Written by Leonid Fadeev

Nutrients are important in ensuring the grapevine thrives and produces quality grapes, which will be used to produce quality wines. Quality grapes are healthy (unaffected by diseases), fully developed and physiologically ripe grapes. Nutrients are elements involved in the vine functioning, its growth, reproduction, berry development and in the photosynthesis process. Nutrients can be divided into two major groups: macro-nutrients (required in large supply) and micro-nutrients (needed in trace amounts). Essential macro-nutrients are nitrogen, phosphorous and potassium while important micro-nutrients are iron, magnesium and boron. Nutrients are required in specific levels. Deficiency of nutrients may reduce vine growth while excess may be toxic to the vine. Thus, grape growers around the world implement nutrient management programs to deal with nutrients’ excess or deficiency. Nutrients are supplied to the vine by fertilizers applied to soil or leaves. Requirements vary due to soil type, grape variety, vine age and rootstock type. This essay will review the most important nutrients for the production of quality grapes giving reasons why they are important.

Nitrogen (N) is a crucial macro-nutrient for grapevine growth and berry development. It is an essential element in chlorophyll development in leaves required for photosynthesis. Vines typically gain nitrogen by the break-down of organic matter in soil, such as leaves, canes, shoots and grass cuttings. Nitrogen excess is more common than deficiency. However, deficiency may cause low vigour, poor grape production and yellowing of leaves and green tissue due to reduced photosynthesis. According to Smart R.E. for a Sauvignon Blanc vineyard in Tasmania, nitrogen deficiency and water stress led to reduced shoot vigour, yield and flavour expression in grapes and wines. It may also lead to defoliation resulting in late ripening, smaller berries and bunches or total crop loss. To counter nitrogen deficiency, urea can be applied (foliar spray 40-60 g/10 l). Urea (Folur) is used in Domaine des Cassagnoles (Cotes de Gascogne) on Colombard and Sauvignon Blanc (10 units/hectare) to increase Yeast Assimilable Nitrogen and aromatic compounds in the grapes. Excess nitrogen will lead to large green mass, reduction in fruit set, shaded canopy, development of fungal diseases and decrease of colour intensity and total polyphenols in black varieties. In this case, shoot thinning, extra trimming, leaf thinning and spaying can be applied. Complex fertilizers are generally used to ensure sufficient nitrogen. For example, Cote Rocheuse (Krasnodar, Russia) uses Terra-Sorb foliar (two spraying per year, 2 l/ha).

Phosphorous (P) is another macro-nutrient essential for vine growth. Vines typically derive phosphorous from the breakdown of organic matter. It plays a crucial role in almost every plant process and is involved in energy transfer within plant cells that facilitates metabolism and in photosynthetic reactions. The optimal level of phosphorous in the soil is 160-325 mg/kg. Vines seldom suffer from deficiency, but should it happen, it will lead to poor bud and fruit set, reduced vine vigour, yellowing of the interveinal area of basal leaves and premature defoliation. Triple super phosphate is usually added to the soil before planting a vineyard or in existing vineyards (70-120 kg/ha). Diammonium phosphate can also be used (200 mg/vine) to help accelerate root growth in spring. In Australia, the soil is naturally low in phosphorous, so growers apply phosphorus fertilizers which increase fruit set and yield. Excess phosphorous may limit the uptake of other essential elements, e.g. zinc.

Potassium (K) is the third major nutrient required for vine growth and yield. It aids regulation of water movement in the vine. It is also important for berry colour development in black varieties. The optimal level of potassium in the soil is 205-250 mg/kg. Deficiency will result in marginal leaf yellowing in white varieties, marginal leaf reddening in black varieties, uneven berry ripening, reduced bunch weight and premature defoliation. It may also limit calcium and magnesium uptake. Solutions for counteracting potassium deficiency are foliar feed and application of fertilizers containing potash. Potassium sulfate is commonly used to address potassium deficiency (into soil 120 kg/ha; foliar spray 35-45 g/10 l). Excess potassium often results in high pH levels in juice and resulting wines, leading to flabby wines prone to oxidation. It can also cause magnesium deficiency because both elements’ absorption mechanisms are antagonistic. Using low-vigour rootstocks (e.g. RGM, 116-19) typically produces juice with lower potassium levels, while high-vigour rootstocks (e.g. 99R, Rupestris St. George) produce juice with higher potassium levels. Reducing vine shading help balance wine pH.

Iron (Fe) is a micro-nutrient present in proteins used for energy transfer and required for chlorophyll formation. The optimal level of iron in the soil is over 40 mg/kg. Deficiencies typically occur in high pH soils, resulting in stunted growth, diffuse yellowing of young leaves and sometimes whole-leaf chlorosis. To address iron deficiency, liquid fertilizers containing chelated iron are sprayed onto leaves or injected into the soil. For example, Sequestren turbo (ready solution - 800–1000 l/ha) or Brexil Fe (max 1.5 g/l, 2 kg/ha). Foliar treatment with Ferleaf 100 may be used (1 l/ha in 1000 l of water) for local iron assimilation problems caused by wet and cold spring weather. To prevent iron deficiency, suitable rootstocks should be chosen depending on the soil’s calcareous content, e.g. SO4, 41B, Fercal.

Magnesium (Mg) is an important micro-nutrient for photosynthesis. The optimal level of magnesium in the soil is 171-209 mg/kg. Magnesium deficiency will cause chlorosis-like symptoms – leaf yellowing - or may result in damage to bunch stems, thus affecting proper ripening. Choosing the right rootstocks is crucial. For instance, SO4 and Fercal are inefficient at taking up magnesium, hence vines on these rootstocks in light, sandy soils may experience magnesium deficiency. Other solutions are foliar feeds or magnesium sulfate fertilizers (into soil 120-150 kg/ha; foliar spray 35-40 g/10 l). To balance excessive potassium fertilization, Chateau du Trignon (Gigondas) used magnesium chloride (MAG 20) for three successive years as two foliar sprays per season of 3kg/ha.

Boron (B) is a micro-nutrient that is important for vine growth, fruit set and sugar transportation in the vine. The optimal level of boron in the soil is 0.3–0.5 mg/kg. Deficiency can seriously affect vine health, leading to stunted vine growth, death of shoot tips, poor fruit set, irregularly sized berries, distorted shoots and lower yields. Foliar applications of fertilizers containing boron before flowering help prevent coulure in sensitive varieties like Grenache and Merlot. For instance, the Institute Rhodanien on Grenache recommends Boronia LS (2 applications of 1 l/ha). Boron toxicity should only be a problem due to local fertilization excess or using irrigation water with high concentrations of it.

Vines need nutrients to grow and produce quality grapes. The right amount of nutrients is required. Macro- and micro-nutrients should also be in balance. The most important nutrients for a grapevine are nitrogen, phosphorous, potassium, iron, magnesium and boron. In general, nutrient deficiency can result in poor vine growth and grape quality or reduced crop, while excess may poison the vine. Monitoring nutrient levels in the soil and vine leaves, careful and timely addition of corrective measures is important in maintaining vine health, yielding a crop and being economically profitable.

LEONID FADEEV, Stage 2 MW Student, WSET / IWSC “The Future 50” winner, Weinakademiker, DipWSET, WorldSom Magister Sommelier, founder of Vine and Wine Consulting