Greenhouse fertiliser: measure first, then water.
Bloomcabin Journal · Precision at the roots
Greenhouse fertiliser:
measure first, then water.
The best fertiliser is not the richest one. It works when the correctly diluted amount reaches a root zone able to take it up, at the point when the crop genuinely needs more nutrients.
01 / Read Crop stage, formulation and label directions.
02 / Check Moisture, drainage, temperature and previous feeding.
03 / Mix Add water first, measure accurately, apply evenly.
01
The basic rule
Feed the crop – not the calendar.
Growth feels immediate in a greenhouse: tender leaves emerge early, fruit sets under cover, and a vigorous tomato can appear to demand more at every visit. Fertiliser is therefore easily confused with growth momentum. In reality, it is a concentrated nutrient input. Its value depends on dilution, plant demand, root-zone moisture and temperature, light, drainage and reserves already present in the growing medium.
Start with observation rather than a fixed weekly routine. A newly pricked-out seedling in pre-fertilised mix needs different care from a cucumber in a growbag with full foliage and a rising fruit load. Cool, dull weather also reduces water use and nutrient uptake. What suited bright, active growth may be excessive once roots cool down or the medium stays wet longer.
That is why a thoughtfully planned greenhouse helps. In a carefully arranged classic aluminium greenhouse you can position benches, containers, irrigation and ventilation so feeding becomes repeatable rather than improvised. The aim is not a universal recipe, but a calm system that gives each crop only what it can actually use.
Anatomy of a fertiliser label
15–5–30
Declared analysis
N · P₂O₅ · K₂O
Trace elements, form, interval and directions.
What the figures really mean
A nutrient analysis is not a dosing instruction.
The three prominent figures on a fertiliser label state the declared proportions of nitrogen (N), phosphate (P2O5) and potassium oxide (K2O). A 15–5–30 product therefore contains the stated proportions of those nutrient forms. The figures do not mean every greenhouse crop should receive the same amount every week at every stage.
Nitrogen / N Essential for leaf area and growth. Pale older foliage can suggest low supply, but the same appearance may occur in cold, persistently wet medium. Excess encourages soft, lush growth and can upset balanced root, flower and fruit development.
Phosphorus / P Supports young roots and key metabolic processes. A particularly high middle figure is not a shortcut to more flowers. Cold roots can limit uptake even where sufficient phosphorus is present.
Potassium / K Supports water balance and becomes more important for actively fruiting tomatoes, cucumbers, capsicums, aubergines and melons. Higher demand still does not justify an over-concentrated nutrient solution.
Always read the complete label: nutrient declaration, trace elements, crop group, dilution, interval and directions belong together. The analysis describes composition; the instructions explain how the particular product should be used. A stated maximum rate is not an invitation to use it as a precaution.
More than the three main figures
Understand nutrition in layers.
NPK
Primary nutrients
Ca Mg S
Secondary nutrients
Fe +
Trace elements
pH / EC
Check root zones
Primary nutrients
Nitrogen, phosphorus and potassium are needed in comparatively large amounts. In a productive greenhouse, the key question is not which nutrient is best, but whether formulation and concentration suit the crop’s present work: rooting, leaf growth, flowering or fruit development.
Secondary nutrients
Calcium, magnesium and sulphur are often included in complete fertilisers or supplied partly through water and lime. Calcium-related problems in tomatoes and capsicums often result from uneven moisture and restricted transport within the plant, not simply from a missed fertiliser application.
Trace elements
Iron, manganese, boron, zinc, copper, molybdenum and other elements are needed only in tiny amounts but remain essential. Their availability is closely linked to pH. Adding extra trace nutrients without diagnosis can turn a manageable issue into a complicated nutrient lockout.
The growing medium is part of the equation
pH influences nutrient availability. Electrical conductivity, or EC, indicates the total amount of dissolved salts, but not which nutrient causes it. Suitable ranges depend on crop, medium, water and test method. Treat readings as decision support and trend signals, not isolated numbers on a meter.
Demand keeps moving
More growth does not automatically mean more fertiliser.
The curves shown describe relative demand; they replace neither the product label nor crop-specific advice. Light, root volume, temperature, planting density and harvest load all shift the curve. That is why an unchanged feeding schedule rarely suits an entire greenhouse season.
Leaf crops
Fruit crops
PropagationRootingLeaf growthFloweringFruit fillingResting phase
Seedlings
Seed reserves support the first stage of development. Only once several true leaves are present and propagation medium contains few nutrients should a weak, complete nutrient solution be considered – strictly according to the manufacturer’s seedling recommendation. Small cell volumes are especially sensitive to overdosing and salt accumulation.
Establishment
After transplanting, an evenly moist, loose and well-aerated root zone takes priority. New pots, growbags or beds may already contain nutrients. Give roots time to explore the medium before treating slow top growth as hunger.
