Managing nitrogen in vines can be tricky, but understanding some of the complexities involved will help make more informed decisions, as Hutchinsons agronomist Rob Saunders explains.
Nitrogen management is something of a catch-all term that belies the complexity of this vital macronutrient.
Indeed, regenerative agriculture advocate, John Kempf, suggests growers should not regard nitrogen as one simple nutrient, because it is the different compounds that plants interact with, not nitrogen per se. There are four main ones to consider; nitrate, ammonium, urea and amino acids, and each one can produce different physiological responses in plants.
We know, for example, that excess nitrate can result in soft, sappy growth more prone to diseases such as Botrytis, while excesses in soil may alter rooting zone conditions, potentially reducing the availability of other nutrients and disrupting the rhizophagy cycle (the process by which plants access nutrients assisted by soil microbes).
Nitrogen deficiencies can be equally damaging to the health, yield and quality of that year’s crop, and reduce the vigour of canes laid down for next year too.
Given the difficulties of accurately measuring nitrogen, either in soil, or tissue tests, sometimes the first indication of an issue is when growth starts to struggle. This can be most apparent on relatively new sites, where vines grow well for the first three, four, or five years, then suddenly run out of steam, with canes struggling to reach the top wire.
Efficient utilisation
Vines can use a lot of energy processing nitrogen into the essential ‘building blocks’ that plants need, which encompasses proteins, enzymes, amino acids, nucleic acids, and pigments including chlorophyll. The type of nitrogen compound growers apply can make a real difference to the speed and efficiency with which it is utilised.
Foliar amino acid feeds are generally most efficient, as the nitrogen is already in a form that plants can use immediately. This is followed by urea (amine nitrogen), ammonium, and finally, maybe the most problematic form, nitrate, which has to go through energy-hungry processes as it is assimilated into more complex compounds.
It is one reason foliar amino acid feeds and urea can provide a relatively quick boost to help green-up canopies when there are signs of nitrogen deficiency in-season. However, anyone using urea instead of nitrate must remember that urea is more at risk of volatilisation, so soil-applied products must contain a urease inhibitor to counter this.
It is also worth noting that foliar feeds generally work best as a ‘top-up’ to healthy background levels of nutrition from the soil, something that is key to promoting balanced extension growth; the growth that becomes the canes laid down to carry a crop the following season.
Build a solid base
It is key to anticipate deficiencies before they occur by building and maintaining a good base level of nutrition that supports healthy growth, but does not result in overly large canopies or excess soft, sappy, growth. We must also recognise the complex interactions between nitrogen, carbon and soil biology.
Long-term, organic forms of nitrogen are generally more beneficial to a healthy, balanced soil ecosystem than artificial fertiliser, although there is a place for both in a well managed system.
We have seen promising results from using organic products based on pelletised chicken manure, for example, which provides a slow release form of nitrogen, phosphorus and potash, and helps improve soil condition. Indeed, one of the most visible effects after using the pellets has been an increase in the number of worm casts, a key indicator of soil biology.
It is important to remember though, that organic nitrogen sources, such as manure or compost, are not available to plants as quickly as artificial fertilisers, so in some cases, the best option may be a synthetic product to support canopy growth during periods of peak demand.
Ammonium sulphate may be worth considering over ammonium nitrate, as although it is more acidifying to soil, moderate quantities will reduce nitrate loading – and associated risks to soil biology – while still supplying useful nitrogen and sulphur.
Sulphur is key to optimising nitrogen utilisation, so should always be considered when applying nitrogen. As should carbon to help maintain the appropriate C:N ratio, and molybdenum – the latter being part of the nitrate reductase enzyme that plants use to assimilate nitrate. Tests often reveal low molybdenum levels in vines, so it is something to watch.
Ultimately, there is no one-size-fits-all answer to managing nitrogen and every site and soil type will be different. The key is to recognise the complexities and consider all options when planning your nutrition strategy.
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