Building the foundations for a strong and sustainable vineyard

The nutritional needs of vines change as they enter different stages of growth during the season; a peak level of calcium is used just after bud burst, for example, while the uptake of potassium and nitrogen is at its highest after flowering as the fruit sets.

Building the foundations for a strong and sustainable vineyard can be the work of many years, developing the soil’s capacity to capture, retain and, when required, release vital macronutrients and micronutrients for the development of healthy commercial crops which produce quality wine.

Monitoring nutrients through tissue analysis of the vines is an important measure that should be taken regularly, but not an absolute indicator of health. It is quite normal for there to be some variability in vine nutrient requirements across vineyards and over different seasons. There is also a ‘normal’ range for expected leaf nutrient levels, but that estimate of what is ‘normal’ should be used as a reference point. Production goals, vine growth and historical data should also play a part in assessing the nutritional requirements of the plants on any given estate.

Soil nutrition

Good nutrition starts with a solid foundation. Soil health is quite literally the bedrock of a successful vineyard. Analysing the physical, chemical and biological composition of the soil helps to understand nutrient availability and also how that varies from one area of the estate to another, allowing for the precision management of nutrition.

Julian Searle, agronomist with Agrii has asserted that soil structure may be the most important factor when it comes to vine performance – “influencing how vine roots can grow through the soil profile and forage to exploit nutrient reserves,” he writes. Adding organic matter is one way of improving the structure of the soil, alleviating compaction mechanically or adding cover crops with deep roots are alternative approaches.

The key to successful nutrition is balance, as Tim Haywood, owner of Astley Vineyard near Worcester explains. Astley has recently had a successful trial of worm soil, used to help establish newly planted vines. Along with introducing worms, they have been maintaining a good soil structure through the addition of organic material like mowings, prunings and pomace, introducing a wildflower cover crop and avoiding compaction and unnecessary cultivation. But vineyard managers “need not encourage over-rich soil,” Tim said. “Vines enjoy the struggle.”

Astley Vineyard’s regime consists of seaweed as a foliar feed and targeted micronutrients based on soil and petiole tests, mainly boron and copper. The team balance this by ensuring that the vines don’t get stressed by over-cropping. They also prune to a sustainable length of cane and remove excess buds and shoots.

Astley’s philosophy holds that the better the grapes, the better the wine. They aim to pick at ideal harvest numbers – rather than being pressured by poor ripening or disease. The team do not underestimate the old saying: good wine is made in the vineyard.

Where mature vineyards have degraded soil, research from America suggests that applying compost in high concentrations can achieve improved soil fertility and vine performance, without any compromise in grape quality. Here, the application of steer manure (that is young, neutered male cattle raised for beef production) was found to improve levels of some macronutrients including nitrogen, potassium and available phosphorus, whilst decreasing phosphorus fixation.

Getting to know macronutrients

As the name suggests, macronutrients are those required in relatively large quantities by the vines, as opposed to micronutrients which are needed in tiny amounts – although they are no less essential. Let’s focus on some of the key macronutrients, noting that this list is not exhaustive.

Nitrogen

Nitrogen has a significant role to play in all grapevine processes, making it essential for normal growth. It impacts shoot growth and yield as well as the plant’s sensitivity to fungal diseases. It also influences the synthesis of primary metabolites like sugar and organic acids, along with secondary metabolites like amino acids, flavonoids and aroma compounds, so nitrogen’s presence has an important contribution to make to wine quality. A lack of nitrogen can therefore affect all key metabolic functions – hampering shoot and bunch development, for example.

The nutrient is available naturally in the soil, although different types of soils will retain it more effectively than others. Vines need nitrogen in a soluble form to be able to absorb it. Increasing the amount of organic matter in the soil contributes to the amount of nitrogen that is plant-available. It also has the benefit of making heavier clay soils better aerated and increasing water retention in sandy soils.

Many nitrogen fertilisation trials have taken place in vineyards around the world in recent decades. A Spanish study has produced a meta-analysis of 374 such studies to try and better understand the overall trends. Those researchers have focused on the Mitscherlich law of diminishing returns as describing “the set of vine production parameters against nitrogen application rate”. They have identified optimum nitrogen requirements for maximising grape quality in their research, although these recommendations are given for general application without fine-tuning for parameters like vine age and variety.

Essentially, nitrogen fertiliser can support healthy vine development up to a point, after which the plant cannot take up any more of the nutrient and any surplus may be leached out of the soil or lost to the atmosphere. Tipping over the sweet spot can encourage excessive vigour, leading to shading of the fruit without careful canopy management.

To effectively manage nitrogen levels, the nutrient status of the plant needs to be tested. By the time a nitrogen deficiency is visible through leaf discolouration and reduced vigour the damage has already been done, broadly speaking. A regular programme of tissue sampling gives a proactive way around the problem.

Good general soil health promotes the presence of beneficial microorganisms in the ground which will facilitate the natural nitrogen cycle and encourage better nitrogen fixation over several years to reduce reliance on fertilisers. Reducing the use of herbicides and pesticides, adding cover crops and encouraging biodiversity within the vineyard can all support the improvement of overall soil health.

When applying nitrogen, the timing is important to ensure the most efficient take-up. Vines benefit the most from nitrogen fertiliser around the time that they bloom.

A recent review of the role of nitrogen in vine balance and grape composition published in OENO One points out that nitrogen has been used extensively as a fertiliser for generations now, even though the plants only absorb around 30%-40% of what is applied. Their study concluded that “by adapting plant material, soil management and vine balance to environmental conditions, it would be possible for grape growers to improve plant nitrogen use efficiency and minimise nitrogen input in the vineyard.” Researchers also emphasised the need to balance and restrain, since an excess of nitrogen is “as detrimental to wine quality as is nitrogen depletion.”