Leaf growth
Lettuce, herbs, brassicas and young fruiting plants respond to calm, moderate feeding in good light. Pale plants may need nutrients, but they may also be too cold, shaded, rootbound or continually too wet.
Flowering and fruit load
Tomatoes, capsicums, cucumbers, aubergines and melons have increasing needs during fruit set and filling. Focus especially on even watering and label-led feeding now – not merely a stronger concentration with each application.
The dilution bench
Calculate the mix before the watering can reaches the bed.
Concentrations vary substantially between products. Use the manufacturer’s measuring cup, a suitable syringe or scales. The following examples show only the calculation behind a label-compliant mix; they do not set an independent dose.
Label rate × Water volume = required fertiliser amount
Example A / watering can
Dose per litre
Suppose the label states 2 ml per litre of water and your clean watering can holds 5 litres, then measure 10 ml of fertiliser for the fully prepared volume of water. If the label gives a lower seedling dose, that specific manufacturer instruction alone applies.
Example B / larger batch
Dose per ten litres
If the product instruction says 15 g per 10 litres of water, a 30-litre container needs 45 g of fertiliser. Dissolve the product fully and distribute the finished nutrient solution evenly. Do not estimate with an arbitrary oversized spoon.
Example C / dosing equipment
Stock solution and finished nutrient solution
With fertigation, stock-solution concentration and the dosing unit’s mixing ratio must match exactly. Follow instructions for both fertiliser and equipment. A stock solution is deliberately highly concentrated and must never reach plants or the root zone undiluted.
Mixing order: Add most of the required water first. Add the accurately measured fertiliser, stir until fully dissolved, then top up to the final volume. Clearly label any mix not used immediately. Mix different concentrates only where compatibility is expressly confirmed: calcium-containing and phosphate- or sulphate-containing concentrates can form insoluble precipitates.
A precisely arranged classic aluminium greenhouse offers room to separate a clean mixing area from planted benches, keep paths clear and make irrigation lines accessible. This order makes every mix easier to calculate, apply in a controlled way and repeat consistently.
Application method
Choose the method to suit crop and system.
Root feeding
This is the standard approach for most edible greenhouse crops. Apply the correctly mixed solution slowly to an evenly moist root zone so it does not simply run past the pot edge. Whether water should drain through, and how much, depends on crop, medium, water quality, container and possible salt loading.
Foliar feeding
This is a specialised, label-led tool, not a replacement for functioning root nutrition. Use only products expressly intended for foliar application on the relevant crop and follow the stated dilution exactly. Temperature, light, leaf tolerance and drying conditions must also be suitable.
Fertigation
With drip irrigation, fertiliser reaches the plant with irrigation water. This can be efficient and precise for many containers, growbags or specialist crop systems. It does, however, require clean water, correctly set equipment, even emitters and documented monitoring of both nutrient solution and root zone.
Containers, beds and water quality
Salt is the quiet risk in a well-fed greenhouse.
Every soluble fertiliser brings ions into the root zone. As plants take up water and moisture evaporates from the medium surface, dissolved salts can become concentrated around roots. Small pots, growbags, unsuitable irrigation water, poor drainage, high heat and repeated feeding narrow the safe margin.
Containers have limited root volume and usually need closer attention than established beds. A greenhouse bed with mineral soil and compost may buffer more strongly, but is not immune to overfeeding. In both systems, water quality affects the finished nutrient solution: hardness, bicarbonate and existing salts act before fertiliser is added. Collected rainwater is not automatically ideal either.
For recurring problems, a professional water or growing-medium test and traceable pH or EC measurements are usually more useful than another bottle of fertiliser. Regular soil testing is especially useful in greenhouse beds used for years, particularly where compost, manure, organic or mineral fertilisers have been added. An EC reading shows total salt concentration, not the identity of individual nutrients.
Choose a formulation
Organic, soluble or slow-release: choose the behaviour you need.
Organic and slow-acting approaches
Compost, organic soil conditioners and fertilisers of natural origin can support a living medium and release nutrients over time through biological activity. They are valuable components, especially in soil-grown beds and long-standing crops. Release is neither immediate nor perfectly even: temperature, moisture, material quality and soil life all interact. Use soil tests and read nutrient analysis rather than equating “organic” with “impossible to overdose”.
Controlled-release fertiliser can simplify container feeding because nutrients become available gradually according to coating, water and temperature. This is useful when regular mixing is impractical. In a warm greenhouse, release can accelerate, so observation and label checks remain necessary.
Water-soluble complete fertilisers
Water-soluble complete fertilisers allow precise adjustment through concentration and interval. They suit situations where feeding should reflect crop stage, water quality and the measured condition of the root zone. This flexibility is both strength and risk: accurate measurement, complete dissolution and clear records are essential.
