Wine is a product of nature and as such, to reach appropriate levels of quality, it is necessary that grapes used to make it be healthy and ripe. There are two types of ripeness: 1) physiological, when its sugar is in balance with its acids; 2) phenolic, achieved slightly more slowly, when tannins & aromatics have reached their optimum levels. The former is the minimum requirements for wine quality while the latter is the key to high quality. At optimum phenolic ripeness, the tannins will not only be ample but also silky, and the aromatic spectrum will be at its widest. These conditions will enhance the wine’s complexity, and ageing ability, two desired characteristics of a high quality wine.
Although minerals in soil are essential to vine nutrition, studies have shown that there is no direct correlation between them and flavours and other elements of wine quality (Séguin, Bordeaux, 1970). However, nutrients are essential to plant growth and ultimately to the ripeness of the grapes. It can therefore be argued that, indirectly, nutrients are essential to the production of high quality wine.
It is the absorption of minerals that is key. Poor absorption creates imbalances, excesses or shortages, each with their causes and effects on plant growth. It is especially critical in relations to nitrogen and potassium content in the soil as their impact is largest. Imbalances of other nutrients can also cause several problems, especially chlorosis, poor fruit set, and soil toxicity. For each imbalance exists a remedy to support optimum nutrients absorption and reach desired levels of growth and ripeness.
Microscopic root-hairs at the tips of the roots facilitate the absorption of nutrients into the vine. This is influenced by many factors. Soil pH is key and is optimum between 6 and 7. At each end of its spectrum, nutrients will be locked-up. Minerals, when in excess, can also lock-up other minerals. For examples, high potassium levels will lock-up magnesium while high phosphorous will lock-up zinc. Rootstocks can also affect minerals absorption. Among others, rootstock SO4 has difficulty drawing magnesium. Finally, other factors of nature can affect nutrients balance in soil. Negatively, water in excess can increase soil pH and negatively affect nutrients absorption. Positively, humus can provide the soil with the necessary bacteria to improve mineral intake.
Problems with absorption will create imbalances. This will lead to problems with plant growth and ultimately affect sugar and acids production leading to unripe fruit and unbalanced wine.
Nitrogen is one of the most essential elements for plant growth. It is required at all stages of the growth cycle. In spring, it will be necessary for immediate use to ensure growth. In the fall, it will be required as reserve for the following season. Imbalances of its levels in soil will ultimately have a major impact on wine quality. Excesses caused by rich soils and/or excessive fertilization will lead to high vigor, shading, and risks of botrytis and mildew. Shortages will reduce vigor and make the leaves become yellow and pale. In the vineyard, these imbalances will impair sugar production, grape color development, and phenolic ripeness. In the winery, shortages can create winemaking problems and be the source of stuck fermentation and loss of aromatics.
Potassium is the second most important element for plant growth. Imbalances will also have major impact on the quality of the resulting wine. Especially when in excess, potassium neutralizes acidity in the must. Such high pH can result in flabby and unbalanced wine. Excesses are caused by high use of potassium-rich fertilizers such as that experienced in Burgundy in the 70’s and had a major impact on the quality of wine during that period. Another cause is the shade created by high vigor, which makes potassium migrate to the leaves and grapes as they act as storage recipients. Shortages are rare but cold temperature and drought can affect the ability of the vine to draw enough potassium. This leads to chlorosis-like symptoms, which will be reviewed below.
The vine generally requires very small levels of all other nutrients. Their imbalances are rare as most soils have enough reserves. Nonetheless, their excesses or shortages can create major problems. In particular, shortages of iron will lead to chlorosis. Under this condition, the leaves will turn yellow and seriously impair photosynthesis and ultimately negatively affect ripening. Chlorosis can also be caused by shortages of magnesium or potassium and also by high soil pH. Another problem, caused by shortages of zinc or boron, is coulure, or poor fruit set. This leads to irregular berries and ripening. Bunch stem necrosis, caused by low calcium or magnesium, leads to the bunch shriveling and berries to stop developing. Toxic soils caused by over use of fertilizers can also impair ripeness and/or stunt growth. For example, over-use of the “Bordeaux-mixture” which contains copper in Bordeaux has lead to excess in copper in some soil and has been detrimental to the quality of wine from many estates in the area.
Thankfully, several remedies exist to counterbalance excesses or shortages of nutrients to ensure healthy vine growth and favor the production of high quality wine.
Soil management remains one of the most useful actions viticulturist can take.
As such, cover crops can play an important role. With the careful planting, it will be possible to manage the nitrogen intake. For example, clover and lucerne can absorb nitrogen from the air and fix it on their roots. In this process propagate nitrogen in the soil. However, if nitrogen is in excess, it will be better to create competition for it. Grass planted in-between rows is an example of cover crops that can reduce nitrogen in soil. Cover crops can also absorb excess water before harvest and thus limiting its effect on soil pH.
Organic Matter can play an important role in increasing absorption of minerals through the roots. Compost poor in Nitrogen such as mulches from pruning activities can keep the soil rich in bacteria while minimizing nitrogen intake. Alternatively, nitrogen-rich matters such as manure or domestic compost can help in case of nitrogen shortages. Humus can also correct excess copper by increasing microbial activity in the soil.
Encouraged through ploughing, drainage, and cover crops, deep rooting is the ultimate soil management technique. It will reduce extreme variation in nutrient supplies and as well as control the amount of water absorbed by the vine.
Soil analysis will be critical when planning a vineyard. Judicious rootstock selection before planting can help to reach nutrients balance. For example, low-vigor rootstock based on V. Riparia such as 420A can help control nitrogen and potassium intake. Rootstock based on V. Berlandieri such as SO4 or Fercal can help resist chlorosis. If potassium is required, it will be added before planting as deeply in the soil as possible.
Canopy management can help in reducing shading and limit potassium intake.
Fertilizers can be used when shortages exist. However, they must be used in moderation, especially those rich in nitrogen or in potassium. Leaf spraying can also help correct shortages of zinc and chlorosis.
As we saw, high wine quality is dependent on healthy grapes and high levels of physiological and, especially, phenolic ripeness. These conditions are achieved by efficient nutrient absorption as it supports optimum sugar production and aromas development. Excesses or shortages cause problems with ripening in the vineyard and wine making in the cellar. However, each has its own remedies and careful monitoring will be necessary throughout the growth cycle.