Organic liquid fertiliser 5-2-5, a balanced growth feed with magnesium
Liquid fertiliser 5-2-5 is a balanced natural plant food for the main vegetative stretch of the season, the period when a plant is building leaf, stem and root before it turns its attention to flowering. Nitrogen and potassium are matched at 5% each with phosphorus held at 1.8%, and the formulation carries magnesium and trace elements so that a season of feeding does not quietly create a secondary deficiency.
Everything in it is plant-derived, including the magnesium and the full trace element set. That is unusual. Most liquid feeds reach for mineral salts or synthetic chelates for the micronutrients even when the macronutrients come from plant material, and we did not want to.
The nutrients are in true solution rather than suspension, so there is nothing to settle, nothing to shake and nothing to block a nozzle. It works equally well through a watering can, a drip line or a sprayer.
This is the middle feed in a three-stage programme. A rooting feed goes in at establishment and a high-potash feed takes over at fruiting. The 5-2-5 covers the long stretch in between, which for most British gardeners means May through to mid-July.
What to use a 5-2-5 growth feed for
- Vegetative growth on tomatoes, peppers, cucumbers and courgettes the stretch between planting out and first flower, when the plant is building the frame that will carry the crop. Feed here and the plant has something to fruit on later
- Leafy vegetables and cutting salads chard, spinach, kale, lettuce and oriental greens want steady nitrogen with enough potassium to keep tissue firm rather than sappy
- Brassicas, leeks and sweetcorn through the main season long-season heavy feeders that need consistent supply rather than occasional large doses
- Container displays and hanging baskets in build-up the establishment weeks before flowering starts in earnest, when the plant needs bulk before it needs bloom
- Young shrubs, hedging and newly planted perennials first and second season after planting, through spring and early summer
- Weekly maintenance feeding across a mixed plot the one vegetable fertiliser that suits most things at once, which matters when you are watering forty pots and do not want four different bottles
- Preventing magnesium shortfall under heavy feeding potassium competes with magnesium at the root, so including magnesium in the feed itself heads off the interveinal yellowing that heavy potash programmes provoke
Where 5-2-5 sits against the alternatives
Liquid 5-2-5 growth feed
- Balanced N and K for the vegetative stage, phosphorus held low and steady
- Magnesium and trace elements included, so no secondary deficiency creeps in
- True solution, so no settling, no shaking, clean through drip and sprayer
- Plant-derived throughout, macronutrients and micronutrients alike
- Works as a root drench or a foliar spray at the same dilution range
General-purpose synthetic liquid feed
- Cheap and immediately available
- Usually no magnesium and few or no trace elements
- No organic matter, no carbon contribution to soil biology
- Salt load builds in containers over a season of feeding
Dry organic fertiliser
- Steady release over weeks, and builds soil over years
- Slow to respond when a plant needs feeding now
- Harder to apply evenly to established containers
- The right base for the season, and this feed layers on top of it
Use a rooting or phosphorus feed at establishment, 5-2-5 through the main vegetative period, then switch to liquid 3-2-6 as flowering and fruiting begin. The three cover a full season without ever feeding a plant the wrong thing for the stage it is at.
Made with organic ingredients, in small batches. Made by Growers. Backed by Science.
The science behind a 5-2-5 ratio
NPK ratios are marketing shorthand for something real: the relative demand a plant has for each nutrient at a given growth stage. During vegetative growth a plant is laying down protein and enzyme machinery, which takes nitrogen, and running the water and sugar transport to support it, which takes potassium. Phosphorus demand is real but far smaller by mass, because phosphorus goes into membranes and nucleic acids rather than bulk tissue (Marschner, 2012).
A ratio is not a recipe for a plant. It is a statement about which stage the plant is in.
What each element is doing at this stage
Nitrogen, building the machinery
Every protein and every chlorophyll molecule contains nitrogen. At 5% it is the dominant element by demand during vegetative growth, but held low enough that tissue stays firm rather than soft and aphid-prone.
Potassium, running the machinery
Matched to nitrogen at 5%. Potassium regulates stomatal opening, activates more than sixty enzymes and drives sugar transport out of the leaf. Feeding nitrogen without matching potassium produces bulk without structure (Zörb et al., 2014).
Phosphorus, held deliberately low
At 1.8%, enough for continuing root development and membrane synthesis without the surplus that suppresses mycorrhizal association. Heavy phosphate feeding reduces the fungal partnerships plants rely on for phosphorus foraging (Smith and Read, 2008).
Magnesium, the chlorophyll atom
Every chlorophyll molecule has a magnesium ion at its centre. Magnesium is also the nutrient most often displaced by heavy potassium feeding, because the two compete for the same uptake sites.
Trace elements, the small things that stop everything
Iron, manganese, zinc, boron, copper and molybdenum are needed in milligram quantities and bring growth to a halt when absent. Here they are drawn from plant material rather than mineral salts, and including them in the feed avoids the slow drift into deficiency that long container runs produce.
