Microalgae biostimulants in citrus: a guide to foliar use

Introduction: the nutritional challenge of citrus

Citrus growing pursues two objectives that do not always go hand in hand: maintaining regular production of quality fruit and doing so with tight nutritional management that avoids excess fertiliser and looks after the soil. It is in this context that microalgae biostimulants for foliar uptake in citrus come into play: products applied to the canopy, usually mixed into the spray mix, as a complement to fertilisation and not as a substitute for it. This guide explains what they are, what determines whether a foliar application works, at which stages of the crop they are assessed and what is worth checking before buying them.

Citrus — orange, mandarin, lemon, grapefruit — are evergreen trees with a continuous nutritional demand throughout the year and with periods of particular demand around budbreak, flowering, fruit set and fruit growth. When a nutrient is not available at the right moment, the tree may show it through leaf symptoms, through the drop of young fruit or through irregular fruit size. But these same symptoms can have very different causes: a genuine deficiency, a pH or lime problem in the soil, excessive or insufficient irrigation, root asphyxia, salinity, inadequate pruning or simply the fruit load of the season. That is why the first step is never choosing a product, but diagnosing.

Micronutrients such as iron (Fe), zinc (Zn) and manganese (Mn) may have low availability in calcareous or alkaline-pH soils, and this is one of the situations in which foliar application is used as a complementary route. A soil analysis and a leaf analysis help to distinguish a genuine deficiency from a management imbalance, and they are the starting point of any serious programme.

This guide is intended for growers, technicians and advisers who want to assess microalgae biostimulants with sound judgement. It does not include doses or fixed schedules, because these depend on each formulation, the variety, the area and the season: the reference is always the product label and the recommendation of a technical adviser. Nor does it promise results. A biostimulant can contribute to the general vigour of the crop, but it is not a plant protection treatment and does not on its own correct a management problem.

What microalgae biostimulants are and what can be expected of them

Microalgae biostimulants are products made from microalgae biomass by means of extraction or hydrolysis processes. The term "microalgae" is used broadly and usually includes organisms such as Chlorella or Scenedesmus, and also cyanobacteria such as Spirulina, which are technically not algae. For the buyer, what matters is not the trade name, but which organism has been used, how it has been cultivated and how it has been processed, because the final composition of the product depends on this.

Unlike a fertiliser, whose function is to supply nutrients in relevant quantities, a biostimulant is defined by the intention with which it is applied: to contribute to the general vigour of the crop and to its nutritional management. In the European Union, Regulation (EU) 2019/1009, applicable since July 2022, regulates fertilising products and, within them, plant biostimulants. It defines them as products whose function is to stimulate the plant nutrition processes independently of the nutrient content of the product, with the sole aim of improving one or more of the following characteristics: nutrient use efficiency, tolerance to abiotic stress, quality traits or the availability of nutrients confined in the soil or in the rhizosphere.

That definition is useful as a filter. Products with a plant protection purpose belong to another regulatory field, with their own registration and their own requirements; a biostimulant is not a plant protection treatment and must not be presented as such. If a label or a salesperson attributes that function to it, that is a reason to ask for documentation before going any further.

In practice, what is usually proposed with these products is modest and gradual: accompanying the tree during the stages of greatest demand, complementing a well-designed fertilisation programme and supporting management in stressful situations. The response is not uniform. It depends on the previous nutritional status, the variety, the rootstock, the season's climate and the quality of the application. It is therefore advisable to treat any effect as something that must be verified on your own plot and not as something that can be taken for granted.

What technical data sheets usually declare

The composition of a microalgae biostimulant varies according to the species, the cultivation system and the extraction process, and two products with the same generic name can be very different from each other. Technical data sheets usually describe groups of compounds such as the following:

• Amino acids and peptides: organic nitrogen fractions whose profile and content must appear in the product's analysis.
• Polysaccharides and oligosaccharides: carbohydrates of different molecular size whose specific function depends on each formulation.
• Minerals and trace elements: present in variable proportions, which in no case replace the supply from a specific fertiliser when a deficiency has been diagnosed.
• Vitamins and other organic compounds: in quantities that depend on the species and on the processing.
• Adjuvants and preservatives: substances added by the formulator, which must also appear on the label and which, in organic production, must be reviewed with the certification body.

It is worth distinguishing between what the product contains and what it does in the crop. The composition is a piece of data that the manufacturer can substantiate with an analysis; the field effect is an expectation that depends on the context. A rigorous data sheet reports the guaranteed composition, the method of use and the crops for which it is recommended, and avoids absolute promises.

