Foliar Fertilisation in Viticulture: Limited Efficacy of Organic Forms on Yeast-Assimilable Nitrogen in Musts
Photo: Agroscope
In viticulture, application of organic foliar fertiliser at the time of veraison does little to correct a deficiency of yeast-assimilable nitrogen in the must. Applying foliar mineral nitrogen remains a more effective approach.
Sound vine mineral nutrition management is essential to ensure grape yield and quality. Balanced nitrogen nutrition is particularly important not only for plant development, but also for the smooth functioning of the fermentation process and the aromatic development of the wines. The vine depends primarily on soil mineral nitrogen (nitrates and ammonium), whose absorption is strongly influenced by climate and soil type: hot, dry years or compact clay-limestone soils which lead to shallow rooting often result in musts with a lower nitrogen content. Nitrogen nutrition must therefore be managed in an integrated manner, bearing in mind technical choices regarding plant material, soil maintenance and leaf area/yield balance. Despite everything, additional fertilisation may be necessary to maintain long-term productivity.
An application of foliar urea at veraison in addition to fertilisation of the soil in spring is a tried-and-tested manner of enriching the must with nitrogen without promoting plant vigour. Based on the passive absorption of nutrients by the leaf cuticle and stomata, foliar fertilisation enables the rapid absorption and assimilation of mineral nitrogen into amino acids. In organic viticulture, the ban on highly available mineral forms makes it necessary to resort to organic foliar fertilisers, whose actual effectiveness in increasing yeast-assimilable nitrogen in the musts remains uncertain.
Less effective organic forms
The trial – conducted on Chasselas vines at Agroscope (Pully, Switzerland) between 2023 and 2025 – compared five nitrogen fertilisers applied at the time of veraison (vinasse, glycine betaine, Azotobacter salinestris bacteria, plant proteins, mineral fertiliser) to a non-fertilised control. The measures concerned plant development (pruning wood weight, leaf area, chlorophyll index), yield components and must composition at harvest. The results show a strong influence of vintage on vigour, yield and maturity, with a persistent shortage of yeast-assimilable nitrogen (lower than 140 mg N/L) regardless of the fertiliser tested. Mineral foliar nitrogen (ammonium and urea) remains the most effective for increasing must nitrogen concentrations, whilst the organic forms proved substantially less effective, with some even exhibiting phytotoxic effects at the dosages and schedule studied (Figure 1). These results highlight the limitations of an organic foliar application at veraison to correct nitrogen deficiency in the musts and argues in favour of strategies combining soil fertilisation and smaller and more-frequent foliar applications timed earlier in the growing cycle.
Conclusions
- Foliar application at veraison does not always successfully restore the yeast-assimilable nitrogen level in the must to above 140 mg N/L to ensure that fermentation proceeds smoothly.
- The application of nitrogen in mineral form has proven more effective and less expensive than application of the organic formulations tested, the latter of which are assimilated to a lesser extent and may even have major phytotoxic effects on the vine as well as potentially negatively affecting yield.
- Integrated management of nutrition should remain the priority. For organic foliar fertilisers, strategies involving earlier, smaller and more-frequent applications focused on supporting bud fruitfulness and vigour rather than on a late supply at veraison supporting nitrogen concentration in the musts should be explored.
Bibliographical reference
Fertilisation foliaire en viticulture: efficacité limitée des formes organiques sur l’azote assimilable des moûts.
