Greenhouse fertiliser: measure before you pour.
Bloomcabin growing notes / T34
Greenhouse fertiliser:
measure before you pour.
A concentrated feed becomes useful only when it matches the crop, growing medium, weather and roots receiving it. Read both labels, check root-zone moisture, then mix only what you can apply evenly.
01 / Read Crop stage, formula and label rate.
02 / Check Moisture, drainage, weather and recent feeding.
03 / Mix Calculate water, measure feed, apply evenly.
01
The governing rule
Feed the crop, not the calendar.
A greenhouse makes growth feel immediate: tender leaves appear early, fruit sets under glass and a vigorous tomato may seem hungry whenever you pass. Yet fertiliser is not momentum. It is a concentrated nutrient input whose usefulness depends on dilution, crop demand, root-zone moisture, light, temperature, drainage and the reserves already present in the compost or soil.
Begin with observation rather than a fixed weekly ritual. A young transplant in fresh, pre-fed compost needs a different approach from a cucumber filling fruit in a well-rooted grow bag. Cool compost, low light and wet roots slow nutrient uptake during the British spring and autumn; a dose tolerated in bright July conditions may become excessive when growth slows and the medium stays damp.
A deliberate greenhouse makes that judgement easier. In a well-planned Classic Aluminium Greenhouse, benches, pots, grow bags, beds and irrigation lines can be arranged for clear access and regular inspection. Feeding then becomes repeatable rather than improvised. The aim is not a universal recipe, but a calm system that supplies what each crop can genuinely use.
Illustrative label anatomy
7–7–7
Nutrient analysis
N · P₂O₅ · K₂O
Micronutrients, form, interval and directions matter too.
What the numbers actually say
Analysis is not a feeding instruction.
The three prominent figures on a UK fertiliser label identify the declared proportions of nitrogen (N), phosphate (P2O5) and potash (K2O). A 7–7–7 product therefore presents its analysis in familiar oxide-based terms. It does not mean that every greenhouse crop needs the same amount at the same interval throughout the year.
Nitrogen / N Supports leafy, vegetative growth. Too little may leave older foliage generally pale, while too much can encourage soft, lush growth at the expense of balanced roots, flowers and fruit.
Phosphorus / P Supports roots, young shoots and energy transfer within the plant. A high middle figure is not a shortcut to abundant flowers, particularly where the growing medium already contains sufficient phosphorus.
Potassium / K Is particularly relevant to flowering, fruit development and plant resilience. High demand in a cropping tomato still does not justify exceeding the stated concentration.
Read the complete declaration rather than relying on visual symmetry. UK labels may show phosphorus and potassium as oxide values, followed by elemental values in brackets. Check micronutrients, suitability for edible crops, dilution and interval. The exact product label supplies the operational instructions and overrides any generic online schedule, including this guide.
Beyond the main three
Build nutrition in layers.
NPK
Primary macronutrients
Ca Mg S
Secondary nutrients
Fe +
Micronutrients
pH / salts
Medium checks
Primary macronutrients
Nitrogen, phosphorus and potassium are required in comparatively large quantities. The useful question is not which nutrient is best, but whether the formulation and concentration suit the plant’s current work: establishing roots, making leaves, flowering or carrying fruit.
Secondary nutrients
Calcium, magnesium and sulphur form another nutritional layer and may be present in complete feeds, compost or irrigation water. Apparent calcium-related problems in fruiting crops can also involve inconsistent moisture and impaired uptake, so another dose is not automatically the right response.
Micronutrients
Iron, manganese, boron, zinc and other micronutrients are needed in much smaller quantities but remain essential. Their availability can be influenced by the root environment and pH. Applying an extra trace-element product without diagnosing the underlying condition may complicate rather than solve the problem.
The medium belongs in the calculation
Fresh multipurpose or specialist compost may contain sufficient fertiliser for several weeks; an older, heavily rooted grow bag has a much smaller reserve. pH affects nutrient availability, while visible deposits or measured conductivity can indicate dissolved salts. Interpret any reading in relation to the crop, medium, water and testing method rather than treating a meter as a diagnosis by itself.
Demand is a moving line
More growth does not always require stronger feed.
Treat these stages as a relative demand map, not as a dose chart. Available light, compost temperature, root volume, container size, crop variety and harvest load all move the line. During a dull, cool spell, question your assumptions before increasing nutrient input.
Leafy crops
Fruiting crops
PropagateSettle inCanopyFlowerFill fruitSlowdown
Seedlings
Seed reserves and the nutrients in seed compost support the earliest phase. Feed sparingly, if at all, until seedlings are established and those reserves are being exhausted. Young roots are more easily damaged by excess than helped by enthusiasm. Prioritise adequate light, suitable temperature, gentle watering and timely potting on, following both compost and fertiliser labels.
