Discover how ammoniacal nitrogen biostimulates your crops, improves nutrient uptake and reduces losses. Technical guide with doses and applications.
Introduction
Ammoniacal nitrogen is one of the most efficient forms of nitrogen for plant nutrition and crop biostimulation. Unlike nitrate, which is easily leached, ammonium (NH4+) is retained in the soil exchange complex and can be absorbed directly by the roots, activating metabolic pathways that improve growth, flowering and stress tolerance. In a context of growing pressure to reduce chemical fertilisers and optimise nutritional efficiency, understanding the role of ammoniacal nitrogen becomes strategic for professional farmers, agricultural engineers and technical advisers.
European agriculture faces the challenge of producing more with fewer inputs, complying with regulations such as Regulation (EU) 2019/1009 on fertilising products and Regulation 2018/848 on organic production. In this scenario, ammoniacal nitrogen, combined with microalgae biostimulants and fulvic acids, is positioned as a key tool for improving foliar nutrient uptake, crop rooting and soil organic matter. This article analyses its physiological foundations, its agronomic benefits and the best practices for integrating it into sustainable fertilisation programmes.
What is ammoniacal nitrogen and why is it key?
Ammoniacal nitrogen is the reduced form of nitrogen, represented by the ammonium ion (NH4+). It is generated in the soil through the mineralisation of organic matter and biological fixation, but it is also supplied through specific fertilisers. Its importance lies in the fact that it is a source of rapidly assimilated nitrogen, with a lower energy cost for the plant than nitrate. Whereas nitrate must be reduced to ammonium before being incorporated into amino acids and proteins, ammonium can be metabolised directly in the roots, which accelerates the synthesis of essential nitrogenous compounds.
According to a study by Wageningen University, the uptake of ammonium by roots can be up to 30 % more efficient than that of nitrate in soils with a neutral or slightly acidic pH. In addition, ammonium has limited mobility in the soil, which reduces losses through leaching and improves nitrogen use efficiency (NUE). In crops such as olive, citrus or grapevine, the availability of ammonium at key moments (budbreak, flowering, fruit set) can make the difference in yield and harvest quality.
Forms of nitrogen in the soil
In the soil, nitrogen is present mainly as nitrate (NO3-) and ammonium (NH4+). Nitrate is highly mobile and prone to leaching, whereas ammonium is adsorbed onto clays and organic matter, remaining available to the roots. The optimal ammonium/nitrate ratio varies according to the crop and the phenological stage. For example, in greenhouse tomato, a high proportion of ammonium in the early stages of the cycle stimulates rooting and biomass formation, whereas during the fruit-filling stages it should be balanced with nitrate to avoid cationic imbalances.
Soil microbial activity regulates the transformation between both forms. Nitrifying bacteria such as Nitrosomonas and Nitrobacter oxidise ammonium to nitrate, a process that depends on temperature, moisture and pH. In soils with active microbiota, the applied ammonium can be transformed quickly, so its direct biostimulant effect is best exploited with split applications or through formulations that include natural nitrification inhibitors, such as microalgae extracts.
Ammonium as a biochemical signal
Beyond its nutritional function, ammonium acts as a signal that regulates the gene expression of nitrogen transporters and carbon metabolism enzymes. Research published in the Journal of Experimental Botany indicates that the presence of ammonium in the rhizosphere can induce the synthesis of glutamine synthetase and glutamate dehydrogenase, key enzymes in nitrogen assimilation. This biostimulant effect translates into greater root growth and a better response to abiotic stresses such as drought or salinity.
In citrus, for example, the application of ammoniacal nitrogen in spring has shown increases in the density of absorbing roots and greater tolerance to water stress in summer. This effect is enhanced when combined with microalgae biostimulants rich in phytohormones and amino acids, which favour foliar nutrient uptake and the soil microbiota.
Mechanisms of action in the plant
Ammonium uptake by the roots occurs through specific transporters (AMT) that have a high affinity for the ion. Once inside the cell, ammonium is rapidly incorporated into carbon skeletons to form amino acids such as glutamine and glutamate, which are precursors of proteins, chlorophyll and plant hormones. This process, known as nitrogen assimilation, occurs mainly in the roots and requires energy in the form of ATP and reducing power (NADH).
Ammonium assimilation in the roots has a lower energy cost than nitrate reduction, which leaves more resources available for other metabolic processes, such as the synthesis of defence compounds or the growth of reproductive organs. In grapevines, the application of ammonium before flowering has been associated with better fruit set and greater uniformity in berry size, probably due to greater energy availability for cell division.
Effect on root growth
Ammonium stimulates the proliferation of lateral roots and root hairs, which increases the surface area for water and nutrient uptake. This effect is especially relevant in crops with root systems sensitive to compaction or salinity, such as avocado or almond. In field trials in high-density olive groves, the application of ammoniacal nitrogen together with fulvic acids increased fine root density by 25 % and improved the uptake efficiency of potassium and micronutrients.