Foliar absorption in citrus: what determines it

Foliar application consists of spraying a solution onto the leaves so that the compounds dissolved in it penetrate the tissue, mainly through the cuticle, although other entry routes are also involved. It is a route complementary to root nutrition: it is useful when you want to act on the tree at a specific stage without depending on soil availability, which may be limited by pH, lime, texture or moisture.

The citrus leaf cuticle is thick and waxy, which hinders the entry of aqueous solutions. For this reason, the effectiveness of a foliar application depends less on the amount applied than on the conditions under which it is made: how long the leaf remains wet, how the spray liquid is distributed, what state the leaf is in and what the formulation contains. This is an area in which the details of execution weigh heavily.

Some manufacturers of microalgae biostimulants state in their technical data sheets that the product incorporates components that facilitate the distribution of the spray liquid over the leaf. If that property is relevant to your decision, ask the supplier to explain what it is based on and what documentation supports it; do not take it for granted. In any case, a well-formulated product does not compensate for poor coverage or an application under unfavourable conditions.

Another practical aspect is what is intended to be corrected. If the aim is to resolve a micronutrient deficiency confirmed by foliar analysis, what supplies the nutrient is the fertiliser or the specific chelate chosen for that purpose; the biostimulant, at most, may form part of a broader programme. Applying a product without a prior diagnosis can delay the real solution to the problem.

The foliar route and the root route do not compete, they complement each other. Application to the canopy allows intervention at specific stages; soil management — organic matter, moisture, pH, nutrient balance — is what sustains nutrition throughout the season. A programme based only on sprays on poorly managed soil is unlikely to give stable results.

Factors influencing foliar absorption

The efficiency of a foliar application depends on several agronomic and environmental factors, which should be reviewed before each treatment:

• Environmental conditions: high temperature, direct sun and low relative humidity dry the droplet quickly and increase the risk of leaf damage; with wind, moreover, coverage is lost and drift is favoured.
• Time of day: cool hours without intense sun are usually suitable for spraying; the product label may indicate additional restrictions.
• Leaf condition: young and mature leaves do not behave the same, and a leaf under stress, with prior damage or covered in dust responds differently.
• Water quality: the hardness, alkalinity and pH of the spray water can alter the stability of some products; pH adjustment must be carried out according to what the label indicates.
• Formulation: the presence of adjuvants, their type and their admissibility in organic production must be stated in the technical data sheet.
• Coverage and subsequent rain: a uniform distribution of the spray liquid and a dry period after application prevent product loss through washing off.

None of these factors is corrected by applying more product. If conditions are not good, it is preferable to delay the application by a few hours or to another day than to repeat it afterwards.

Key phenological stages for foliar application

Application timing matters because the tree does not demand the same things at every stage. The useful references are not calendar dates, which change with the area, the variety and the season, but the phenological stages: what the tree is doing at each moment. Below are described the stages at which an application is usually considered and what should be taken into account in each one. In all cases, the dose, the spray volume and the intervals are those indicated on the product label and by the technical adviser.

Budbreak and pre-flowering

Before flowering, the tree mobilises reserves to bud and form flowers. This is a stage at which some programmes include support applications, especially if the previous season was demanding or if the tree arrives with a tight nutritional status. Before deciding, it is advisable to review the leaf analysis and the general condition of the tree, and to check on the label whether the product allows applications at this stage.

Flowering

During flowering, particular care must be taken. It is a sensitive stage and coincides with the activity of bees and other pollinators. Consult the label and local regulations on applications during flowering and, if you decide to apply, do so outside foraging hours. If there is no clear reason to intervene, the sensible thing is not to.

Fruit set and early fruit stages

After flowering, the tree goes through a settling phase in which some of the young fruits drop naturally. This is a stage at which many growers want to support the tree, but also one in which it is easy to confuse the causes: the drop may be due to crop load, weather, irrigation or nutrition. Before attributing a drop to a deficiency, it is advisable to rule out the other factors. Applications at this stage should avoid the hours of intense sun to limit the risk of damage to the leaf and to the young fruit.

Fruit growth

During fruit growth, the demand for potassium, calcium and other nutrients increases, and competition between the fruit and the vegetative shoots intensifies. At this stage, basic management — regular irrigation, balanced fertilisation, crop load control — carries more weight than any complementary application. If a foliar application is included, it must be consistent with the fertilisation programme and respect any interval indicated on the label before harvest.

Post-harvest and recovery

After harvest, the tree must rebuild reserves for the following season, especially after an abundant crop or a period of water or heat stress. This is a stage at which some agronomists propose support applications together with base fertilisation. It is advisable to assess the condition of the leaves, the intensity of the previous harvest and the other tasks planned before deciding.

