2.6.1.c What are the main mineral elements and the consequence of deficiencies?

Potassium or potassium oxide (K2O)

The potassium ion (K+) is one of the most important minerals for the vine growth cycle. It is involved in many physiological mechanisms. It has an effect on vigour and yield as well as the vine's longevity. This cation regulates the opening of the stomata and facilitates resistance to drought.
It also increases photosynthesis and the transportation of sugars to the berries. It is therefore a quality factor.
In the soil, potassium is found in three forms: free, adsorbed and combined.
The free and adsorbed forms can be assimilated by the vine; they only represent a maximum of 2% of total potassium. Their ratio is 1% free forms to 99% adsorbed forms.

These two forms represent only 10 to 20% of the plant's K+ supply; the rest comes from non-exchangeable potassium (combined) which is slowly released by mineralisation.
Deficiency factors: if the soil experiences a period of drought, a high pH or an excess of Ca2+, the free potassium can be adsorbed to the clay plates and therefore becomes temporarily unavailable to the plant. Excess nitrogen can also be a problem.
Potassium's assimilation is linked to the potassium/magnesium ratio. If the latter is present in too large a quantity, it blocks the assimilation of potassium and vice versa. A healthy ratio is between 3 and 5 according to leaf analysis and 0.8 in soil analysis (K20/MgO ratio), depending on the soil type. Above that level, potassium should be added. Below that level, magnesium should be added.

Symptoms generally appear at the end of July and sometimes as early as May in acute cases.

• Yellowing of the white grape varieties and reddening of the red grape varieties with a blurred edge of the leaf blade (appearing on the young leaves then progressing from top to bottom)
• Leaves appear shiny and thick
• Edges of the leaf blade curve upwards
• Peripheral necrosis and premature leaf drop
• Appearance of browning during ripening

Correction: a foliar application at the onset of symptoms will correct symptoms during the year. Potassium should then be added to the soil. If the reason is an excess of magnesium, it is also necessary to reduce magnesium inputs.
Over-fertilisation with potassium can lead to various problems: for example, if there is an excess, potassium takes the place of Mg2+ and Ca2+ ions in the clay-humus complex which are then leached out. This represents the decalcifying and anti-magnesium action of potassium inputs. Moreover, many plants seem to favour the absorption of potassium to the detriment of calcium. This is the phenomenon of selective absorption.

Magnesium

Magnesium is part of the composition of chlorophyll and participates in the intracellular ionic balance of the vine, i.e. it enables certain acids to be neutralized. In soil analyses, it is quantified in its form MgO (magnesium oxide).
Deficiency factors: as previously explained, it has a strong antagonism with potassium regarding its assimilation by the plant; inputs should therefore be calculated with that in mind. Its assimilation is only marginally affected by lack of water, unlike potassium, however, too much water can reduce its assimilation.

Excessive liming can also lead to a magnesium deficiency.
Substantial temperature variations block its internal migration in the plant, which can lead to induced deficiencies. In such cases, the only solution is a foliar application.

Symptoms: appear at the end of July and sometimes as early as flowering.

• Appearance on the lower leaves before the symptoms move upwards (the opposite of potassium)
• Yellowing of the leaf blades and interveinal zones before reddening
• Delayed growth
• Drying and premature falling of foliage for the most severe deficiencies
• Drying of the stalk during ripening

Correction: foliar application as soon as symptoms appear to correct the year's deficiency. Then adapt fertilisation according to the K/Mg ratio. Most soil improvers contain magnesium and potassium, their ratio in the soil improver is the key to choosing the most suitable product.
Magnesium can also be added in the form of magnesium oxide to regulate the pH.

Calcium

Calcium, like magnesium, plays a role in the intracellular ionic balance. It is the most important element for the cell's buffering capacity and the cell walls' elasticity.
In the soil, it plays a crucial role in the formation of the clay-humus complex because it allows the flocculation of clay and humus. Calcium applications increase the saturation of the CEC if it is not optimal, thus providing a greater quantity of exchangeable ions. Moreover, it is easily detached from the clay-humus complex in favour of potassium, which enables a good availability of these elements.

Deficiency factors: in certain cases, such as where the soil is too compacted or saturated with water, calcium assimilation is reduced. This can cause vine growth problems or accentuate coulure. Moreover, excessive magnesium, potassium or sodium quantities also cause assimilation problems.
Correction: usually in the form of lime. If you do not want to increase the pH, foliar application or soil application in a very soluble form is possible.

Nitrogen

Nitrogen is a major element in plant development. It is present in many compounds such as amino acids or chlorophyll. Nitrogen plays a fundamental role in the vigour and yield of vines. It is mainly absorbed during fruit set and véraison. Vines have modest needs in nitrogen, about 25kg/year/ha. Nitrogen is usually provided through the mineralisation of organic matter. If the biological activity in the soil is well managed, it is not necessary in most cases to add nitrogen. A compost-type input may suffice.