Establishment
After transplanting, prioritise an evenly moist, aerated root zone. A fresh pot, grow bag or prepared bed may already contain ample nutrition. Give roots time to occupy the medium before interpreting modest top growth as hunger, especially in cool spring weather.
Leaf production
Salads, herbs and young fruiting plants benefit from steady availability under good light. Pale foliage may indicate limited nutrition, but it may equally reflect cold compost, shade, compaction, damaged roots, waterlogging or a rootbound pot.
Flower and fruit load
Tomatoes, peppers, cucumbers, aubergines and other fruiting crops become more demanding while setting and filling fruit. Increase attention to even watering and label-directed feeding at this stage—not simply the strength of each application.
The mixing bench
Calculate the batch before opening the concentrate.
Rates vary considerably between products. Use the exact measuring device, weight or injector ratio specified by the manufacturer. The examples below demonstrate label-based arithmetic only; they do not establish a rate for another feed, crop or growing medium.
Label rate × water volume = fertiliser required
Example A / hand watering
Rate per litre
If the label states 5 ml per litre and you need a final volume of 8 litres, measure 40 ml of concentrate for the complete batch. If that label specifies a lower rate for young plants, use its young-plant instruction instead.
Example B / smaller batch
Rate by water volume
If directions state 10 ml per 2 litres, a 6-litre batch requires 30 ml. Mix fully and distribute the finished solution evenly. Do not improvise with capfuls or an unmarked oversized jug.
Example C / injector
Stock and final solutions
With fertigation, stock concentration, injector performance and irrigation volume together determine what reaches the roots. Follow the fertiliser and equipment instructions precisely, use a suitable fully soluble product and monitor distribution. Concentrated stock solution must never be applied directly to plants.
Mixing order: begin with most of the required water; add the accurately measured fertiliser; agitate until fully dispersed; then top up to the final volume and mix again. Use a syringe, jug or measure reserved for garden work. Mix products together only when their labels explicitly permit it, and avoid keeping an unnecessary surplus that may later be applied to plants which do not need feeding.
A well-proportioned Classic Aluminium Greenhouse provides room to separate a clean mixing position from planted benches, preserve clear aisles and keep irrigation accessible for inspection. That physical order makes accurate delivery easier to repeat and reduces the temptation to pour an unwanted remainder into the nearest pot.
Delivery method
Choose the route that suits the crop and system.
Root feeding
This is the normal route for most greenhouse food crops. Apply correctly diluted solution around the root zone when the medium is suitably moist, working at a pace it can absorb. Aim for even distribution without splashing foliage, leaving dry pockets or allowing pots to stand in nutrient-rich run-off.
Foliar feeding
This is a specialist, product-specific tool rather than a substitute for sound root-zone nutrition. Use only when the label permits foliar application, at the stated dilution. Avoid hot, bright conditions, and do not spray edible crops, flowers or stressed plants unless the product expressly allows that use.
Fertigation
Supplying soluble fertiliser through irrigation can serve larger collections of pots and grow bags efficiently, but it requires more measurement, not less. Keep filters and emitters clean, verify even delivery and account for the stock concentration, injector performance and total irrigation volume.
Containers, beds and water
Salt is the quiet risk in a well-fed greenhouse.
Every soluble fertiliser increases the concentration of dissolved nutrients around roots. As plants take up water and moisture evaporates, those salts may become concentrated in the medium. Small pots, grow bags, inconsistent watering, poor drainage, high summer temperatures and repeated feeding all narrow the margin for error.
Containers hold a limited volume of compost and can dry out or become saline quickly, so they need closer observation than greenhouse beds. Bed soil offers a larger buffer, but repeated tomato and cucumber crops still remove nutrients and salts can accumulate where drainage is poor. Structure, organic matter, crop rotation and testing all belong in long-term bed management.
Water is part of every fertiliser batch. Its starting composition affects the solution before concentrate is added, while watering frequency determines how nutrients move through the medium. If crops repeatedly disappoint, a soil, water or compost test can be more informative than another bottle. Use any pH or conductivity result alongside drainage, root health, crop stage and feeding history.
Formulation choice
Choose organic, soluble or controlled-release by behaviour.
Organic and soil-building approaches
Composts, well-rotted manures and plant- or animal-derived fertilisers can improve soil structure, support soil life and release nutrients over time. They are particularly useful when building or renewing greenhouse beds. Availability is not instant or perfectly uniform, however; microbial activity, temperature, moisture and material quality all matter. Read the nutrient analysis and consider what is already present rather than assuming that an organic input cannot be overapplied.
Controlled-release fertiliser can reduce mixing frequency by supplying nutrients gradually. Release varies with temperature and must be included in the total feeding plan before any liquid supplement is added. A warm greenhouse may alter release, so convenience does not remove the need for observation.
Soluble, responsive approaches
Soluble feeds act quickly and suit actively cropping vegetables in containers and grow bags. They allow the programme to respond to plant stage and root-zone condition, but that responsiveness demands accurate dilution, complete mixing and a clear record. Their concentration should never be estimated by eye.