Summary by phenological stage

Stage	What happens in the tree	What to check before applying
Budbreak and pre-flowering	Mobilisation of reserves and flower formation	Leaf analysis, general condition of the tree and product label
Flowering	Sensitive stage, with the presence of pollinators	Label and local regulations on applications during flowering; time of application
Fruit set	Natural settling of the young fruit load	Other possible causes of drop (weather, irrigation, crop load, nutrition); hours of lower sunshine
Fruit growth	Greater nutrient demand and competition between fruit and shoots	Consistency with fertilisation and irrigation; intervals indicated on the label
Post-harvest	Rebuilding of reserves for the following season	Condition of the leaves, intensity of the previous harvest and compatibility with base fertilisation

Note: this table provides guidance on what to review at each stage and does not replace the product label or the recommendation of a technical adviser.

Preparation of the spray solution and good application practices

The dose of a microalgae biostimulant depends on the commercial formulation, its concentration, the crop stage and the purpose of the application; there is no single dose valid for all products. The reference is always the one stated on the label, together with the spray volume and the interval between applications indicated by the manufacturer. If anyone proposes a regimen different from the label, ask for it in writing and consult your adviser before applying it.

Before treating the whole farm, carry out a test on a small number of trees and observe the leaf and young fruit for a few days to rule out damage. This step is especially important if you are going to mix the product with others or if you change formulation.

The preparation of the spray solution requires attention to water quality. Hard water or water with high alkalinity can affect the stability of some products; in that case, the label will indicate whether the pH must be corrected or an approved conditioner used. The order in which products are added, the agitation and the time the spray solution waits in the tank also matter: prepare only what you are going to apply and do not leave it standing.

Mixtures deserve caution. Some products — including certain oils, copper compounds or other preparations with a strong reaction — may be incompatible with certain formulations or increase the risk of leaf damage. Before mixing, consult the labels of all the products involved and carry out a compatibility test in a small container.

As for the equipment, what matters is that the spray solution reaches the leaves and fruits inside the canopy uniformly, where the young leaves and developing fruits are concentrated. Check the condition of the nozzles, calibrate the machine and adjust the pressure and volume to wet well without reaching continuous dripping. Avoid windy days and do not apply if rain is forecast soon.

Always respect the safety measures on the label: protective equipment for the applicator, cleaning of the equipment and management of empty containers. The fact that a product is of natural origin does not mean it requires no precautions.

Checklist before applying

• Is there a prior diagnosis (observation, leaf or soil analysis) that justifies the application?
• Is the crop stage one of those permitted by the product label?
• Has compatibility with the other products in the spray solution been checked?
• Are the day's conditions (temperature, sun, wind, forecast of rain) suitable?
• Is the equipment calibrated and does it guarantee uniform coverage?
• Have the date, product, batch and application conditions been recorded?

How to assess the field response: fruit set, size and water stress

One of the difficulties with biostimulants is that their effect, when it exists, is difficult to separate from the other factors of the season. Fruit set, size, colouring and juice content depend on the weather, irrigation, fruit load, pruning, rootstock and the nutritional status of the tree. That is why it is not honest to promise a specific result, and it is equally not honest to rule it out without having tested it properly.

In fruit set, what is observed is the proportion of flowers that reach fruit and the subsequent drop of young fruits. Variations from one season to another are normal and, in many varieties, are affected by alternate bearing. An apparent improvement in a single year may simply be due to that alternate bearing or to the weather during flowering.

Fruit size is influenced by the load per tree, irrigation and nutrient availability during fruit growth. If more fruit is thinned in one plot than in another, or if irrigation is uneven, differences in size cannot be attributed to the product. The same applies to colouring and juice content, which change with variety, exposure and harvest date.

With regard to water stress, it is worth being clear: no biostimulant replaces irrigation or the practices that conserve soil moisture. In areas with recurrent drought, the priority is water management — irrigation efficiency, soil cover, root status. Some manufacturers declare that their product contributes to tolerance to abiotic stress, which is one of the characteristics covered by Regulation (EU) 2019/1009; that claim must be supported by the manufacturer's documentation and verified under the conditions of each holding.

How to set up a comparison plot

A reliable way to find out whether a product contributes anything on your holding is to compare it with untreated trees under the same conditions. Some design guidelines:

• Choose homogeneous plots: same variety, rootstock, age, soil and irrigation system.
• Leave control trees: untreated, distributed among the treated ones and not concentrated in an area different from the rest of the plot.
• Keep the same management: fertilisation, irrigation and pruning the same in both groups, so that the only difference is the product.
• Define what you are going to measure: for example, fruit set per tree, fruit drop, size, colouring or yield, using the same method in treated and control trees.
• Record everything: dates, product, batch, dose applied according to the label, weather and incidents.
• Repeat over several seasons: a single year can be misleading because of alternate bearing and weather conditions.