A moderate nitrogen deficiency is favourable to grape quality. From a practical point of view, high availability of nitrogen leads to vigorous growth of vegetation and therefore potentially more pruning and a greater risk of cryptogamic diseases.
Factors favouring deficiency: a lack or excess of water can reduce the assimilation of nitrogen. Its availability strongly depends on the soil's biological activity; cold soil or soil with little organic matter is not favourable to the availability of nitrogen.

Deficiency symptoms:

• Low vigour and low foliage density.
• Leaf yellowing
• Small leaves
• Poor lignification
• Early leaf drop

Consequences of excess:

• Excessive yields
• Decrease in grape quality
• Increased coulure and millerandage
• Increased susceptibility to grey rot

Correction:
Foliar supplementation as soon as symptoms appear is feasible to correct the year's needs.
To estimate its availability, in addition to soil analysis, the vigour of the plots must be examined either by observation or by more precise measurements (NDVI weight and diameter of branches etc.).
If nitrogen is not available, it is possible to encourage mineralization by ploughing to aerate and decompact the soil and/or to add more organic matter with a rapid mineralization rate. If there is grass cover, it may be necessary to remove it.
If too much nitrogen is available, grassing can be a solution to create competition for the vines.

Phosphorus

Phosphorus is a source of cellular energy (in the form of ATP) and is involved in the formation of membranes in the form of phospholipids. Phosphorus is often present in sufficient quantities for normal growth of the vine. In France, almost no cases of deficiency have been observed, even in the case of a presumed deficit via soil analysis.

Moreover, it is regularly added in small quantities via most of the soil improvers used to increase the proportion of organic matter. It is therefore not usually taken into account in the decision-making process.
The only time phosphorus is applied is before planting in order to have well-balanced soil, with no deficiencies.

Trace elements

These elements are only necessary in very small quantities in the vineyard, but they are nonetheless indispensable.
A deficiency in one or more of these elements can lead to major physiological problems as they play a role in photosynthesis, respiration and/or energy transportation. Deficiencies remain quite rare except for iron chlorosis which is relatively common. Manganese and boron deficiencies are also possible. One of the most effective solutions is a foliar application, especially for manganese.

Manganese

Manganese is a cofactor in various enzymatic reactions (photosynthesis and nitrate reduction).
A deficiency leads to a decrease in vigour and yield, an increase in coulure and reduced sugar content in the berries due to less photosynthesis.
Factors favouring deficiency: soils that are too calcareous or too poor (shallow, leached soils) as well as when there is too much iron. Drought accentuates manganese deficiency.

Symptoms:

• Yellowish-green mottling of adult leaves in the middle part of the branches before véraison.
• Blackish necrotic spots on branches

Correction: foliar application around flowering is the only effective solution. In the soil, manganese oxidizes and quickly becomes unavailable to the vine.

**Boron

Boron plays a very important role in the transportation of sugars and during fertilisation, flowering and fruit set.
Boron deficiency leads to poor lignification and an increased risk of coulure and millerandage. A deficiency is considered to exist when its concentration in the soil is less than 0.15 mg/kg.
Factors favouring deficiency: acidic or too calcareous soil favours boron deficiency problems. If soil contains too much water, there is a risk of boron not being available.

Symptoms :

• If the deficiency is prior to flowering, increased coulure.
• Shortening of internodes
• Necrosis of tendrils in severe cases and formation of a "witches' broom" on the tip of the main shoot, i.e. many small secondary shoots
• Diffuse leaf yellowing, crisscrossing lines on the leaf surface and leaf edges curling downwards

Correction: soil or foliar application two to four weeks before flowering if rapid intervention is necessary. If the soil is acid, the pH must be raised by liming.

Zinc

Zinc is involved in protein and nucleic acid synthesis. This deficiency has never been observed in France. It remains exceptional and only affects soils with a very high pH and/or excessive phosphate fertilization.

**Iron

Iron is involved in chlorophyll formation and therefore plays a fundamental role in photosynthesis. It is involved in various oxidation-reduction reactions such as the conversion of nitrates into amino acids.
There are two types of iron chlorosis, true iron chlorosis due to a lack of iron in the soil and induced iron chlorosis due to poor iron assimilation by the vine or a problem with iron transportation in the vine after it has been assimilated.
Factors favouring chlorosis: assimilation problems come from soil pH, lack of aeration, excessive amounts of water or a lot of active limestone. The transport problem comes from a lack of citric acid in the vine or a problem with iron solubilisation when it arrives in the leaves. Lack of citric acid is often caused by excessive vigour.

Symptoms:

• In spring, a discolouration that does not affect the veins (first stage) is observed, the colour goes from yellow to ivory without affecting the veins
• In the second stage, the yellowing affects the veins, the discoloured areas become necrotic and the leaf edges dry up.
• At the most severe stage: total leaf necrosis, the young leaves are very small and become discoloured very quickly. The plant has a stunted appearance.

Correction: chlorosis must be prevented. Good tillage with the correct pH and a good proportion of organic matter (well-humified organic matter increases the bioavailability of iron) generally prevents these types of symptoms. If not, iron should be added to the soil for the long term by applying ferrous sulphate below the soil surface (to avoid contact with the air and thus oxidation).