The combination with microalgae biostimulants, such as those developed by Ecoganic, enhances this effect thanks to the presence of polyamines and betalains that favour cell division at the root apex. This synergy is particularly useful at transplant stages or for the recovery of root systems damaged by abiotic stress.
Interaction with carbon metabolism
Ammonium assimilation competes with photosynthesis for carbon skeletons, but it can also stimulate photosynthetic activity by increasing the demand for sugars in the roots. This dynamic balance is known as the carbon-nitrogen (C/N) ratio and is fundamental for balanced plant growth. An adequate supply of ammonium can improve the net photosynthetic rate and biomass accumulation, provided that toxicity from excess does not occur.
In greenhouse tomatoes, optimal ammonium levels in the nutrient solution (around 20-30 % of total nitrogen) have shown increases in fruit production and a higher concentration of soluble solids. However, proportions above 50 % can induce imbalances with calcium and magnesium, so careful management is recommended.
Agronomic benefits of ammoniacal nitrogen
The benefits of ammoniacal nitrogen go beyond simple nutrition. Its ability to improve nitrogen use efficiency, reduce losses through leaching and stimulate root growth make it a key tool for sustainable agriculture. In addition, its interaction with soil microbiota can favour the availability of phosphorus and micronutrients by slightly acidifying the rhizosphere and solubilising poorly available compounds.
In woody crops such as olive or citrus, the application of ammonium at strategic times (budbreak, flowering, fruit set) can improve fruit set and reduce flower abortion. This is due to a greater availability of amino acids and energy in the reproductive organs, which translates into better fruit retention and more stable production.
Reduction of nitrogen losses
Nitrate leaching is one of the main causes of aquifer contamination and fertilisation inefficiency. Ammonium, being retained by the exchange complex, significantly reduces this risk. In sandy soils, where leaching is more likely, the use of fertilisers with a significant proportion of ammonium can increase nitrogen use efficiency by up to 20 % compared with exclusively nitric fertilisers.
Furthermore, the combination with natural nitrification inhibitors, such as the polyphenols present in microalgae extracts, can slow down the transformation of ammonium to nitrate, keeping nitrogen available in the root zone for longer. This strategy is especially useful in long-cycle crops and in regions with high rainfall.
Improvement of harvest quality
Ammoniacal nitrogen also influences fruit quality. In grapevines, balanced nutrition with ammonium can increase the concentration of anthocyanins and polyphenols in the grape, improving colour and oenological potential. In citrus, an increase in juice content and a reduction in titratable acidity have been observed when ammonium is supplied in the early stages of fruit development.
In avocado, the application of ammonium together with microalgae biostimulants has shown increases in fruit size and greater uniformity in ripening. These effects are attributed to better nutrition of the developing tissues and greater tolerance to heat stress during filling.
Practical application: rates, timing and routes
The application of ammoniacal nitrogen must be adjusted to the crop, the soil type and the phenological stage. As a general reference, it is recommended that ammonium represents between 20 % and 40 % of the total nitrogen applied in most herbaceous crops, and up to 50 % in woody crops during the vegetative growth phase. In fertigation, typical rates range between 5 and 15 kg of ammoniacal N per hectare per week, depending on crop demand.
The application route may be root-based, by fertigation or direct application to the soil, or foliar, using formulations containing stabilised ammonium. Foliar nutrient uptake is faster but limited in quantity, so it is recommended for correcting specific deficiencies or for stimulating specific processes such as fruit set. In foliar applications, ammonium concentrations must not exceed 0.5 % to avoid phytotoxicity.
Key phenological timings
The timings of maximum efficiency for the application of ammoniacal nitrogen are:
- Budbreak and vegetative growth: stimulates the formation of leaves and shoots, and improves rooting.
- Pre-flowering: promotes fruit set and reduces flower abortion.
- Fruit set and fruit development: improves fruit retention and final size.
- Post-harvest: helps to replenish reserves and prepare the plant for the next cycle.
In olive, for example, an application of ammonium at pre-flowering (April-May in the northern hemisphere) can increase fruit set by 15 % in years of abundant flowering. In citrus, post-harvest application (autumn) improves the accumulation of reserves and spring budbreak.
Application routes and precautions
Fertigation is the most common route for applying ammonium in intensive crops. It is recommended to split the weekly rates and to monitor the electrical conductivity (EC) and pH of the nutrient solution. A pH below 5.5 can favour ammonium uptake but also aluminium toxicity in acid soils. In foliar applications, it is important to carry out the spraying during cooler hours and to avoid mixing with alkaline products.
It is essential to avoid excess ammonium, as it can induce calcium and magnesium deficiencies through cationic antagonism, as well as phytotoxicity in young seedlings. Monitoring the nitrogen content in sap or in plant tissues can help to adjust rates precisely.
Interaction with biostimulants and organic matter
The combination of ammoniacal nitrogen with microalgae biostimulants and fulvic acids can enhance its beneficial effects. Fulvic acids, for example, increase the soil's cation exchange capacity and improve ammonium retention, reducing losses through volatilisation. In addition, they favour the uptake of micronutrients such as iron, zinc and manganese, which are essential for nitrogen metabolism.
