Organic slow-release fertiliser notes

By Joe, Founder of Dr Forest · September 2026 · 8 min read

Edited 28 Sep 2026

The hub for this topic is the page: Slow-release organic fertiliser (meal vs coated, when All Purpose 4-4-4 fits, honesty limits). This post keeps the longer garden notes. Default bag: All Purpose 4-4-4.

The short hub answer lives on our slow-release organic fertiliser page. This post keeps the longer garden notes on meal vs coated, cold soil, and what “up to 8 weeks” means on a bag.

In short

Slow release vs controlled release

Coated granules start as soluble fertiliser wrapped in sulphur or polymer. Water vapour enters, salt dissolves, nutrient diffuses out. Thickness and temperature set the pace (Lawrencia et al. 2021). In the ground it is less tidy than the bag number: Ransom et al. saw a 180-day polymer urea drop >80% of its N by day 112 under fluctuating temperatures — and surface-applied polymer products dump still faster.

What the shell is made of matters. A 2025 analysis of two shop-bought "slow release" feeds found polyethylene/polyacrylic and polyacrylamide-type coats — neither rots in soil. Nutrients leave; plastic stays and fragments. From 17 October 2028 the EU 90% / four-year coating biodegradability test applies to coating polymers after the feed finishes — it is not in force yet; treat it as a regulatory direction of travel, not today’s UK shop-shelf rule.

An organic feed has no shell to leave. Nitrogen in alfalfa meal is protein; phosphorus in bone meal is calcium phosphate. Roots wait on mineralisation. Speed = speed of the soil food web that day.

Why nothing happens in March

That breakdown depends on temperature, and sharply so. Miller and Geisseler kept 55 Californian farm soils at 5, 15 and 25 °C for ten weeks and measured how much nitrogen came out of each. They summarised it as a Q10 value, which is simply how many times faster the process runs for every 10 °C the soil warms. Their soils averaged 2.87, with regional means running from 2.67 to 3.15.

Which explains the most common complaint about organic granular fertiliser. You spread it in March over a cold bed, wait a fortnight, and see very little. Nothing has failed. Cold soil slows mineralisation sharply — it does not stop it (Miller and Geisseler measured mineralisation even at 5°C) — so release crawls until the bed warms. By late May the same granules are releasing several times faster. Annoying if you wanted an instant green-up, and quite useful on a mixed plot, because the release picks up at roughly the point the plants start asking for more.

Moisture works on the same terms. Dry soil suspends the process almost entirely, which is why every rate card tells you to water it in.

Slow, in an organic feed, describes the soil rather than the granule.

What up to 8 weeks means on the bag

The Dr Forest Organic All Purpose Fertiliser 4-4-4 carries a stated figure of up to 8 weeks. That number describes a blend of release rates rather than one rate. Ten inputs, each on its own clock.

Six of the ten inputs, ordered fastest to slowest. Shares are the proportion of the mix as stated on the All Purpose 4-4-4 product page.

An initial soil-mix charge feeds most plants for up to 8 weeks. The fast inputs are working within days. The slow ones are still going at week ten. If your compost was rich to start with, you will usually be reapplying after 2 to 4 weeks anyway, because the plant draws down what was already there long before the bag has finished.

Rates for the All Purpose 4-4-4, since they set the size of the charge you are metering out:

• Soil mix before planting: 5–10 g per litre of soil or compost, worked in thoroughly.
• Top dress during growth: 2–4 g per litre of soil, scattered on the surface and watered in.
• Beds, borders and allotments: typically 100 g per m² every 4–6 weeks in the growing season; use up to 300 g per m² once on a new or tired bed — do not repeat that top rate every four weeks. Work lightly into the top 5–10 cm.
• A level tablespoon holds roughly 12–15 g. A handful is roughly 40–50 g.

Feeder group changes those numbers. Light feeders such as peas, beans, carrots and Mediterranean herbs want the lower end and will run to leaf or fork if you overdo it. Heavy feeders such as tomatoes, courgettes, brassicas and anything growing in a pot want the upper end, and want it more often.

The same slow-release build runs through the crop-specific blends, if you would rather match the ratio to one crop than feed the whole plot from one bag. Tomato Fertiliser 3-4-6, Fruit & Vegetable Fertiliser 4-5-6 and Rose & Shrub Fertiliser 5-3-5 are all granular organic feeds on the same soil-temperature clock. The ratio differs; the release mechanism does not.

Why a blend staggers its release on purpose

A single-ingredient feed has one release curve, and you get whatever shape that ingredient happens to have. Bone meal on its own delivers phosphorus and calcium slowly and very little else. Alfalfa meal on its own front-loads nitrogen and carbon, then tails away. Sulphate of potash is soluble and moves in days, while most of the nitrogen and phosphorus wait on soil biology — so do not read the blend as a “no salt build-up” feed in the coated-CRF sense. Each is useful on its own terms, and each leaves a gap. The dry amendments guide runs through what the individual materials do.

Stacking them is the point of a multi-input blend such as the All Purpose 4-4-4. Something is becoming available at most points across those eight weeks, instead of everything arriving in week one and nothing arriving in week six. The soil also gets a spread of different materials to work on rather than just one, which suits ground you are trying to keep alive.

The trade-off is honest enough. You lose precision. A single soluble input you can time. A blend you cannot, because several materials are breaking down at once, at their own speeds, in weather nobody controls.

What you cannot predict

Anyone handing you a week-by-week release table for an organic feed is guessing. The variables that move it are the ones no bag can know: soil temperature through the season, moisture, how much organic matter and biology your soil already carries, particle size, and whether the material was worked in or left on the surface.

Ransom's coated products missed their own stated timings by as much as 68 days under real temperature swings, and those are engineered to a specification with a measurable coating thickness. An organic feed has no specification to miss in the first place. What it has is a direction of travel, which is the useful thing to plan around. Slow at the start of the season, faster through the middle of it, and always slower than a liquid.

Judge it over weeks rather than hours. If you need a plant to respond by Thursday, a granular organic feed is the wrong tool and a liquid is the right one. If you want a bed that keeps feeding while you get on with something else, a granular blend like the All Purpose 4-4-4 is what that looks like. For the wider definitions, what is a fertiliser covers the ground.

Related: See the Organic All Purpose Fertiliser 4-4-4

Spread it on a cold March bed and it will look like little is happening, because release has slowed sharply. Put your hand in the same soil in June and the same material will be feeding hard. The bag has not changed. The soil has warmed up.