Cation competition, and why the ratio matters
Potassium, calcium and magnesium are all cations taken up through overlapping pathways. Push one hard and you suppress the others. This is real and well documented, but it does not support the older Base Cation Saturation Ratio theory, which held that soils should be balanced to fixed cation percentages. That framework has not survived testing. Sufficiency of each nutrient matters far more than the ratio between them (Kopittke and Menzies, 2007).
The practical version is simpler. Do not feed one cation hard without covering the others. Which is why magnesium is in this formulation rather than sold separately.
Solution rather than suspension
Nutrients here are dissolved, not suspended. That has three consequences. Uptake is immediate, because roots and leaf cuticles absorb ions rather than particles. Nothing settles out in the bottle. And the same product can be used as a foliar spray, because foliar uptake happens through cuticular pores measured in nanometres, far too small for suspended mineral particles.
Organic and mineral feeding read honestly
Across 537 experiments in grasslands and croplands, organic fertilisation raised aboveground biomass by 56% against 42% for mineral fertilisation, and only the mineral treatments reduced plant diversity (Shi et al., 2024). A separate synthesis of 7,859 paired observations from 551 field experiments found fertiliser lifted crop yield by 30.9% and nutritional quality by 11.9%, with potassium, magnesium and micronutrients doing much of the quality work, and combined organic plus mineral inputs performing best of all (Ishfaq et al., 2023). Vegetables were the most responsive crop group in that dataset.
That last finding is the argument for this feed rather than a straight NPK. Potassium and magnesium together, with the micronutrients present rather than assumed. None of it makes a liquid feed a complete system on its own. It makes it a good partner to a dry organic base.
References
- Marschner, P. (ed.) (2012). Marschner's Mineral Nutrition of Higher Plants, 3rd edition. Academic Press.
- Zörb, C., Senbayram, M. and Peiter, E. (2014). Potassium in agriculture, status and perspectives. Journal of Plant Physiology, 171(9), 656–669.
- Smith, S.E. and Read, D.J. (2008). Mycorrhizal Symbiosis, 3rd edition. Academic Press.
- Kopittke, P.M. and Menzies, N.W. (2007). A review of the use of the basic cation saturation ratio and the "ideal" soil. Soil Science Society of America Journal, 71(2), 259–265.
- Shi, T-S., Collins, S.L., Yu, K., Peñuelas, J., Sardans, J., Li, H. and Ye, J-S. (2024). A global meta-analysis on the effects of organic and inorganic fertilization on grasslands and croplands. Nature Communications, 15, 3411.
- Ishfaq, M., Wang, Y., Xu, J., Hassan, M.U., Yuan, H., Liu, L. et al. (2023). Improvement of nutritional quality of food crops with fertilizer: a global meta-analysis. Agronomy for Sustainable Development, 43(6), 74.
How to use liquid 5-2-5 growth feed
Measure with a syringe or a small measuring cup rather than pouring by eye. The concentrate is acidic at pH 3.5 to 4.5, which keeps the nutrients soluble. Always dilute into water and never apply neat. Keep the cap tight between uses to stop moisture loss.
Application rates
Root drench, standard feeding
Delivers roughly 150 to 300 mg/l each of nitrogen and K₂O, which is standard horticultural feeding strength. Use the lower end for young plants and general border work, the upper end for established heavy feeders in full growth.
Root drench, open ground and plot feeding
Same dilution, applied once a week rather than at every watering. Suits allotment beds, brassicas, leeks and anything growing in soil rather than compost.
Foliar spray
Spray to run-off in early morning or evening, never in strong sun. Useful when cold or waterlogged soil is limiting root uptake, or to correct a visible deficiency more quickly than a root feed manages.
Mixing
- Half-fill the watering can or sprayer with water first.
- Measure the dose with a syringe and add it to the water.
- Top up to full volume and stir or agitate briefly.
- Apply the same day, because diluted feed is not worth storing.
- Rinse equipment through with clean water afterwards.
Where it sits in the season
| Stage | Feed | Roughly when |
|---|---|---|
| Establishment and rooting | Liquid phosphorus feed | Transplanting, first 5 to 6 weeks |
| Vegetative growth | This feed, liquid 5-2-5 | May to mid-July for most crops |
| Flowering and fruiting | Liquid 3-2-6 high-potash feed | From first flower or fruit set |
| Deficiency correction | Single-nutrient liquid feeds | As and when symptoms show |
Never feed a plant that is already wilting and dry. Water it plain first, let it recover for a few hours, then feed. Concentrated nutrient solution applied to a dry root ball draws water out of the roots rather than into them.
Storage
Cap tight to prevent moisture loss. Store dark and dry between 10 and 25 °C. Shelf life 18 months to 2 years. Do not let it freeze.
What to pair it with
Pair this with the rest of the Dr Forest liquid range. Liquid Seaweed Biostimulant covers alginates and natural growth promoters that a straight nutrient feed does not. Liquid Gypsum supplies calcium and sulphur, the two things most liquid feeds leave out. For a full-season plan, see the Dr Forest feeding programme. If you are chasing down yellowing leaves rather than feeding to schedule, start with why tomato leaves turn yellow.