A sensible system may combine a suitable medium, a modest baseline feed where appropriate and carefully managed soluble nutrition for a productive tomato, cucumber or pepper crop. Avoid accidental overlap: rich compost, controlled-release granules and frequent liquid feeding can readily add up to excess, even when every individual product appears familiar.
For crops kept under cover for several seasons, make the structure part of the growing plan. The adaptable proportions of the Classic Aluminium Greenhouse can support separate propagation, container and fruiting-crop zones. This makes it easier to inspect each group, keep light feeders apart from heavily cropping plants and avoid applying one mixture indiscriminately across the entire greenhouse.
The smallest useful tool
Keep a record that informs the next decision.
A greenhouse feeding record need not resemble a laboratory notebook. It simply needs to prevent accidental repeat applications and connect plant response with conditions. Record crop zones separately: a tray of March salads is not a July tomato grow bag, while an overwintered citrus in low light has a different appetite again.
Date
Crop + stage
Feed + rate
Water / pH / salts
Plant response
12 May
Tomato / first flowers
Label-rate tomato feed
Evenly moist; drainage checked
Active; no edge scorch
19 May
Lettuce / harvest week
Light feed or none
Compost evenly moist
Good colour and texture
Note significant weather changes too. Several dull days, a sudden hot spell, increased irrigation or a ventilation problem can alter uptake quickly. When organising water, feed and access inside the Classic Aluminium Greenhouse, use water purposefully, deliver it close to the roots and keep drainage visible. An orderly arrangement helps reveal uneven application, blocked drippers and surplus run-off before they become recurring problems.
When selecting the structure itself, consider workflow as carefully as appearance. A configurable Classic Aluminium Greenhouse can become a quiet operating system for the garden: clear bench zones, accessible pots and grow bags, space for a clean watering can and enough room to inspect leaves, compost and drainage without stepping over plants.
FAQ
Questions that deserve a measured answer.
How often should I feed greenhouse plants?
There is no reliable UK interval for every crop, compost and product. Follow the exact label, then consider plant stage, available light, temperature, irrigation frequency and growth rate. A seedling, leafy salad, fruiting tomato and dormant citrus should not share one calendar. In cool, dull conditions, uptake usually slows even if the planned feeding day has arrived.
Can one fertiliser feed tomatoes, herbs and lettuce?
A complete fertiliser may be labelled for several crops, but formulation, dilution and frequency can differ. Fruiting tomatoes have substantial demand while carrying trusses, whereas seedlings and leafy crops are more easily damaged by excessive concentration. Separate crop zones where practical and follow the product’s directions for each permitted use.
Should I add fertiliser whenever I water?
Not by default. Some carefully monitored fertigation systems supply low-level nutrition through irrigation, but they depend on accurate stock mixing, equipment performance, drainage and root-zone monitoring. For many home greenhouses, periodic label-directed feeding is clearer and easier to review. Do not adopt continuous feeding simply because plants require more frequent plain water during hot weather.
What is the quickest remedy for yellow leaves?
First inspect the pattern and the growing conditions. Yellowing may indicate limited nutrition, but it can also result from roots that are too wet, dry, cold, compacted, damaged or rootbound, as well as unsuitable pH, pests or disease. Excess potassium may interfere with magnesium uptake too. Check feeding history and where symptoms began before selecting a corrective product.
Can I feed when the compost is dry?
Avoid applying concentrated feed to a severely dry root zone. Re-wet the compost gently and evenly with plain water, allowing moisture to distribute before feeding later if the crop and label warrant it. Equally, do not feed a waterlogged pot reflexively: oxygen-deprived roots cannot use nutrients effectively.
Sources & further reading
Read the label, then inspect the roots.
These UK horticultural resources support the principles in this guide, from understanding fertiliser declarations and plant uptake to watering, fertigation and peat-free nutrition. Product-specific rates, permitted uses and safety directions must always come from the exact fertiliser and growing medium in use.
More from Bloomcabin: greenhouse gardening guide for beginners · water-wise greenhouse gardening guide · UK greenhouse setup guide
The next pour, calibrated
Three checks before the solution reaches the roots.
Begin with the crop, inspect the root zone and obey the label. A greenhouse designed around access, ventilation and orderly watering creates the conditions for making those small decisions accurately, day after day and through the changing British growing year.
Crop stage
Seedling, canopy, flower, fruit or seasonal slowdown?
Seedling, canopy, flower, fruit or seasonal slowdown?
Root-zone condition
Moist, aerated, draining and not recently overfed?
Moist, aerated, draining and not recently overfed?
Label rate
Measured precisely for the final water volume?
Measured precisely for the final water volume?
For a considered growing space built around access, observation and practical crop care, discover the Classic Aluminium Greenhouse.
