Strategies for nutrition – Potato Review
Andrew Goodinson considers the merits of foliar feeding and biostimulants this season ...
Meeting nutrient demand of potato crops is key to yield, tuber weight, tuber size and quality, but there may be times when, despite the best thought-out fertiliser strategies, high or low levels of one nutrient will result insufficient levels of another, resulting in a need to balance crop nutrient availabilities.
Crop development from insufficient N, P and K fertiliser applied in the seedbed is not easily repairable, and can affect crop potential, uniformity, quality and resilience.
“Before you start, it is always a good idea to assess the variety that you are growing and its prospective nutrient demand, tuber setting potential and whether it is determinate or indeterminate,” Andrew said.
“The best management strategies ensure all the basics of crop nutrition are correct before refining them with supplementary applications such as foliar feeding.”
Andrew recommends careful analysis of soil indices to see what is already available, as well as any manures applied and match this to crop needs. “Some nutrients are leached out of soils over winter especially on sandy soils, such as nitrogen, sulphur, potash and magnesium so account of these losses should be made. pH is important as it can affect the availability of some nutrients, especially at low levels.
“If you suspect a deficiency, it is always advisable to find out exactly what it is and target your approach rather than applying a generic multi-nutrient product.”
Understanding crop needs at particular times have improved thanks to nutrient testing. For example SAP testing means it is now possible to analyse old and new leaves to evaluate how the plant is functioning and what it requires more accurately. This helps growers achieve better nutritional balance.
Nonetheless, tissue testing still has its place, and Andrew works out local trends by taking them across different farms on the same day in the same area. Reflecting on the dry spell in 2022, he points out the importance of K and Mg for cell turgidity, cell wall strength and nitrogen efficiency.
“During the dry spell last year, using tissue testing, we were able to identify crops which were low in phosphate, this was due to the root system not functioning well enough to extract the nutrient from the soil.”
However, some nutritional elements are not always easy to evaluate and rectify, particularly when potassium (K) levels are found to be low. “There are times when it appears to be available in the soil, yet the tissue sample show it to be deficient, possibly due to interaction with other elements such as boron and calcium.
“Potatoes need potash (K) throughout the growing season, and at the peak between uptake can be 10kg/ha/day. “Once a deficiency shows then the damage to the plant is occurring, despite a seedbed fertiliser application it is wise to top up the K, within the plant, with a foliar feed.
“Another thing to consider is if availability of one nutrient in the soil can lead to deficiency in another. For example, because we place potash in the seedbed, it can lead to an early magnesium (Mg) deficiency, as it is easier for the plant to uptake the K+ion rather than the Mg 2+ion.
To address this, he routinely uses a foliar feed with Mg, which is applied from the rosette stage – often with the first blight spray – until the end of bulking.
“If the tuber setting potential looks low, phosphites can stimulate root development leading to improved tuber numbers.”
Phosphites can be applied in the furrow at planting or at the rosette stage. However, he notes, environmental conditions such as cold, dry weather at tuber initiation (TI) will result in the potato plant going into survival mode.
“Plants undergo a number of physiological changes to cope with biotic and abiotic stress, and their survival depends on adaptation to a varying environment – and if they are stressed at TI they may abort daughter tubers resulting in lower tuber numbers.”
Nitrogen drives yield, and he recommends putting most of the Nin the seed bed, and then topping it up at tuber initiation if necessary.
“Of course, the amount of N taken up by the plant depends on the variety and whether it is determinate or indeterminate. If it is growing well it can uptake 5-10kg/N/day. Such crops may require top dressing at tuber initiation, after this growth stage the crop undergoes the highest crop demand for N.
While phosphate is also a crucial element, the overall crop demand is lower than for N.
“If your analysis shows the soil to be Index 3 or above, you may find there no need to apply any. However, it is often a good idea to place a protective granule such as Primary P in the ridge at planting. This close contact in the rhizosphere of the tuber increases root growth to enable better scavenging for phosphate in the surrounding soil, especially in soils with low organic matter.”
Alternatively, an application of monoammonium phosphate (MAP) or diammonium phosphate (DAP) can help by placing the important nutrient close to the roots to improve root development.
“Decisions on whether to apply foliar feeds need to consider whether the nutrient can be absorbed and translocated to where it needs to be. Calcium (Ca) is important in the tuber to keep the skin elastic and prevent cracking, but because the size of the molecule makes mobility difficult, it is difficult to ensure it gets to the right place, and also it does not move from the leaf to the tuber.”
Discovering whether there is a response to foliar feeding has always been one of the challenges, but with better understanding of how micronutrients work, they can be targeted more effectively.
“Applications need to be based on what the crop needs and the moment it needs it, but sometimes it can be difficult to get accurate assessments. When you are trialling new products, and you think you see a response, what you need to look for is whether there is a consistent response in different situations and seasons. The more you do to set the plant up early, the better it will be.”
