beneficial for the soils, as together they help prevent ridges from slumping, capping and then cracking open.

Remedying the amount of Ca available inthe ground can be done with chicken manure but as it is also very high in N and P, and this can upset the nutrient balance, said Andrew.

Foliar-applied nutrients have won an important place in nutrition strategies, and timing is crucial to get the best possible efficacy, with applications made at the rosette stage, TI and the onset of tuber bulking.

“Prior to potential foliar applications, it is a good idea to do some leaf sap petiole testing so you know the what is going on in the crop and what it needs,” he says, adding that tissue tests are best done in the morning.

Nonetheless, he reflects, it is important to tailor fertiliser according to need, and avoid older practices that may have applied 1000kg/ha of a 14:14:21 blend of N,P,K to coverall bases.

“If you have grown a cover crop before potatoes, there may be some residual P in the roots, but any N captured will be used by the soil microbes, leaving only a little residual N for the following crop.”

Effective weed control in the early stages of crop development is vital for achieving a high yield, says Andrew. Anything that affects early crop growth can threaten quality and yield. Herbicide applications too close to emergence can also damage crops, particularly varieties that are not metribuzin-tolerant.

Crops were affected more than usual last year because of the poor seedbed conditions which resulted in plants struggling to develop roots and were unable to bounce back from damage.

“Ideally you would apply a residual two weeks after planting then return closer to emergence with a contact herbicide (Gozai), or apply a residual and the contacts together prior to emergence.”

Whatever strategy is chosen, it needs to be followed through effectively, said Andrew. “Cleavers and bindweed can smother the canopy, while others, such as oilseed rape volunteers, fat hen, and thistles compete with the crop for nutrients, water and light, affecting yields and also making harvesting more difficult.

“Further problems can be caused by those in the same crop family. For example black nightshade (Solanum nigrum), can also host some of the same pests that affect potatoes such as Rhizoctonia.”

Herbicide costs have gone up, but ensuring the right fit to the on-farm conditions and soils is still important. “The aim should be not to use post emergence broad leaf herbicides, as there is a risk of affecting foliage and crop growth.

“Soil type should be an important driver of your herbicide choice too. Herbicides move more quickly through sandy soils, so their persistence is shorter. Choice of herbicide is limited, and Bentazone (which is post-emergence) is effective with cleavers and other broad-leaved weeds is under review because of the risks of it entering water courses. This will limit the already depleted toolbox even further, and there is nothing new in the wings,” said Andrew.

“Managing grass weeds with a contact herbicide is increasingly difficult, and there are occasions when we resort to an application of glyphosate as a pre-em within 10 days of planting. This can still do a reasonable job, but timing is critical because glyphosate is very mobile and can damage shoots below the surface of the ridge.”

He said some mechanical weeding is now being done with potatoes grown in the regenerative agriculture system.

“The seed beds are cultivated and then re-ridged, which is a useful activity because it helps to mineralise nutrients. After planting there is often heavy rainfall and the ridge caps and by re-ridging this avoids the problem at the same time as removing the weeds,” he said.

“Although the activity could encourage other weeds to germinate, as the potato is already planted and getting ahead, these should soon be smothered by the canopy.”

Skimping on rhizoctonia risk assessments may put the crop at risk, warns Andrew. Also known as black scurf, the fungus causes stem canker, which reduces sprout development and causes stolon pruning, affecting the number of daughter tubers. Cold, wet soils make the crop more vulnerable to infection.

He recommends washing and examining the seed potatoes when they arrive on-farm and treating them with a fungicidal seed dressing if necessary. Home-saved seed can help growers save money on the costs of seed, but seed health remains key and proper testing should still be done.

“There are varietal differences in the number of tubers produced, so if you are growing a variety such as Shepody, which does not produce many tubers, compared to Maris Peer which produces a lot of tubers, seed size and plant spacing become important.”

Where treatment is necessary, the sector is moving away from on-planter powder dressings, and towards liquid applications, which must be applied on a roller table on a grading line and this can be done at the seed source or on arrival at the farm, he says.

Rhizoctonia is not always possible to see and there can be sub-clinical infections, said Andrew. Sometimes the runner hyphae can be invisible as well as typical symptoms of growth cracks, a crazy paving-type skin blemish or symptoms that look like black ash on the tuber surface. As a result, growers need to think of the bigger picture and how that might affect the risk of the pathogen affecting crops.

For example, knowledge of field history is useful when making decisions on whether to apply an in-furrow treatment for soil-borne rhizoctonia, particularly when the previous potato crop had been harvested more than four weeks after defoliation.

“Last year’s cold wet soils in spring saw more of the disease than in other recent seasons, but there were other factors coming in to play, such as physiological age and plant establishment, which made the plants more susceptible to stolon pruning,” said Andrew.

Symptoms on emerging crops can be confused with free living nematode (FLN) damage, and he notes that some agronomists consider that FLN feed on roots infected by rhizoctonia, making an accurate diagnosis more difficult.

“Testing for FLN pre-planting would help clarify the situation, but this cannot be done at the same time as PCN testing. Common-sense practical routines together with appropriate fungicide applications on the seed tubers or in-furrow can help reduce the threat of seed and soil-borne pest rhizoctonia.

“Seed treatments can also delay emergence, although not establishment, they prevent infections of other pathogens black dot and silver scurf. Using all the information available is key to making informed decisions on what is right in a particular season to help get the most from your crop.”