Organic dry amendments: the 3 most gardens need

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

Quick answer. Most gardens need just three things: compost or well-rotted manure, a nitrogen meal such as alfalfa, and a potassium source such as sulphate of potash. The compost brings organic matter. The other two are organic dry amendments: solid feeds such as plant meals, ground rock and fermented bran that you dig in or scatter on top. Add phosphorus, calcium or magnesium only when a soil test says you are short. Meals feed slowly through soil life over several weeks, while mineral salts such as sulphate of potash work within days.

I don't keep any single meals myself. I use my own blends, such as All Purpose 4-4-4 and Bloom 2-8-4, which I formulate and blend myself, so the amendments are already in them in the right proportions. My advice on mixing your own from single meals is to leave it. You need to know the nutrient profile of each meal and work out how much of each nutrient ends up in the finished mix, and that is difficult to get right. If you do want to mix your own, use somebody's tried and tested recipe. Make one up yourself and you will probably run into issues.

Take two sacks as an example. Alfalfa meal runs at roughly 2.5-0.2-2.5 [1], and micronised rock phosphate is 31% phosphate (P2O5) on the pack. Mix a kilo of each and on paper you get about 1.3-15.6-1.3, so around twelve times as much phosphate as nitrogen, going into soil that often has plenty of phosphorus already.

What counts as a dry amendment

Organic here means materials from plants or animals, plus minerals dug from the ground. Compost and manure are the bulky organic soil amendments. Dry amendments are the concentrated end, used by the handful rather than the barrowload, so they add very little organic matter and you still need compost for that.

How they differ from bottled feeds

The difference is the route the nutrient takes. A liquid feed goes from bottle to root and washes through within days. A meal goes from sack to soil life to root, and that middle step is what people mean by a living soil: you feed the soil, and the soil life feeds the plant. Mineral salts skip the middle step. Bottled extras such as amino acid biostimulants work on the plant at small doses, so they sit alongside a dry feed rather than replace it.

How fast a meal works depends on the material and the weather. The more carbon it carries for each unit of nitrogen, the slower it releases, and soil life slows right down when it is cold or dry. A 10 °C drop in soil temperature cut nitrogen release from alfalfa pellets by about 20% [1].

The six families of dry amendments

Most dry amendments fall into six groups. You do not need one from every group.

Family	Example inputs	What it does	Most gardens?
Nitrogen meals	Alfalfa meal, malted barley, plant protein meals	Leafy growth; feeds soil microbes as it breaks down	Yes
Potassium and minerals	Sulphate of potash, rock dust (basalt)	Fruit and flowers; a broad spread of minerals	Usually
Phosphorus	Rock phosphate (mined); bone meal (animal-derived)	Roots, flowering and fruit set	If soil is low
Calcium and magnesium	Gypsum, Epsom salt, kieserite	Fixes a shortage a soil test has found; gypsum also helps sodic clay hold its structure	Test first
Biology and carbon	Bokashi, biochar, worm compost	Food and a home for soil life; holds moisture	Optional
Biostimulants	Seaweed meal, humic and fulvic acid	Growth boosts at small doses; little feed in them	Nice to have

A home soil test kit shows pH and rough nitrogen, phosphorus and potassium levels, which tells you which rows you need. A lab test is more exact.

Many UK garden soils that have been fed for years already hold plenty of phosphorus, so test before you add more. My All Purpose 4-4-4 and Bloom 2-8-4 both contain bone meal for slow phosphorus, and the share is on each product page. If you'd rather keep animal inputs out, micronised rock phosphate is the mined alternative, released slowly as roots and soil acids work on it.

You'll read that soil needs a fixed ratio of calcium to magnesium. Kopittke and Menzies went back through the evidence and found that, within the range most soils fall in, the balance of calcium, magnesium and potassium makes little difference to fertility, and chasing it wastes money [2]. Use gypsum or Epsom salt when a soil test shows a shortage, and leave the ratios alone.

