Fertilizers: fertilizer or soil amendment, how to choose

French law (Rural Code, article L255-1) defines fertilizing materials as products meant to feed plants or to improve the physical, chemical and biological properties of soils. A fertilizer feeds the plant: it supplies directly usable nutrients, nitrogen (N), phosphorus (P) and potassium (K), plus magnesium, sulphur compounds, calcium and trace elements. A soil amendment feeds the soil: it improves its structure, humus, water-holding capacity, pH or microbial life. In French, “fertilisant” covers both; growing media such as potting soil form a separate family.

The difference matters when you choose. On a soil low in humus, part of the fertilizer is lost, because the soil holds nutrients poorly: its CEC, or cation exchange capacity, is low. That is why you look after the soil first, then the plant.

An organic fertilizer is made from plant or animal materials. Its nutrients are not soluble straight away: soil life releases them gradually, faster when the soil is warm and moist. It feeds the plant over time and keeps soil biology active. A mineral fertilizer supplies soluble nutrients, available at once, but nothing for the soil, and whatever the plant does not take up can be leached. An organo-mineral fertilizer bridges the two: an organic base, composted in our case, and a mineral share for higher content and faster availability.

In France, synthetic fertilizers are banned for non-agricultural uses from 1 January 2027 at the latest (Climate and Resilience Act, article 269): private gardens, local authorities and private grounds. Fertilizers allowed in organic farming are not affected. Details are in our article France’s 2027 synthetic fertilizer ban.

The three figures on a fertilizer, such as 4-8-10, give its nitrogen (N), phosphorus (as P₂O₅) and potassium (as K₂O) content, as a percentage of weight. 100 kg of a 4-8-10 therefore supply 4 kg of nitrogen, 8 kg of P₂O₅ and 10 kg of K₂O; spread at 500 kg/ha, it provides 20 kg of nitrogen per hectare. Then come the secondary nutrients: magnesium (MgO), sulphur (SO₃) and calcium (CaO).

For an organic soil amendment, look at the organic matter (OM), the ISMO, or organic matter stability index, which shows the share able to become stable humus, and the C/N ratio: low, the amendment releases its nitrogen quickly; high, it breaks down slowly. Example: Germi’One® shows 55% OM, an ISMO of 59 and a C/N of 19.

Finally, the standard (NF U 42-001 for fertilizers, NF U 44-051 for organic soil amendments, NF U 44-551 for growing media) and the claim “allowed in organic farming”: a fertilizing product does not carry the organic label, it is either allowed in organic farming or not, under Regulation (EU) 2018/848.

Our amendments and organo-mineral fertilizers are built on Mazor®, the result of seven months of controlled composting with the patented Cronops® process. Plant oilseed cakes, grape and olive pulp, coffee grounds and, depending on the formula, sterilized animal materials are transformed: carbon is stabilized, the C/N ratio falls, pathogens are eliminated and the microflora develops. That is what sets a composted material apart from the same material in its raw state.

Patented Cronops® process · FR2103847

In the garden, the Marcel le Jardinier range brings together organic soil amendments and organo-mineral fertilizers for vegetable gardens, fruit trees, citrus and the whole garden, all allowed in organic farming.

For professionals, the professional range and our crop pages: vineyards, orchards, vegetables, field crops, pastures, turf and green spaces, horticulture.

For a formula under your brand or calculated from your soil: toll manufacturing and private label.