Key nutrients for tuber bulking


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Don't overlook nutrition says specialist


In terms of establishing final yield and quality, the tuber bulking phase is one of the most important stages of crop growth for potatoes, and the role of crop nutrition during this period should not be overlooked.


Chris Bond, FMC Commercial Technical Manager for crop nutrition, explains that growers in most parts of the country have experienced good conditions for establishment and will want to capitalise on that potential, especially given the high input prices the industry is currently experiencing.


“The tuber bulking phase not only determines the size of the tuber, but also the final yield potential. So, ensuring the crop has the right nutrition will allow the tubers to grow to a marketable size and ensure the necessary internal quality and help to build starch content.”


According to Chris, any phase of crop growth where biomass is accumulated, whether above or below ground, requires a lot of energy and potatoes are no exception. “Phosphate is important at this stage because of its role in the formation of ATP, which drives plant energy at a cellular level,” he said.


“Once tubers have formed, the roots of the potato plant generally become less efficient at extracting nutrients from the soil, as the crop is focusing on the tubers rather than fine root production.”


Therefore, unless there is a pool of very readily available phosphate in the soil, it may not be taken up by the plant, especially in cold conditions, although these are less likely in the summer months.


“Foliar phosphate applications can be beneficial in this circumstance as they can bypass the soil and go straight to the plant, helping the crop through the bulking process, improving tuber size, marketable yield, as well as tuber quality.”


If plants are showing classic symptoms of phosphate deficiency, such as a purpling on the leaves and stems, then it is probably too late to prevent limiting effects on tuber development. Growers are therefore encouraged to assess phosphate levels and availability throughout the season to ensure any foliar phosphate is applied as soon as required.


Maintaining bulking also requires efficient photosynthesis. Magnesium and manganese are important in allowing the plant to build the chemical compounds that it needs to export to the tubers.


Mr Bond points out that magnesium is an essential element in chlorophyll, and manganese is required for chlorophyll production. “You aren’t going to have that active green canopy if you’ve got a magnesium or a manganese deficiency.


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“You really want a large productive canopy, but magnesium can become less available in drier soils, meaning there’s a higher risk of deficiencies in non-irrigated crops,” he said.


“Shortages are more likely through the summer months and can result in a shortage of available magnesium in the soil, as it depends on water to move into the roots. Therefore, boosting availability by applying it through the leaf can be beneficial.”

But the most important nutrient at this time in the crop’s development could arguably be potassium.


“Potassium is very important in controlling water movement in all crops, but especially potatoes, due to its important function in stomatal control and plant-water regulation,” said Chris.


“You need water to keep the plant healthy and actively growing, and you obviously need a lot of water to build the tubers. The last thing that you want is for crops to run out of water and stop growing.”


Potassium is also involved in the activation of a number of enzymes, including starch synthase, which is crucial in the formation of starch, the key component in the tubers. Potassium also helps manage the export of photosynthates and other compounds from the leaves where they are formed to the tubers, where they play a role in bulking.


“Potassium deficiency can lead to problems with internal blackening in the tubers, making them unsaleable,” warns Mr Bond. “As well as affecting tuber quality directly, due to a shortage of starch, potatoes that are low in potassium can be more susceptible to damage during harvest, so you might get more bruising or damage during lifting and handling. This could be a potential risk factor for the variety Sagitta, as it often shows a deficiency a potassium.


“A number of foliar nutrition products can be applied to help support the supply of all of these crucial nutrients, but for the supply of potassium, growers should consider KuruS®, which contains a highly soluble form of potassium to support plant growth and is a clear liquid fertiliser which is already in solution, making it easy to use and mix into the spray tank.”


He advises using KuruS® at the start of the bulking phase, based on soil or tissue analysis, with a follow-up application after 10-14 days if required. As well as potassium, the product contains sulphur which is necessary for nitrogen utilisation and protein formation.


“At this stage of crop growth KuruS®  should be used as a foliar spray, and many growers choose to combine it with their blight control programmes at a particularly busy time.


“It is physically and chemically compatible with a wide range of agrochemical products, but as with any tank mixes, growers should consult the latest information from their adviser regarding its compatibility with particular crop protection products,” he said.


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