A sensible system can combine methods in a planned way: a structurally stable medium, moderate base nutrition where appropriate, and controlled soluble top-up feeding for heavily fruiting tomatoes, cucumbers or capsicums. The total amount matters. Several feeding systems must not accumulate unnoticed.
Where crops grow under cover for extended periods, greenhouse structure should also form part of the feeding plan. Adaptable classic aluminium greenhouses allow separate zones for propagation, fruiting crops and larger beds. This makes it possible to separate light feeders from crops with heavy fruit load – and avoid the imprecise habit of treating every area alike from one container.
The smallest useful tool
Keep records for the next decision.
A feeding log need not become a laboratory journal. It should prevent duplicate applications and connect plant responses with weather, irrigation or a product change. Record each crop zone separately: lettuce trays are not tomato beds, basil in a pot is not a cucumber in midsummer, and seedlings follow their own rhythm.
Date
Crop + stage
Product + dose
Water / EC / pH
Plant response
12 May
Tomato / early flowering
Complete fertiliser to label
Watered; drainage checked
Active, no marginal necrosis
19 May
Lettuce / harvest week
Weak feed or none
Medium evenly moist
Colour and firmness normal
Also record weather changes. Several dull days, a heat spell with more frequent watering, impaired ventilation or a new water source can quickly alter the result of a feeding strategy. An accessible system for classic aluminium greenhouses supports the same principle: use water deliberately, direct it near roots and plan the layout so surplus drains visibly rather than remaining unnoticed in containers or difficult-to-reach places.
When choosing a greenhouse, the workflow deserves as much attention as external design. A configurable classic aluminium greenhouse can become a calm operating system for the garden: clearly separated benches, reachable containers, a clean place for water and meters, and enough room to inspect leaves, root zones and drainage without stepping over plants.
FAQ
Questions that deserve a balanced answer.
How often should I fertilise greenhouse plants?
There is no reliable universal interval. Start with the product label and, within those directions, consider crop stage, medium, light, temperature, watering frequency and growth. A seedling, leafy herb, fruiting tomato and resting container plant should not follow the same weekly plan.
Can I feed tomatoes, herbs and lettuce alike?
A complete fertiliser may suit several crops if its nutrient declaration and label directions fit. Formulation, dilution and interval can still differ. Fruiting crops often need more during fruit filling, while seedlings and leafy crops react sensitively to overly concentrated solutions. Separate crop zones whenever practical.
Should I fertilise every time I water?
Professional systems sometimes use a low, continuous nutrient supply. This requires close control of water quality, dosing equipment, emitters, drainage and EC values in both nutrient solution and root zone. In a home greenhouse, periodic label-compliant feeding with suitably spaced plain-water applications is often clearer. Do not begin continuous feeding without understanding the medium’s salt response.
How can I fix yellow leaves quickly?
Identify the pattern and site conditions first. Evenly pale older leaves can indicate nitrogen shortage. Yellowing can also result from waterlogging, cold roots, low light, unsuitable pH, pests, disease or salt stress. Check roots and moisture, and have pH and EC tested if problems persist before feeding more heavily.
Can I fertilise dry growing medium?
Not as a rule. Very dry medium often distributes nutrient solution unevenly, and locally high concentrations can place extra strain on stressed roots. First moisten the rootball slowly with clean water. Feed later only when moisture is even, the crop is growing actively and the product instructions call for an application.
Sources and further reading
Read the label. Then check the root zone.
These New Zealand, German and international specialist resources underpin the principles described here. They cover product-label safety, soil testing, water quality, EC monitoring, nutrient labelling, vegetable feeding and raising seedlings. Specific quantities must always come from the label of the product actually being used.
More from Bloomcabin: Classic aluminium greenhouses · Buy a greenhouse for years ahead · Bloomcabin aluminium greenhouse FAQ
- LWG Bavaria – fertilising in the garden
- North Rhine-Westphalia Chamber of Agriculture – fertigation and EC control
- North Rhine-Westphalia Chamber of Agriculture – garden fertilising by need
- New Zealand EPA – gardening products and label safety
- University of Florida IFAS – greenhouse vegetable fertilising
- Utah State University Extension – raising vegetable seedlings
- LWG Bavaria – soil testing and successful fertilising
The next watering round, precisely dosed
Three checks before nutrient solution reaches the roots.
Start with the crop. Check the root zone. Follow the label. A greenhouse planned for easy access, airflow and orderly irrigation creates the conditions for making these small decisions reliably, season after season.
Crop stage
Seedling, leaf growth, flowering, fruit or resting phase?
Seedling, leaf growth, flowering, fruit or resting phase?
Root-zone condition
Moist, airy, free-draining and not recently overfed?
Moist, airy, free-draining and not recently overfed?
Label rate
Measured exactly for the final water volume?
Measured exactly for the final water volume?
For a permanently orderly, individually planned growing space, discover the classic aluminium greenhouse.
