Introduction
Nutrition is a major part of grape production with implications for vine growth, berry composition and ultimately juice quality for wine making. The nutritional requirements of grapevines vary according to variety, production levels, age of the vines and whether they are grafted to rootstocks. This article outlines the most important factors that influence wine grape nutrition. Firstly, information is given on soil fertility and the factors that affect soil nutrient supply.
Then the most significant functions of nutrients in grapevines to maintain optimal plant development are provided. This is followed by methods to assess vine nutrient status, either by visual observation or laboratory testing. Finally, information is presented on fertilisers (inorganic, organic) that are most suitable for the nutritional requirements of grapevines.
Soil fertility
The properties of the soil have important effects on the amount and availability of nutrients stored in the soil. The unique characteristics of soils strongly influence nutrient uptake by the vine roots. Soil tests are aimed to match the availability of nutrients in the soil by using chemical extraction methods.
Soil characteristics
Soil depth, soil texture and soil structure are important in determining the soil’s ability to hold water which influences nutrient availability. The amount and availability of nutrients is affected by the soil organic matter content, soil moisture and the composition and chemistry of inorganic soil components. Soil nutrients exist in organic and inorganic forms, but the latter are the major short term source of nutrients. Organic forms must be mineralised into soluble inorganic forms before they can be absorbed by plant roots. Rates of mineralisation by soil micro-organisms are affected mainly by soil temperature and pH, and are very low in dry temperate climates.
Soil pH
The soil nutrient availability is strongly dependent on the soil pH (Figure 1) and the optimum soil pH range for vines is between 5.5–8.0. Outside of this range an excess of certain elements and a deficiency of others is found. For example, soil with a pH < 5.5 is likely to contain an excess of aluminium ions. In contrast, deficiency is induced in iron, manganese and copper under alkaline soil conditions (pH > 8.0).
Soil nutrients
Assessing soil nutrient levels in the vineyard before planting and during full production of vineyards is useful when determining fertiliser requirements. An approximate guide of the adequate range of key soil chemical properties which is made up of important nutrients for vine growth and elements that influence soil condition (Table 1).
Table 1. Recommended adequate ranges of soil nutrient, carbon, EC and pH for established grapevines#.
*Proportion of the total exchangeable cations; # Data is from several literature sources
Macronutrients
The elements N, P, K, Mg, Ca and S are defined as such on concentration basis in plant tissue. N, Mg and S form part of the chlorophyll molecule and are for the production of sugars by the photosynthetic process. The elements N and S are components of proteins, while Ca is part of cellular structural components. P is important for energy transfer and cell membranes. K has an important role in carbohydrate metabolism and transport, and the regulation of water movement.
Micronutrients
Other essential elements Fe, Mn, Zn, B, Mo and Cu are in this category, since they are generally present in lower concentrations. Fe, Mn, Zn and Cu are involved in the assimilation process. Fe and Mn activate enzymes and Cu is present in enzymes. Zn and Mo have a role in the N metabolism, while B has an important role in carbohydrate metabolism.
Vine development
Several nutrients have an important role in the reproductive and vegetative development of grapevines. Shoot growth in spring is influenced by the availability of N, P and K and micronutrients. The sufficient supply of B and Cu ensures cane maturation and a good supply of N and Fe avoids premature leaf fall. N and P are important for inflorescence initiation and development, while the supply of the micronutrients Zn, B and Mo are important for fruit set. Berry growth is influenced by N availability after fruit set and K and Mg are important nutrients during the berry ripening process.
Grape composition
The nutrient composition of grapes influences the winemaking process and the wine produced from these grapes. The supply of the macronutrients N, P and K alter the must composition, excessive K results in high juice pH and low colour in the ferment and wine. The amounts of nitrogenous compounds present in the must as amino acid and ammonium have a critical role in the fermentation process and the final wine composition. The timing of N application and the seasonal water supply varies amino N concentration in the juice.
Assessing nutrient status
Assessing grapevine nutrient status is important for verifying the requirements for fertiliser application in a vineyard to ensure optimal vine productivity. To meet the nutritional requirements of vines, the assessment of vine appearance (qualitative) and tissue nutrient content (quantitative) is essential. Visual deficiency (or toxicity) symptoms become apparent only after the productivity declines.
Qualitative assessment
Observations of shoot vigour and leaf deficiency symptoms are important indictors of vine nutrient status. Most macronutrient deficiency symptoms appear first on the older leaves (except N and S), while most micronutrient deficiencies appear on the younger leaves (except Mn). The difference between the occurrences of deficiencies (leaves of different age) relates to nutrient mobility.
Quantitative assessments
Nutrient standards are developed through the assessment of the relationship between nutrient content in petioles and vine performance (Figure 3). For the determination of vine nutrient status it is recommended to collect about 100 petioles opposite the basal bunches at flowering (50% cap fall) and compare the analytical nutrient results to the recommend standards (Table 2).
Figure 3. Timing of petiole sampling and performance indicators.
AUTHORS:
Bruno Holzapfel, Leo Quirk, Ron Hutton and Jonathan Holland