Rock dust is the slowest of the lot and suits light, sandy ground best. It nudges the pH up, so check the pH first if your soil is already near neutral or alkaline.

Go carefully in the biology aisle. Bagged mycorrhizal inoculants (powders of root fungi) give patchy results in real soil, so check whether your plants use them first in what are mycorrhizal fungi. Biochar on its own made no difference to crop yield on average in temperate regions such as the UK, against a 25% average gain in the tropics [3]. Worm compost earns its place: averaged across studies, vermicompost raised commercial yield by 26%, with the best results when it made up 30 to 50% of the soil volume [4]. Feed the life you already have with compost, and with kitchen scraps pickled in bokashi bran.

Which organic dry amendments do you need?

Most gardens need three, in this order:

1. Compost or well-rotted manure, for the organic matter the meals don't supply.
2. A nitrogen meal. Alfalfa meal is the gentle choice and keeps the soil life fed.
3. A potassium source. Sulphate of potash works within days, and rock dust is the slow option for light soil.

The phosphorus row and the calcium and magnesium row wait for a soil test. Bokashi or worm compost is a fair extra once the three are in.

If you'd rather not weigh out separate sacks, a blend such as All Purpose 4-4-4 covers the three main nutrients in one bag. The Mineral Mix, a soil conditioner, is another of my own blends. The full range is in the organic fertilisers collection.

Rock dust, rock phosphate and bokashi can go on in autumn. Hold the nitrogen meals until spring. Cold soil releases little [1], and whatever the soil life does free up over a wet winter can wash out before anything is growing to use it. Then top dress through the season as the plants need it.

Common questions

What are organic dry amendments?

Organic dry amendments are solid, slow-release inputs you mix into soil or scatter on top as a top dress. Meals feed the soil life that releases their nutrients to roots over weeks, while a few, such as sulphate of potash, dissolve and feed the plant within days.

What is the difference between dry amendments and liquid feeds?

A liquid feed is already dissolved, so a plant can use it straight away, but it washes through within days. A dry amendment feeds more slowly, and meals feed the soil life as well as the plant.

Which dry amendments do I actually need?

For most outdoor gardens, three things cover the ground: compost or well-rotted manure as the base, a nitrogen meal for growth, and a potassium source or rock dust for fruit and minerals. Add phosphorus, calcium or magnesium only when a soil test shows a shortfall.

What are the best dry amendments for nitrogen?

Plant meals such as alfalfa and malted barley are reliable slow-release nitrogen sources that also feed soil microbes. Keep to the rate on the pack, because too much nitrogen gives soft, floppy growth.

Do dry amendments work for potassium?

Yes, sulphate of potash is a concentrated dry potassium source that works within days. Rock dust such as basalt supplies potassium more slowly, alongside a spread of other minerals.

When do you apply dry amendments?

Nitrogen meals go on in spring, when the soil is warming, and slow minerals such as rock dust and rock phosphate can go on in autumn. Work them into the bed before planting, or scatter them as a top dress and let rain and worms carry them down.

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

Sources cited

1. Reid, K. & Morcom, T. (2023). Nitrogen mineralization from organic fertility sources. Fact sheet, Perennia, October 2023. ofcaf.perennia.ca
2. Kopittke, P.M. & 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 to 265. doi.org/10.2136/sssaj2006.0186
3. Jeffery, S., Abalos, D., Prodana, M., Bastos, A.C., van Groenigen, J.W., Hungate, B.A. & Verheijen, F. (2017). Biochar boosts tropical but not temperate crop yields. Environmental Research Letters 12(5): 053001. doi.org/10.1088/1748-9326/aa67bd
4. Blouin, M., Barrere, J., Meyer, N., Lartigue, S., Barot, S. & Mathieu, J. (2019). Vermicompost significantly affects plant growth. A meta-analysis. Agronomy for Sustainable Development 39(4): 34. doi.org/10.1007/s13593-019-0579-x