Greenhouse fertilising: measure first, then water.
Bloomcabin Journal · Precision at the root
Greenhouse fertilising:
measure first, then water.
The best fertiliser is not the richest one. It works when the correctly diluted amount reaches a receptive root zone at the point when the crop genuinely needs more nutrients.
01 / Read Crop stage, formulation and label directions.
02 / Check Moisture, drainage, temperature and the last feed.
03 / Mix Add water first, measure accurately, apply evenly.
01
The basic rule
Feed the crop – not the calendar.
Growth is immediately visible in a greenhouse: tender leaves emerge early, fruit sets under cover, and a vigorous tomato can seem to demand more at every inspection. Fertiliser is therefore easily confused with growth momentum. In fact, 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 young plant in pre-fertilised compost needs different feeding from a cucumber in a growbag with a full canopy and developing fruit. Cool, overcast weather also lowers water use and nutrient uptake. What suited bright, active growth can become excessive when roots cool down or the medium remains wet for longer.
That is why a deliberately planned greenhouse is useful. 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 absorb.
Anatomy of a fertiliser label
15–5–30
Declared analysis
N · P₂O₅ · K₂O
Trace elements, form, interval and directions.
What the numbers really mean
A nutrient analysis is not a dosing instruction.
The three prominent figures on a fertiliser label show the declared levels of nitrogen (N), phosphate (P2O5) and potassium oxide (K2O). A 15–5–30 product therefore contains the stated proportions of these 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 may indicate insufficient supply, but the same appearance can occur in cold, persistently wet medium. Too much encourages soft, lush growth and can upset the balanced development of roots, flowers and fruit.
Phosphorus / P Supports young roots and key metabolic processes. A particularly high middle number is not, however, a shortcut to more flowers. Cold roots can restrict uptake even where sufficient phosphorus is present.
Potassium / K Supports water regulation among other functions and becomes more important for actively fruiting tomatoes, cucumbers, peppers, aubergines and melons. Greater demand still does not justify an over-concentrated nutrient solution.
Always read the complete label: nutrient declaration, trace elements, crop group, dilution, interval and application directions belong together. The analysis describes composition; the product directions explain how that particular fertiliser should be used. The stated maximum amount is not an invitation to use it as a precaution.
More than the three main numbers
Understand plant nutrition in layers.
NPK
Primary macronutrients
Ca Mg S
Secondary nutrients
Fe +
Trace elements
pH / EC
Check root zones
Primary macronutrients
Nitrogen, phosphorus and potassium are needed in comparatively large amounts. In a productive greenhouse, the key question is not which nutrient is best. It is whether the formulation and concentration suit the crop's current work: root formation, 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 peppers often arise from uneven moisture and restricted movement within the plant – not simply from one missed feed.
Trace elements
Iron, manganese, boron, zinc, copper, molybdenum and other elements are needed only in tiny quantities, yet remain essential. Their availability is closely linked to pH. Adding extra trace nutrients without a diagnosis can turn a manageable issue into a complicated nutrient lockout.
The growing medium is part of the equation
pH affects nutrient availability. Electrical conductivity, or EC, shows the total amount of dissolved salts, but not which nutrient is responsible. Suitable ranges depend on crop, medium, water and measurement method. Treat readings as decision support and trend signals, not as an isolated number on a device.
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 shift the curve. That is why an unchanged feeding plan rarely suits an entire greenhouse season.
Leaf crops
Fruit crops
PropagationRooting inLeaf growthFloweringFruit fillingResting phase
Young plants
Seed reserves support the first development stage. Only once several true leaves are present and the propagation medium contains very few nutrients should a weak, complete nutrient solution be considered – solely according to the manufacturer’s stated young-plant recommendation. Small cell volumes are especially sensitive to overdosing and salt accumulation.
Rooting in
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 interpreting slow top growth too quickly as hunger.
Leaf growth
Lettuce, herbs, brassicas and young fruiting plants respond to steady, moderate feeding in good light. Pale plants may need nutrients, but they may also be too cold, shaded, root-bound or kept permanently too wet.
Flowering and fruit load
Tomatoes, peppers, cucumbers, aubergines and melons need more during fruit set and filling. Pay particular attention now to even watering and label-directed feeding – not merely a stronger concentration at every application.
The dilution bench
Calculate the mix before the watering can reaches the bed.
Concentrations vary considerably between products. Use the manufacturer's measuring cup, a suitable syringe or scales. The following examples show only the calculation method for a label-compliant mix; they do not set a dosage of their own.
Label amount × water volume = fertiliser amount required
Example A / watering can
Dose per litre
If the label states 2 ml per litre of water and your clean watering can holds 5 litres, measure 10 ml of fertiliser for the full prepared water volume. If the label gives a lower dose for young plants, that specific manufacturer direction alone applies.
Example B / larger mix
Dose per ten litres
Where the product direction is 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 a random 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 the 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 precisely measured fertiliser, stir until fully dissolved, then top up to the final volume. Clearly label any mixture not used immediately. Mix different concentrates only where compatibility is explicitly confirmed: calcium-containing concentrates and phosphate- or sulphate-containing concentrates can form insoluble precipitates.
A precisely arranged Classic aluminium greenhouse provides space to separate a clean mixing area from planted benches, keep paths clear and run irrigation lines where they remain accessible. This layout makes every mix easier to calculate, apply in a controlled way and repeat consistently.
Application method
Choose the method to suit the crop and system.
Root application
This is the standard 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, and how much, depends on the crop, medium, water quality, container and possible salt load.