Microalgae biostimulants, such as those developed by Ecoganic, contain phytohormones (auxins, cytokinins, gibberellins), amino acids and polysaccharides that stimulate root growth and soil microbial activity. When applied together with ammonium, they can improve assimilation efficiency and reduce the stress associated with nitrogen fertilisation.
Synergy with beneficial microorganisms
Soil microbiota plays a crucial role in the nitrogen cycle. Nitrogen-fixing bacteria, such as Azotobacter and Rhizobium, and arbuscular mycorrhizal fungi can improve the availability of ammonium and its uptake by roots. The application of biostimulants containing these microorganisms, or that favour their proliferation, can increase the nitrogen use efficiency of ammoniacal nitrogen.
In field trials in organic vineyard, the joint application of ammonium and a microalgae biostimulant increased mycorrhizal colonisation by 30 % and improved the uptake of phosphorus and potassium. This effect translates into greater resilience to water stress and better grape quality.
Mixing recommendations
To maximise the synergy, it is recommended to apply the ammoniacal nitrogen in fertigation and, 24-48 hours later, to carry out a foliar application of microalgae biostimulants. This sequence allows the plant to assimilate the ammonium and then receive the hormonal signals that enhance growth and defence. It is important to avoid direct mixing in the fertigation tank without checking compatibility, as some extracts can alter the pH and the stability of the solution.
In organic farming, the use of ammonium of organic origin (for example, from algae extracts or from certified animal origin) is permitted under Regulation EC 2018/848, provided that the rates and withdrawal periods are respected. EC 2019/1009 certification guarantees that fertilising products comply with safety and efficacy requirements.
Field cases in Mediterranean crops
In organic olive groves in southern Spain, the application of ammoniacal nitrogen through fertigation during budbreak (March–April) has shown increases in olive production of up to 12% in years of low rainfall. The key lies in the rapid assimilation of ammonium and its effect on root growth, which improves soil exploration in search of water.
In citrus, the combination of ammonium with microalgae biostimulants in post-harvest has reduced leaf drop and improved spring budbreak. In a trial on Navel orange trees, the application of 10 kg of ammoniacal N per hectare in October, followed by a foliar spray of microalgae in November, increased the number of flowers per branch by 18% compared with the control.
Wine grapevine
In organic vineyards, the application of ammonium at pre-flowering (May) together with a microalgae biostimulant improved fruit set and reduced berry shatter in sensitive varieties such as Tempranillo. Must analyses showed an increase of 0.5 °Brix and a higher concentration of total anthocyanins, which translated into wines with better colour and structure.
It is important to adjust the doses according to vineyard vigour and soil type. In calcareous soils, ammonium can compete with calcium, so it is recommended to supplement with foliar calcium chelates if deficiency symptoms are observed.
Greenhouse tomato
In greenhouse tomato, the proportion of ammonium in the nutrient solution must be managed with precision. A study by the University of Almería recommends not exceeding 30% of total nitrogen in ammoniacal form during the fruit-filling phase, to avoid imbalances with potassium and calcium. However, in the first weeks after transplanting, a proportion of 50% can stimulate rooting and reduce post-transplant stress.
The application of microalgae biostimulants in the irrigation water, together with ammonium, has shown improvements in fruit uniformity and a reduction in the incidence of blossom-end rot, associated with calcium deficiencies.
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FAQ
What is the difference between ammoniacal and nitric nitrogen?
Ammoniacal nitrogen (NH4+) is a reduced form that the plant can assimilate directly at a lower energy cost, whereas nitric nitrogen (NO3-) must first be reduced. Ammonium is retained in the soil and is less subject to leaching, but in excess it can be toxic. Nitrate is more mobile and available, but more prone to losses.
Can ammoniacal nitrogen be used in organic farming?
Yes, provided it comes from sources permitted by Regulation EC 2018/848, such as seaweed extracts, compost or certified manures. It is important to verify the product's certification and to respect the doses and withdrawal periods. Combining it with organic biostimulants can improve its efficiency.
What is the recommended dose of ammoniacal nitrogen?
It depends on the crop and the soil. As a guide, in fertigation between 5 and 15 kg of ammoniacal N per hectare per week are usually applied, representing 20-40 % of total nitrogen. In foliar applications, do not exceed a concentration of 0.5 % to avoid phytotoxicity. Carrying out soil and sap analyses to adjust the doses is advisable.
Can ammoniacal nitrogen improve fruit set?
Yes, ammonium favours fruit set by providing energy and amino acids to the reproductive organs, reducing flower abortion. Applications before flowering have shown increases in fruit set in olive, grapevine and citrus. The synergy with microalgae biostimulants enhances this effect by improving nutrient uptake and translocation.
How can ammonium toxicity be avoided?
Avoid excessive doses and maintain a balance with nitrate and other cations such as calcium and magnesium. Monitor the pH and EC of the nutrient solution, and split the applications. In the event of toxicity symptoms (chlorosis, marginal necrosis), reduce the ammonium proportion and apply leaching with irrigation water. The incorporation of organic matter and biostimulants improves the soil's buffering capacity.
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