How long does organic slow-release fertiliser last?

By Joe Barrington, founder of Dr Forest · Published September 2026 · Updated 6 November 2026

Quick answer. Organic slow-release fertiliser is a dry feed made from plant meals, bone meal, manure and ground minerals, and soil life frees the nutrients as it breaks that material down. There's no plastic coat. In warm, moist soil a meal-based feed keeps releasing for up to three months. A potting dose of a blend like All Purpose 4-4-4 does most of its feeding in the first 8 weeks: the quick parts work within days, and the bone meal keeps releasing a little after that. In cold March soil very little happens, so water it in and give it time.

In my experience a slow-release feed lasts up to three months, and you see it tail off before then.

See the organic fertilisers collection, including All Purpose 4-4-4.

Slow release vs controlled release

Coated "controlled-release" fertiliser is soluble feed inside a shell of sulphur or polymer. Water seeps through the shell and dissolves the feed, which then diffuses out at a pace set by the coating's thickness and the soil temperature [1].

The months printed on a bag of coated fertiliser are a rough guide. In an outdoor trial in Utah, a coated urea sold as a 180-day product had released over 80% of its nitrogen by day 112 when worked into the soil, and coated granules left on the surface passed 80% within 35 days [2].

When Isakov and colleagues analysed the coats on two widely sold brands, both were plastic polymers [3]. The feed dissolves out and the empty shell stays in the soil.

An organic feed has no shell to leave behind. The nitrogen in alfalfa meal is protein, and the phosphorus in bone meal is calcium phosphate. Soil microbes break the protein down into nitrogen that roots can take up, a step called mineralisation, so the feed works as fast as your soil life does.

Why organic slow-release fertiliser does nothing in March

Soil microbes slow right down in the cold, so a feed spread on a cold bed or allotment plot in March can look idle for a fortnight. That goes for every organic feed, from alfalfa meal and bone meal to pelleted manure and blends like the All Purpose 4-4-4, because each one waits on the same microbes. Warmth makes a big difference: in 55 Californian farm soils, nitrogen release from the soil's own organic matter ran nearly three times faster for every 10 °C of warming [4]. So by late May the same granules are working much harder, just as most plants start to grow fast. The timing suits vegetable gardening, where most crops put on their growth from late spring into summer. Growing notes for those crops are in the plant growing directory.

Dry soil slows it down a great deal too, so water the feed in after you spread it. Don't add a second dose because the first seems to be doing nothing. It's still there, waiting for the soil to warm.

What up to 8 weeks means on the bag

The Dr Forest Organic All Purpose Fertiliser 4-4-4 feeds most plants for up to about 8 weeks from a potting dose. The bag holds ten ingredients, each on its own clock. The plant nitrogen and sulphate of potash work within days. Alfalfa meal, a third of the mix, takes days to a few weeks, and flaxseed meal a few weeks. Bone meal and gypsum keep releasing a little phosphorus and calcium after the 8 weeks are up. I explain the three numbers in my guide to 4-4-4 fertiliser.

Here's where the nitrogen sits. The plant nitrogen, an amino acid material made from molasses at 13% N, is 18% of the mix, so it supplies roughly 2.3 of the bag's 4% nitrogen, more than half. Alfalfa, at a third of the mix and roughly 2.5% nitrogen, adds about another 0.8. So most of the nitrogen is at the quicker end of the blend, which is why the first top dress in a pot goes on at 6 weeks, before the potting dose runs down.

After that first top-up, the product page says to feed pots every 4 weeks, or every 2 to 3 weeks for hungry plants, and beds every 6 weeks while plants are growing, up to three times a season. Watch the plant as well as the calendar: when the lower leaves fade from deep green to pale, or growth slows, the last dose is running down. How much to weigh out each time is in my guide on how to top dress, and the potting rates for light and hungry plants are in the same 4-4-4 fertiliser guide.

Tomato Feed 3-4-6, Fruit & Vegetable Fertiliser 4-5-6 and Rose & Shrub Fertiliser 5-3-5 are built the same way, so they also slow down in cold soil. The tomato feed is a milled powder you top dress every 3 weeks, so don't copy the All Purpose timings onto it. For when to start and how often, see my guide to the best fertiliser for tomatoes in pots.