Foliar feeding
This is a specialist, label-led tool, not a replacement for functioning root nutrition. Use only products explicitly 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 the irrigation water. This can be efficient and precise for many containers, growbags or specialised growing systems. It requires clean water, correctly adjusted equipment, even drippers and documented checks 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 adds ions to the root zone. As plants take up water and moisture evaporates from the medium surface, dissolved salts can concentrate around roots. Small pots, growbags, unsuitable irrigation water, poor drainage, intense 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, but is not immune to overfeeding. In both systems, water quality affects the final nutrient solution: hardness, bicarbonate and existing salts all contribute 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 readings are usually more useful than another bottle of fertiliser. Germany’s North Rhine-Westphalia Chamber of Agriculture recommends regular soil testing for long-used greenhouse beds, especially where compost, manure, organic or mineral fertilisers have been added for years. An EC value shows total salt concentration, not the identity of individual nutrients.
Choose a formulation
Organic, soluble or controlled-release: choose the behaviour you need.
Organic and slow-acting approaches
Compost, organic soil improvers 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-based 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 note the nutrient analysis rather than equating “organic” with “impossible to overdose”.
Controlled-release fertiliser can simplify feeding in containers because nutrients become available gradually according to coating, water and temperature. This is useful where regular mixing is impractical. In a warm greenhouse, however, release can accelerate; observation and label checks remain essential.
Water-soluble complete fertilisers
Water-soluble complete fertilisers can be finely managed through concentration and interval. They suit situations where feeding should respond to crop stage, water quality and the measured condition of the root zone. This flexibility is both strength and risk: exact measurement, full dissolution and clear records are required.
A sensible system can combine methods in a coordinated way: a structurally stable medium, moderate base nutrition where professionally appropriate, and controlled soluble top-up feeding for heavily fruiting tomatoes, cucumbers or peppers. Total input is what matters. Several feeding systems must not accumulate unnoticed.
When crops grow under cover for longer periods, the greenhouse structure should also form part of the feeding plan. Adaptable Classic aluminium greenhouses allow separate zones for propagation, fruit crops and larger beds. This makes it possible to separate light feeders from crops with heavy fruit loads – and avoid the imprecise habit of feeding every area equally from one container.
The smallest useful tool
Keep records for the next decision.
A feeding log need not be a laboratory journal. It should prevent duplicate applications and help link 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 January, and young plants follow their own rhythm.
Date
Crop + stage
Product + dose
Water / EC / pH
Plant response
12 November
Tomato / early flowering
Complete fertiliser to label
Watered; drainage checked
Active, no marginal necrosis
19 November
Lettuce / harvest week
Weak feed or none
Medium evenly moist
Colour and firmness unremarkable
Also note weather changes. Several cloudy days, a hot spell requiring more frequent watering, impaired ventilation or a new water source can quickly change how a feeding strategy performs. An accessible system for Classic aluminium greenhouses supports the same principle: use water consciously, guide it close to roots and plan the layout so excess can drain visibly rather than remaining unnoticed in containers or hard-to-reach areas.
When choosing a greenhouse, workflow deserves as much attention as external appearance. A configurable Classic aluminium greenhouse can become a calm operating system for the garden: clearly separated benches, easy-to-reach containers, a clean place for water and measuring tools, plus 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 dependable universal interval. Start with the product label and, within those directions, consider crop stage, medium, light, temperature, watering frequency and growth. A young plant, leafy herb, fruiting tomato and dormant container plant should not follow the same weekly schedule.
Can I feed tomatoes, herbs and lettuce in the same way?
A complete fertiliser may suit several crops if its nutrient declaration and labelled crop directions are appropriate. Formulation, dilution and interval can still differ. Fruiting crops often need more during fruit filling, while young plants and leaf crops react sensitively to over-concentrated solutions. Separate crop zones wherever 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, drippers, drainage and the EC of both nutrient solution and root zone. In a home greenhouse, periodic label-compliant feeding with suitable clear-water applications is often easier to manage. Do not begin continuous feeding without understanding the medium's salt response.
How can yellow leaves be fixed 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 assessed for persistent problems before increasing fertiliser.
May I fertilise dry growing medium?
Not as a rule. Very dry medium often distributes nutrient solution unevenly; locally high concentrations can put extra strain on stressed roots. First moisten the root ball slowly with clean water. Feed only later, when moisture is even, the crop is actively growing and product directions call for an application.
Sources and further reading
Read the label. Then check the root zone.
These German specialist resources and international advisory sources underpin the principles described. They support soil testing, water quality, EC monitoring, nutrient labelling, vegetable feeding and raising young plants. Specific quantities must nevertheless always come from the label of the product actually used.
More from Bloomcabin: Classic aluminium greenhouse · Buy a greenhouse built for years · Bloomcabin aluminium greenhouse FAQ
- LWG Bavaria – fertilising in the garden
- North Rhine-Westphalia Chamber of Agriculture – fertigation and EC monitoring
- North Rhine-Westphalia Chamber of Agriculture – needs-based home-garden fertilising
- German Fertiliser Regulation – declaration and nutrient labelling
- 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, air movement and orderly irrigation creates the conditions for making these small decisions reliably, season after season.
Crop stage
Young plant, leaf growth, flowering, fruit or resting phase?
Young plant, 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 dose
Measured exactly for the final water volume?
Measured exactly for the final water volume?
Discover a Classic aluminium greenhouse for a permanently orderly, individually planned growing space.
