Grapevine Nitrogen Nutrition: Measures and Interpretations

Grapevine nitrogen nutrition is a key parameter for achieving production targets in terms of yield and wine quality. Here we set out the current assessment methods and their interpretation thresholds for sustainable management.

In the Swiss context, where vineyards increasingly tend to be grassed and fertilisers are increasingly expensive, nitrogen deficiencies are occurring with growing frequency. Grapevine nitrogen nutrition is a long-term process: the background effects of the previous year and expectations for the following year must be borne in mind. A major change in growing practices such as the complete and simultaneous cessation of chemical weeding and nitrogen fertilisation can lead to a marked nitrogen deficiency in two to five years, depending on the situation, with repercussions for yield and wine quality. Several years are often needed to reestablish a nutritional balance. A good grasp of grapevine nitrogen nutrition status is essential for the long-term production of high-quality grapes.

Taking the time to observe the vine

This is probably the simplest and least expensive thing to do! Several visual symptoms are readily identifiable and provide a good indication of vine nitrogen nutrition status.

Soil analysis: benefits and limitations in viticulture

As for total nitrogen in the soil, it is not a good indicator of grapevine nitrogen nutrition status, since organic nitrogen must first be mineralised to be assimilable by the vine. On the other hand, soil analysis enables the control of other factors influencing mineralisation dynamics and nitrogen availability over the course of the growing season.

Leaf analyses

Leaf analyses are mainly used in addition to other observations, for research purposes to observe the effect of a particular practice on plant nutrition, or to confirm a mineral deficiency/excess symptom. These tests are expensive, and must be interpreted with caution.

Chlorophyll index assessment – a fast, non-destructive measure

Chlorophyll index is measured with devices such as the N-tester (Yara, Oslo, Norway) and the SPAD 502 (Konica Minolta, Nieuwegein, Netherlands). The chlorophyll content of the leaves correlates well with their nitrogen content, provided there are no other symptoms of disease or mineral deficiency. Consequently, the chlorophyll index can be used to diagnose plant nitrogen nutrition status.

Grape must at harvest – the most accurate indicator

The yeast-assimilable nitrogen (YAN) content of grapes at harvest is a key parameter in both viticulture and oenology. Knowing this parameter allows winemakers to anticipate vinification conditions, as well as fertilisation for the following year. Given the major role played by YAN in must fermentability and wine flavour development, it is surprising that it is not systematically included in grape-must analyses in the same way that sugar content or titratable acidity are.

Early determination of the nitrogen concentration of grape must from veraison (the start of berry ripening) is a good indicator of future grape-must nitrogen concentration at harvest. This may be useful for purposes of foliar fertilisation at the start of grape ripening, to correct grape-must-assimilable nitrogen content at harvest.