All techniques › Soil biology & amendments
Compost
Adding stabilised organic matter to the soil. Good long-term European evidence for structure, infiltration and yield stability; the water-storage figure everyone quotes is wrong by an order of magnitude; and compost tea is a different, weakly evidenced product.
The short answer
Compost builds soil organic carbon, and that is what buffers a bad year. In the French QualiAgro trial, biowaste compost every two years added 23.9 t of carbon per hectare over 21 years; a global analysis finds cereal yields rise with soil carbon until about 2%. On five rainfed almond farms in Spain, compost was the only floor system that beat tillage on the accounts.
It does not store the water people say it does. The best synthesis puts the gain at about 1.16 mm of available water per 100 mm of soil for each 1% of soil carbon, roughly 3.5 mm over a rooting depth. Compost helps rain get into the soil through structure and infiltration; it barely changes how much the soil can hold.
Skip the tea. Across 516 case studies compost tea is unpredictable, adds no organic matter, and has no drought mechanism. Turn the heap instead.
The trade-off first
Compost's cost is almost entirely haulage. The material itself can be near-free at the plant gate (€0 to 2 a tonne was typical across the EU in an older survey, up to €14 with a spreading service) or €50 a tonne plus VAT ex works from a Spanish supplier in 2025, and transport is 25 to 70% of the delivered cost (UK supplier, quoting the advisory body ADAS, undated). At 20 t/ha that near-free plant-gate material works out at roughly €50 to €400 a hectare delivered, depending on how far away the plant is — our arithmetic. Quality compost at the Spanish supplier's €50 a tonne is a different proposition: about €1,000 a hectare at the same rate, before VAT and transport, also our arithmetic. That spread is the honest answer.
The version that pays soonest is the one made from what is already on the farm: prunings, straw, manure, crop residues. Free input predicts adoption on every page of this site, and compost is no exception. Europe's entire compost and digestate output, about 15.8 million tonnes a year across 18 countries, would cover 0.7% of arable land at 30 t/ha (European Compost Network, an industry body), so bought-in compost cannot be the whole answer anyway.
What it is
Three products travel under one name, and only the first is graded "good" on this page.
- Compost: a bulk organic amendment at tonnes per hectare. Green-waste compost, biowaste (food and garden) compost, farmyard-manure compost, or on-farm compost of residues.
- Compost tea and extracts: water steeped with compost, sometimes aerated and fed with molasses, sprayed at litres per hectare. Dealt with at the end of this page and on its own claims page.
- Digestate, sewage-sludge products and mixed-waste compost: different rules and different contaminants. Excluded from EU-fertiliser-grade compost.
How it helps against drought
Infiltration and structure. Repeated compost applications improved the aggregate stability of a cultivated silty loam in the QualiAgro trial in France (Agriculture, Ecosystems & Environment). Rain that infiltrates instead of running off is the first drought benefit, and it is the same mechanism as mulching working from inside the soil. The compost industry's compilation of long-term trials reports a 24-year trial with 5 to 15% lower bulk density and roughly 350% higher infiltration rate at 15 and 45 t/ha a year against mineral fertiliser, and a 12-year trial with 5 to 7% higher water capacity in medium to heavy soils (European Compost Network fact sheet, industry-compiled from conference presentations; treat as reported, not verified).
Water-holding capacity, modestly. The claim that each 1% of soil organic matter holds 20,000 gallons an acre, or its metric cousins, is usually attributed to a 1994 paper that never derived it. Minasny and McBratney (2018), from 60 studies and over 50,000 measurements, found a 1% increase in soil organic carbon raises available water capacity by about 1.16 mm per 100 mm of soil, most in sands and least in clays, and concluded that "arguments for sequestering carbon to increase water storage are questionable" (abstract). Over a 30 cm rooting depth that is about 3.5 mm, or 35 m³ a hectare, after years of applications. Real, and not a reservoir. The full story is on the claims page.
Yield stability through soil carbon. A meta-analysis of 840 observations found maize and wheat yields rise with soil organic carbon and level off at about 2%; reaching regional targets could lift yields 10 ± 11% for maize and 23 ± 37% for wheat as a best case, with 5 to 7% less nitrogen fertiliser (Oldfield et al. 2019). Soil carbon buffers the bad years, and compost is the fastest legal way to add it.
