Organic fertilisation: Ensuring and supporting plant growth

Your plants need fertilisers. Organic fertilisers are the right solution to ensure that the optimal conditions for their growth are met. What are organic fertilisers? How do they work? All about these “vitamins” for plants.

Organic fertilisers ensure that the soil’s nourishing function is maintained for crops so that they are in optimal growth conditions. The main function of organic fertilisers is to provide plants with nutrients via soil micro-organisms. In addition to this function, organic amendments will also improve or maintain the physical and biological qualities of the garden soil. The soil is like a digestive tract for the plant, so it is essential to feed it with organic matter.

Knowledge of plant nutrition

Until the 1830s, the scientific community thought that plants were fed by organic matter. Then Liebig’s mineral theory showed that plants assimilate nutrients in mineral form.

A few decades later, in 1913, Carl Bosch found a way of industrially synthesising ammonia from atmospheric nitrogen, which made it possible to manufacture nitrogen fertilisers and gunpowder: this was the explosion of mineral fertilisers. It was only at the end of the 20th century that the importance of organic fertilisation was realised and it was understood that the soil was not just an inert medium serving as an anchor for plant roots.

Soil fertilisation is crucial for plant growth, for defence against pests and for resistance to abiotic stresses. Speaking of human nutrition, Hippocrates wrote 2,500 years ago: “Let your food be your medicine. This is especially true for plants, as they are unable to move around in search of food!

What happens if there is no organic fertiliser in the garden?

If no organic fertiliser is added to the garden, the soil will lose humus over the years, by about 2% per year, due to the natural process of mineralisation (see Figure 1). If there is no compensation through the addition of organic matter, the nutrient and water storage function of the clay-humus complex will decrease and part of the fertiliser added, once mineralised, will be leached by rainwater and carried to the water table. The soil will become less lumpy and more prone to compaction. Microbial life will also suffer. The clay-humus complex is a kind of larder for the plant (Figure 4).

The plants in the garden draw nutrients from the soil, which are then exported through the crops: fruit, vegetables, flowers, prunings, grass clippings. The system does not work in a closed circuit like in a forest. It is therefore necessary to return to the soil what has been taken from it: this is maintenance fertilisation. Without it, year after year, the soil will become poorer. On the other hand, if the soil is deficient in certain nutrients, it is possible to apply fertiliser to correct the deficiencies in certain elements. This is called corrective fertilisation. The best way to know your soil is to identify its needs by carrying out an analysis (see Figures 2, 2.1 and 2.2).

Organic fertilisers

For the nutrients they contain to be assimilated, organic fertilisers must be mineralised by soil micro-organisms. This step is done progressively, in accordance with the needs of the plant, since the factors that stimulate it – temperature, humidity and aeration – are the same as those needed by the plant for its growth. The risks of root burn and leaching to the water table are practically nil because the release of nutrients is not sudden as with synthetic mineral fertilisers (Figure 5). Because of the richness and diversity of the raw materials they contain, natural organic fertilisers enrich the soil with many other nutrients that they intrinsically contain.

They are formulated from different raw materials of plant and/or animal origin that have undergone only physical (grinding, heating, dehydration) or enzymatic transformations by natural fermentation, so as to provide the nutrients in a proportion adapted to the specific needs of the plant. The raw materials used include: feather meal, horn powder, bone powder, fish bone meal, guano, coffee and cocoa cakes, olive pulp, beet pulp, etc. Organic fertilisers provide nitrogen, phosphorus, potassium, calcium, magnesium and various trace elements depending on the raw materials of which they are composed.

REGULATIONS

In order to be marketed in France, organic soil improvers and fertilisers must comply, respectively, with the NF U-44051 and NF U-42001 standards or be approved as fertilising materials (list available on the official website: https://ephy.anses.fr/ in the MFSC category, fertilising materials and crop supports).

Organic amendments

Organic amendments should be mixed into the soil or spread on the surface (called “mulching”), but never buried deeply because they need oxygen to transform into stable humus. They provide nutrients in smaller proportions than fertilisers (less than 3% for each of the three elements N, P and K), but their primary functions in fertilisation are broader than those of fertilisers. In fact, they act on the physical, chemical and biological properties of the soil by providing organic matter of plant origin, which will be transformed into humus.

The addition of organic amendment ensures a lumpy structure to the soil, thus facilitating the circulation of water and air. It gives more consistency to sandy-silty soils. It breaks up clay soils, which facilitates drainage, cultivation and root penetration. The stable humus resulting from this addition will form the clay-humus complex which will prevent erosion by the entrainment of the clay in solution in water. The soil’s capacity to store water is then improved: plants will better resist periods of drought.

Air circulation is good and aerobic bacteria will benefit from favourable conditions for their multiplication. The stable humus resulting from the addition of organic amendment will slowly mineralise and progressively release nitrogen (each year, 2% of humus is mineralised, see Figure 1 and Figure 3). Humus combines with iron to form natural chelates. Thus protected, the iron will no longer be blocked by the limestone. The risk of chlorosis is avoided. The clay-humus complex gives the soil a buffering capacity that attenuates pH variations and allows the storage and exchange of soluble nutrients in the soil solution.

The organic amendment stimulates biological processes in the soil. In particular, it has a beneficial effect on the multiplication and diversification of micro-organisms, thus enabling the control of soil pathogens by antagonism and the production of antibiotics (notion of “barrier flora”), and the production of organic molecules that can be assimilated by plants.

Denis Jardel
Technical Director of SBM life Science

WHAT ARE THE CRITERIA FOR CHOOSING A GOOD ORGANIC AMENDMENT?
The organic matter content is expressed as a percentage of the raw product. The higher the rate, the richer the amendment: there is no point in buying water! It is necessary to choose organic amendments whose raw materials have a high humic potential, rich in cellulose and lignin (the two main precursors of humus) such as coffee or cocoa cakes, olive pulp, cereal straws, herbivore manure (horse or sheep manure, which is one of the richest in dry matter and nutrients). The transformation into stable humus is much faster than with amendments based on tannic plant products: bark, sawdust, plant shreds, grape marc, peat, etc.

Source : jardindefrance.org