As the nutrient base of each field is slightly different and potatoes respond to different nutrients from other crops such as combinables, Andrew recommends evaluating any deficiencies and then taking a targeted approach.
“For stressed crops last year, concentrated seaweed products as a foliar feed helped the crop cope with high temperatures and lack of water.”
Biostimulant products have to pay for themselves, emphasises Andrew, noting that they are increasingly viewed as an addition to the tools in the grower’s armoury because of their potential to improve crop quality and yields and reduce input costs and pesticide use.
“Ideally, there should be a planned programme which will help across the different stages of crop development, such as tuber initiation, tuber set and tuber bulking in potatoes.”
However, growers need to be clear about what they want from their biostimulants, and what they hope to achieve from using them. Before choosing which one to apply, they should identify any weaknesses of the crop which a biostimulant can help ameliorate, emphasises Andrew.
“Biostimulants should be considered as being part of a programme, as they are not a stand-alone silver bullet. They can be used to improve tolerance to abiotic stress, such as heat, cold, drought or salinity, but you have to apply it sufficiently early, and while they can be absorbed quite quickly, you have to wait for the stimulated root system to grow.”
Different biostimulants have activity in different areas of the plant, he explains, noting that they can improve germination and root structure development to vegetative growth.
“Bio-stimulants/biologicals are becoming popular as seed treatments, and as a powder or a liquid spray that is put on the seed potatoes on a roller table, while others go in as a drench at planting alongside a fungicide such as Amistar (Azoxystrobin) or Allstar (Fluxapyroxad).
“There are a number of commercial products in the market, and many of them contain numerous bacillus species, which stimulates an increase in root hairs which improve water and access to nutrients to help rapid root growth.
“It is important to use the label rate to get a response; doing trials with less than full rates are unlikely to have any effect at all as there will not be sufficient quantities of the biostimulant to make a difference.”
He notes that phosphite is often used to stimulate root mass and root exudate while it is not a biological product itself it will increase root mass and nutrient exchange.
He has found some benefit using it alongside molasses and seaweed as a drench for his asparagus crops, the molasses acts as a feedstock for the microbiome and the seaweed supplies a plentiful supply of hormones to the plant, this combination may have potential for use on potatoes too.
Also, once you have started, to get the best effect from the soil biology, you then need to give them time to breed and multiply so they can continue to work throughout the season, he said. This helps plants remain healthy and optimise their growing cycle so the crop has the best chance of meeting its genetic potential.
Andrew and his colleagues at Hutchinsons are working with various partners to understand the mechanisms, timing and efficacy of different biostimulant products to help growers optimise nutrient efficiency in their potato crops.
Together they have been evaluating biostimulants from a range of companies.
“These products have the potential to offer more resilience against environmental stress or up-rate the plant’s processes, usually by increasing the nitrogen reductase within the plant, and increasing energy for growth,” Andrew said.
Reflecting on the low blight pressure in the 2022 season, Andrew said the worst blight year in the last 20 years was back in2007, but he warns against complacency.
“With 36_A2 is on the increase again across Britain, comprising 53% of the outbreaks and incidence of 6_Al up again to 35%, it is clear that these are particularly fit and aggressive strains,” he said.
“The 36_A2 clone was first identified in the potato starch production regions of northern Germany and the Netherlands in 2014 and then found in GB crops in England in 2017. There are no reported changes insensitivity to a selection of key fungicide active ingredients, so its dominance and spread is likely to be due to fitness and aggressiveness.
“But changing blight populations can result in changing sensitivities so well thought out control strategies are a must.”
The recent rise of 43_Al is causing unease because, although it has not so far been found in Britain, it has proliferated in Denmark and has been found in Belgium and the Netherlands. The strain has been found to be insensitive to one of the most important actives used for late blight control, mandipropamid.
While it is possible that not all outbreaks of this strain will show this insensitivity, Andrew is concerned that other actives within the same fungicide group may also be at risk.
“These developments highlight how important it is to rotate chemistry to prevent resistance developing.”
He encouraged growers to sign up as Blight Scouts for the Fight Against Blight, which is run by the James Hutton Institute. Samples are sent in by the Scouts and analysed to detect in-season blight strains, so growers and agronomists can adjust their strategies if necessary.
The service, originally funded by AHDB Potatoes, has been supported by the Scottish Government since the demise of the levy body, and is expected to go ahead with a new funder in 2023.
Comparing for blight control options in GB with those available in Denmark, Andrew notes that the Danes have more limited choice because cyazofamid and mancozeb are not authorised for use there.
But it is not all bad news and there are some new blight control products in the wings; including a new systemic fungicide, says Andrew. “It is particularly good on new growth, and works well at the stage of rapid canopy expansion.”
In addition to fungicide choice, he emphasises the need to choose the right spray nozzle. “I find the 3D Ninety nozzles and IDTA04 Flat Fan give the best coverage, because you can set them to cover the leaves in the top, middle and bottom of the canopy, and also the underside of the leaf.”
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