What the evidence shows
A 21-year European trial
QualiAgro, Feucherolles, France, run by INRAE with Veolia since 1998 on a Paris-basin Luvisol in a wheat and maize rotation. Amendments go on every two years on wheat stubble at a target of 4 t of carbon per hectare until 2013, then 2 t (AnaEE France). After 21 years the extra soil organic carbon over the unamended control was 23.9 t/ha under biowaste compost, 15.5 under farmyard manure and 14.7 under municipal-waste compost (SOIL, 2024), about 0.2 to 0.5 t of carbon a hectare a year. The added carbon was less stable than the soil's native carbon, so it persists for decades but declines if applications stop.
Farm accounts
On five rainfed organic almond farms in Granada and Almería at 300 to 400 mm a year (trials 2016 to 2018), compost at about 6 m³/ha gave the highest net revenue of four floor systems, €1,349/ha a year, against €1,126 for conventional tillage, while no-till with an unmown wild understory gave the lowest at €415 (De Leijster et al. 2020). The authors call compost "the most promising to be scaled up". It is the only farm-account evidence on this site, and it favours compost. Full case on the case studies page.
Meta-analyses
Organic amendments raised crop yield 27% over mineral fertiliser in a global meta-analysis (Soil Biology & Biochemistry, 2018). A 2026 synthesis reports 45% higher yield with compost alone and 63% with compost plus mineral fertiliser (Processes, 2026; full text not accessible to us, so reported rather than verified); another puts the pooled effect nearer 15% (Hayat et al. 2026). The spread is the finding: degraded, low-carbon soils respond most, and mineral nitrogen alongside compost matters.
Most compost meta-analysis data is global and horticultural; the European long-term evidence is French, Spanish and German. The mechanism transfers because adding organic matter is not climate-specific; the rate of benefit is slower where dryness limits biological activity.
Nitrogen: slow, and sometimes negative in year one
Compost is not a fertiliser in the year you apply it. For biowaste and green-waste composts, 5 to 15% of the total nitrogen becomes available in the first year and 2 to 8% a year after that (Agronomie, 2003). Yard-trimming composts can immobilise nitrogen, releasing only about 5% of what was applied over 84 days, while poultry-manure composts release 15 to 30% (Lazicki et al. 2020). A high-carbon compost spread before a cereal can lock up the nitrogen that crop needed. Apply after harvest, and look for a carbon-to-nitrogen ratio near 25:1 to 30:1.
Where it fits
| Context | Fit | Why |
|---|---|---|
| Orchards, vineyards, olive and almond groves | Best | Pruning residue on site; no tillage window to fit around; the only farm-account win in the file |
| Dry Mediterranean land below the cover-crop line | Good | Adds organic matter without a living cover competing for water |
| Arable, applied after harvest | Good | Structure and infiltration; carbon plateau benefit at ~2% SOC |
| Arable, applied before a cereal | Caution | First-year nitrogen lock-up with high-carbon composts |
| Farms far from a compost plant, no own feedstock | Weak on cost | Haulage is 25–70% of delivered cost |
| Anyone expecting stored water | Wrong reason | ~1.16 mm per 100 mm per 1% carbon; the benefit is infiltration and carbon, not volume |
What it costs
Every figure with its country and year; the list is thin and partly vendor-sourced.
| Item | Figure | Country, year | Source |
|---|---|---|---|
| Quality compost, bulk, ex works | €50/t + 10% VAT, transport extra | Spain, 2025 | supplier listing, vendor |
| Agricultural compost price range | €0–2/t, up to €14/t with spreading service | EU, ~2008 | EU compost-use report, old |
| Spreading | £2.50–3.00/t | UK, undated | compost supplier quoting ADAS, vendor |
| Haulage share of delivered cost | 25–70% | UK, undated | same |
| On-farm composting of farm waste | €65.19/t with mobile shredding | SE Spain, 2026 | Agriculture (MDPI), abstract only |
| Labour, almond compost floor system | €396/ha/yr (tillage €386.50) | Spain, 2016–18 | De Leijster et al. 2020 |
Rates. Trials and markets cluster at 10 to 30 t/ha fresh per application, every one to three years. QualiAgro's 4 t of carbon every two years is roughly 15 to 20 t fresh; the almond farms used about 6 m³ every year.