Potassium

Potassium is essential to both vine and berry growth, so plants need large amounts both in the spring when the season’s growth begins and further along in the year, when the grapes develop and ripen.

As well as impacting potential growth, a deficiency of potassium can also lead to rapid water loss from the leaves, since the nutrient helps the plant to regulate the opening and closing of stomata, the small pores on the underside of the leaves which control transpiration. Interesting research was published in Biology in 2020 that highlighted the impact on photosynthesis caused by deficiencies in potassium and magnesium, which ultimately leads to decreased biomass. The researchers observed limitations to growth both above and below ground where nutritional deficiencies had compromised photosynthesis.

As with any nutrient, potassium levels can and should be judged through a combination of soil analysis, plant tissue evaluation and visual checks for signs of deficiency in the foliage. Necrosis in the leaves or chlorosis around the leaf veins are both indicators that there is a problem. It is important to use a combination of assessment tools to check the availability of potassium because the amount recorded in the soil may not be representative of what is available for plant uptake. Taken on its own, soil assessment may give a false sense of security.

As with nitrogen, an excess of potassium can lead to excessive vigour, with all the problems that poses for grape quality. A balance needs to be struck. An early spring application of potassium can help to bolster new growth, while an application just after harvest and before leaf drop can restore what was removed when the grapes were harvested. Where a deficiency has been noted, a foliar application around veraison can give the plant the boost it needs at a key time for development.

Phosphorus

Phosphorus holds the key to vine growth. As well as playing a role in photosynthesis, it is a component of cell membranes and supports the movement of sugars and carbohydrate storage in the vine. Without it, vigour is reduced and the leaves can yellow and drop. A phosphorus deficiency can lead to poorly formed buds at the start of the season and therefore poor fruit set later on.

Organic phosphorus in the soil comes from dead plant and animal residues and soil micro-organisms, but it is not plant-available. Those micro-organisms play a vital role in transforming organic phosphorus into its inorganic form, which is accessible to plants for nutrition. The other key source of phosphorus in the soil is in minerals. Rain and erosion can unlock it and wash it into the soil, but equally excessive water can leach the precious nutrients away.

The efficient uptake of phosphorus increases where crop nutrition has a good general equilibrium. For example, the application of ammonium forms of nitrogen with phosphorus will increase the uptake compared to a phosphorus-only fertiliser applied separately. The absorption of phosphorus is also negatively impacted by poor soil aeration, low soil temperature or excessive soil moisture. A compacted soil decreases the volume of soil penetrated by the vine’s roots, so this will also limit their access to nutrients.

Once again, the law of diminishing returns comes into play, as an excess of phosphorus in the soil can result in a reduced nutrient uptake overall and create excessive vigour which will negatively impact the development of the berries.

Research from the University of Leeds, Lancaster University, the University of Technology Sydney, the Agri-Food and Biosciences Institute and the Centre for Ecology and Hydrology has reported that phosphorus use across the UK agricultural sector is currently extremely inefficient. It is used excessively and unsustainably, with runoff from farms and wastewater treatment plants leading to pollution across our lakes and rivers. Researchers say that to become less dependent on phosphorus supply it is necessary to recover phosphorus that has already been leaked into the environment using emerging technologies.

Their findings have been published in the UK Phosphorus Transformation Strategy, which is available to read online:
https://zenodo.org/records/7404622#.Y5rgSnbP2Uk

Magnesium

Magnesium is the main component of chlorophyll as well as having a role in the synthesis of proteins. Low levels of magnesium can cause premature fruit drop at harvest time and cause yellowing or redding around the edges of leaves, which will appear in the middle or towards the end of the season. Accordingly, magnesium is best applied to the soil as a long-term strategy. Foliar sprays can correct deficiencies during the season, but yield may already have been impacted.

Soil composition can play a role in the availability of magnesium. Extreme pH levels – too high or too low – can lead to magnesium deficiency, and problems have also been observed where soils contain high levels of potassium. Tissue sampling is the best way to check that magnesium levels within the plant are in a healthy state of balance and to flag whether correctional action is required.

Technology: The role of image analysis and AI

In late 2022, Australian researchers at Charles Sturt University announced the part their work had played in developing a smartphone app that can easily diagnose grapevine nutrition deficiencies.

The work has been commercialised in partnership with global agtech company, Deep Planet, forming one aspect of their VineSignal app, which gives users a diverse set of data about their vineyard to support nutrition management strategies. The Charles Sturt team’s contribution included developing image analysis algorithms that train artificial intelligence to assess vine leaf symptoms using photos taken on a standard smartphone camera. The research was intended to help vine growers overcome the tricky problem of symptom confusion.

Behind the app, the University team have built up a library of disease images. They have automated the way user images are processed and checked against this library to make sure the tool is as fast as possible, giving immediate results. While the technology is still in beta testing, it is clear that this is a new phase in the application of technology to nutritional monitoring in the vineyard environment.

Brush up on your knowledge

Plumpton College, a UK centre of excellence in wine education, offers an online course in vine nutrition. It is composed of a video of a WineSkills workshop along with a PDF of course materials which is aimed at vineyard managers and other senior vineyard staff.

This vocational course evaluates the use of tools such as soil analysis and plant observation and discusses the use and application of organic matter, mineral fertilisers and other nutrient replacement methods.