With a record like this, you and your adviser will be able to decide with your own data whether the product deserves a place in the programme. If no differences are observed, that is useful information too.

Regulatory framework and organic certification

In the European Union, Regulation (EU) 2019/1009 governs fertilising products bearing the CE marking, including plant biostimulants, and has applied since July 2022. Anyone placing a product on the market under that framework must have technical documentation and a conformity assessment supporting compliance with the requirements of the relevant category and with what the label declares. Depending on the country, a product may also be sold under national fertiliser legislation, so it is advisable to ask the supplier by which route it is marketed and to request the documentation that proves it.

For organic production, the use of inputs in the European Union is governed by Regulation (EU) 2018/848 and its implementing rules. The fact that a biostimulant is of natural origin or that a commercial data sheet describes it as suitable for organic farming is not enough: it must be confirmed with the certification body that the specific product, with all its components and adjuvants, is admissible in your system. Do this before buying and keep the confirmation in writing.

Outside the European Union, each country has its own agricultural authority and its own register of fertilisers and biostimulants, and the organic certification schemes of the destination markets have their own lists of inputs. Before using a product, confirm with the country's official authority and with the certification body whether it is registered and whether it is admissible in your production system and for the market to which the fruit is destined.

What to ask the supplier

When selecting a microalgae biostimulant, it is reasonable to require the following from the supplier:

• Complete technical data sheet, with the guaranteed composition, the origin of the raw material and the method of use.
• Batch analysis, with the parameters declared by the manufacturer.
• Documentation proving the route by which the product is marketed: CE marking in accordance with Regulation (EU) 2019/1009 or the corresponding national registration.
• Written confirmation of its admissibility in organic production, issued or validated by your certification body.
• Documentation supporting the effects declared, with a clear explanation of the conditions under which they were obtained.
• Safety data sheet and recommendations for use and storage.

A supplier who provides this information transparently and acknowledges the limits of the product is usually a better reference than one who promises results without nuance.

Frequently asked questions

What are microalgae biostimulants and what are they used for in citrus?

They are products made from microalgae biomass that are applied with the aim of contributing to the general vigour of the crop and to its nutritional management. They are not a main-source fertiliser nor a plant protection treatment. The effects that each product may declare depend on its formulation and on the regulatory framework, so it is advisable to read the label and request the documentation from the supplier.

At which crop stages is a foliar application considered?

They are usually proposed around phenological stages such as budbreak and pre-flowering, fruit set, fruit growth and post-harvest. Flowering requires caution because of the presence of pollinators. The specific timing depends on the variety, the area and the season, and must be defined in line with the product label and the recommendation of a technical adviser.

What dose of microalgae biostimulant is used in citrus?

There is no dose valid for all products: it depends on the formulation, its concentration and the crop stage. The dose, spray volume and interval stated on the product label are applied, and any adjustment must be discussed with a technical adviser before application.

Can they be used in organic farming?

Only if the specific product is admissible in the production system. In the European Union the framework is Regulation (EU) 2018/848, but confirmation must be requested from the certification body before buying and applying. Outside the European Union, the country's agricultural authority and the certifier of the destination market must also be consulted.

Do they replace foliar fertilisers or base fertilisation?

No. A biostimulant complements a fertilisation programme, but it does not replace fertilisation nor does it on its own correct a micronutrient deficiency. If there are symptoms, the right approach is to diagnose with leaf and soil analysis and choose the specific fertiliser or chelate required.

Can it be mixed with other products in the same spray tank?

Compatibility depends on each formulation. The labels of all the products involved must be consulted, a compatibility test carried out in a small container, and a few trees treated first to rule out leaf damage before applying to the whole farm.

What should be requested from the supplier before buying?

A technical data sheet with the guaranteed composition and the origin of the raw material, batch analysis, documentation proving the marketing route, written confirmation of its admissibility in organic farming if working under certification, and documentation supporting the effects declared. Absolute promises should be treated with suspicion.

Conclusion and practical recommendations

Microalgae biostimulants can be a complement within a well-designed nutrition programme, especially when used with a clear objective, at the right stage and with careful application. They do not replace diagnosis, fertilisation, irrigation or soil management, and they are not a plant protection treatment.

The practical recommendations can be summed up in a few ideas: diagnose before applying, follow the label, choose the timing by phenological stage rather than by the calendar, check the compatibility of mixes, confirm with the certification body if working in organic farming, and verify the effect on your own plot with control trees and over several seasons.

If you would like to assess incorporating these products into your citrus programme, you can request your free quote and discuss your case with the Ecoganic technical team.