Timeline. Nitrogen from year two. Structure and infiltration after two or three applications. Soil carbon measurable at five years and substantial at ten to twenty. Yield stability follows carbon. Compost is a decade-scale investment that pays some rent along the way, which is the opposite of earthworks.
Commitment
Reversible. No landform change, no permit, no capital beyond a spreader or a contractor. Stop applying and nothing collapses; the added carbon declines over years and the structural gain fades. The obligation runs the other way from a pond: you can stop whenever you like, but you have to keep going to keep the benefit.
How to start
- Sample the soil first: organic carbon, texture, phosphorus, potassium, heavy metals. You want to see a change in three years, and you want to avoid loading phosphorus.
- Compost what you already have. Prunings, straw, manure, residues. Transport is the cost that kills bought-in compost; your own has none.
- If you buy, buy graded: EU-fertiliser-grade compost, or the national quality mark (PAS 100 in the UK, RAL in Germany), and read the analysis rather than the brochure.
- Apply after harvest, not before a cereal, to avoid year-one nitrogen lock-up. On perennials, spread in autumn under the canopy or in the inter-row and let the mulch sit on it.
- Start on one field or one block of trees at 10 to 20 t/ha, keep a control strip, and measure the same things in year three.
- Skip the tea. Spend the brewing time turning the heap.
Risks and failure modes
Nitrogen lock-up in year one with high-carbon green-waste compost. Yield can dip before it rises.
Contaminants in bought compost. EU-fertiliser-grade compost under Regulation (EU) 2019/1009, in application since 16 July 2022, may be made only from separately collected biowaste, green waste and category 2 and 3 animal by-products; sewage sludge and mixed municipal waste are excluded. Cadmium is capped at 1.5 mg/kg dry matter for organic fertilisers; glass, metal and plastic over 2 mm at 5 g/kg dry matter in total, with plastic tightening to 2.5 g/kg from July 2026. The regulation sets no limit for microplastics under 2 mm and no PFAS limit (European Compost Network on the regulation; Fertilizers, 2025). QualiAgro found that the equivalent of over 100 years of urban and animal waste applications had "minimal effect on plant uptake of potentially toxic elements" (Agriculture, Ecosystems & Environment, 2016), which is reassuring on metals and silent on PFAS and plastics.
PFAS. The documented route onto farmland is sewage-sludge biosolids, mainly in the United States so far; biosolid-derived compost releases PFAS (PMC, 2025). EU-grade compost excludes sludge; national "soil improver" grades may not. Ask what went in.
Phosphorus loading at repeated high rates, and weed seeds and pathogens in immature compost.
Overselling the water benefit. A farmer who buys compost to store 200,000 litres a hectare will not get it.
Compost tea
Graded separately, and weak. The most comprehensive synthesis, 39 studies and 516 case studies from 2010 to 2025, finds compost tea has "biological potential" but "substantial variability that limits its reliable deployment"; the field "cannot consistently forecast when a given compost tea will achieve commercially relevant control versus marginal suppression"; aeration is "secondary to feedstock quality"; bacterial and viral diseases are unstudied (Applied Soil Ecology, 2026). Adding molasses to aerated tea can regrow E. coli O157:H7 and Salmonella, a food-safety risk on salad and fruit (WSU literature compilation). The online literature is dominated by brewers, consultancies and inoculant vendors. Compost tea adds no organic matter, does not change water-holding capacity, and has no drought mechanism. It has a claims page, not a place in the library.
Claims checked. 1% organic matter holds 20,000 gallons an acre and compost tea suppresses disease.
The expensive cousin. Biochar adds permanent carbon and, on sandy soils, a large water-holding gain, at €400–600 a tonne (Germany, 2024–25) and 10–30 t/ha; the same texture rule applies, and compost does the organic-matter job for a fraction of the price.
Three ways to hold more water, one with a deadline. Compost, biochar and hydrogels all claim to raise water-holding capacity. Compost is the only one of the three with European farm-account evidence behind it, and the only one with neither a cost problem nor a compliance date.