Why a blend staggers its release on purpose

A single ingredient has one release curve. Bone meal on its own gives phosphorus and calcium slowly and little else, while alfalfa meal front-loads nitrogen and then tails away. Those curves can sit far apart: over 120 days in two Georgia soils, feather meal released 41 to 77% of its nitrogen and pelleted poultry litter 0 to 22%, and the choice of feed counted for far more than temperature [5]. The dry amendments guide covers the single materials. If you use alfalfa on its own, my guide to alfalfa meal as a fertiliser gives the rate.

A blend like the All Purpose 4-4-4 lays several curves over each other, so something is becoming available at most points across those eight weeks. You lose precision, though: you can't time a blend to the day.

What you cannot predict

No one can give you an accurate week-by-week release table for an organic feed. It depends on soil temperature and moisture, how much life and organic matter your soil already has, particle size, and whether the feed was worked in or left on top. Even the growing medium matters: in a 102-day lab test, feather meal and a meal-based pellet released about 59% of their nitrogen in peat-based potting mix, against about 47% in a sandy loam [6]. So a feed in a pot tends to run a little faster than the same feed in a bed.

What you can plan around is the pattern: slow early in the season, faster in the middle, and always slower than a liquid. The range also includes the All-Purpose Booster 5-5-5. If you want a bed that keeps feeding while you get on with something else, use a granular slow-release plant food like the All Purpose 4-4-4. I compare granular, liquid and slow-release feeds more generally in what is a fertiliser.

Frequently asked questions

What is an organic slow release fertiliser?

It's a dry feed whose nutrients are released by soil life as it breaks the material down. It's made from plant meals, bone meal, manure and ground minerals, and it runs faster in warm, moist soil than in cold.

Does slow-release fertiliser contain plastic?

A meal-and-mineral organic feed shouldn't, because it has no coat. Many coated "controlled-release" granules do: the shell is a polymer that stays in the soil after the feed has gone.

Does organic fertiliser work in cold soil?

Only slowly. In 55 Californian farm soils incubated at 5, 15 and 25 °C, nitrogen release from the soil's own organic matter rose exponentially with temperature, so a March dose can look idle for a fortnight and picks up as the soil warms in May. That lag is normal in organic gardening, because the feed has to wait for soil life to wake up.

How long does organic slow-release fertiliser last?

Up to three months in warm, moist soil, though a potting dose of a blend like All Purpose 4-4-4 does most of its feeding in the first 8 weeks, with the quick parts working within days and the bone meal releasing a little after that. In pots the first top-up goes on at 6 weeks, before the dose runs down, and my guide on how to top dress gives the amounts.

Joe Barrington, founder of Dr Forest in Stockport. Joe formulates and blends his own feeds, including All Purpose 4-4-4 and Bloom 2-8-4, and stocks bought-in products alongside them.

Sources cited

1. Lawrencia, D., Wong, S.K., Low, D.Y.S., Goh, B.H., Goh, J.K., Ruktanonchai, U.R., Soottitantawat, A., Lee, L.H. & Tang, S.Y. (2021). Controlled release fertilizers: a review on coating materials and mechanism of release. Plants 10(2): 238. doi.org/10.3390/plants10020238
2. Ransom, C.J., Jolley, V.D., Blair, T.A., Sutton, L.E. & Hopkins, B.G. (2020). Nitrogen release rates from slow- and controlled-release fertilizers influenced by placement and temperature. PLOS ONE 15(6): e0234544. doi.org/10.1371/journal.pone.0234544
3. Isakov, V., Vlasova, E., Forer, V., Kenny, J. & Lyulin, S. (2025). Analysis of slow-released fertilisers as a source of microplastics. Land 14(1): 38. doi.org/10.3390/land14010038
4. Miller, K.S. & Geisseler, D. (2018). Temperature sensitivity of nitrogen mineralization in agricultural soils. Biology and Fertility of Soils 54(7): 853 to 860. doi.org/10.1007/s00374-018-1309-2
5. Ibiapina de Jesus, H., Cassity-Duffey, K., Dutta, B., da Silva, A.L.B.R. & Coolong, T. (2024). Influence of soil type and temperature on nitrogen mineralization from organic fertilizers. Nitrogen 5(1): 47 to 61. doi.org/10.3390/nitrogen5010004
6. Pulliam, J.J., Cassity-Duffey, K. & Cabrera, M. (2025). Effect of biochar on the nitrogen mineralization of commercial organic fertilizers in both mineral soil and organic potting media. Nitrogen 6(3): 71. doi.org/10.3390/nitrogen6030071