The Soil Fertility Cycle

Technical articles - Soil and fertilization

Organic matter autonomy in vegetable farming Climate resilience Soil cover Biodiversity Soil regeneration

The plant owes its existence to the soil and the plant cover contributes to soil formation and its evolution.

Under the action of climate, microorganisms, and plants, the parent rock breaks down and releases clays and mineral salts. At the same time, the organic matter, whether plant or animal, is returned to the soil in the form of humus and combines with the products of the parent rock to form what is called the clay-humus complex, which forms the fertile layers of the soil.

The organic matter cycle

Organic matter is matter produced by living beings. It composes the living and dead biomass.

When organic matter is integrated into the soil, in the litter, it is subjected to the action of soil organisms. These intervene in two ways : part of the OM will be processed and humified, thus added to the existing humus stock, while another part will be used by the soil fauna for feeding, thus mineralized and constituting a source of food for plants. Primary mineralization occurs directly on the deposited organic matter, secondary mineralization when microorganisms attack the humus.

The mineralization process is when nutrients become available to plants. They then grow and produce OM in turn : thus, a self-sustaining system exists, which is largely surplus as nutrients are stored in the soil.

Moreover, the system is also beneficial for soil fauna and flora that participate in the biotransformation of organic matter : the humus accumulating in the litter provides them shelter and food. The organic matter cycle is therefore a true ecological symbiosis based on a virtuous and building circle.

The multiple role of soil biological life

It is the soil biological life that performs the work of humification and mineralization. The biological activity of a soil is inversely proportional to the humus mineralization rate. The more intense the soil life, the slower the mineralization. The more intense the biological activity of a soil, the more it mobilizes organic matter and stores it in a stable form. Hence the antagonism : natural humification prevents too rapid mineralization. Without soil life, there is no humification and thus no natural mineralization. Instead, organic matter fossilizes and no longer decomposes. Excessive humus mineralization leads to rapid biological degradation of soils and thus loss of fertility. Humification and mineralization are therefore two complementary biological phenomena.

But soil biological life does much more!

In a handful of fertile soil, there are up to 5 to 6 billion living beings, constituting a rich flora and fauna at all scales. Soil biological life strengthens the overall biodiversity of the farm and provides many other agronomic services.

Nutrition

Soil biological life links mineral and plant. It first enables rational and sustainable exploitation of organic matter, notably because it slowly returns it to plants. Moreover, plants benefit from the services of nitrogen-fixing bacteria present in the soil or in the plant (for example in legumes) that provide this element. Similarly, mycorrhizal fungi, thanks to their network of fine filaments, act as an extension of plant roots and thus facilitate access to essential nutrients and minerals such as phosphorus.

Protection

Soil biological life also improves the health of crops. Mycorrhizal fungi stimulate plants' natural defenses while microorganisms consuming available food in the rhizosphere limit pathogen development.

Soil health

Biological activity builds soil structure and shapes the soil by participating in the formation of humic layers, through the work of earthworms of course but also bacteria, which bind soil particles thanks to their biofilm, a kind of protective glue. Some fungi do the same with glomalin. Plant roots can then develop freely, allowing the existence of a protective plant cover, facilitating air circulation and carbon storage. Finally, with the soil loosened, water management is almost flawless.

What are mycorrhizal fungi? They are fungi living through symbiosis with a plant. The association occurs at the roots. The fungi benefit from the shelter and food the plant offers, in exchange for which the plant multiplies its exchange surface with its immediate environment. The fungi thus prospect for water and nutrients present in the soil. They also protect roots from pathogens by their mycelial sheath enveloping the roots. There are two groups of mycorrhizal fungi: ectomycorrhizae, which develop around the rootlets of herbaceous and woody plants, and endomycorrhizae, which develop partly outside the roots but also inside the rootlets without attacking the cells.

Organic matter in its soil

Fresh organic matter or dry matter?

Since fresh organic matter can absorb and lose water depending on climatic conditions, agronomists have taken to measuring organic matter dry (without water) to better quantify it.

OM rate and soil ratio

The organic matter rate represents the proportion of OM in the soil. Generally, the average rate over a 0-30cm soil horizon is considered. (Care must be taken to mix the different first soil horizons well when sampling soil for laboratory analysis).

Carbon and OM rates in French soils
Data on 0-30cm	Carbon quantity (t/ha)	OM quantity (t DM /ha) (OM = 1.72 x C)	OM rate (% OM = OM / 3500t soil /ha)
French average	43	73	2.1%
Forest and grassland	80 ± 35	137	3.9%
Vineyard and orchards	35	60	1.5%
Arable crops	43	73	2.1%
Average 10 farms MSV Normandy	100	175	5.0%

Studies of grassland or forest show these systems return 20 t/ha of dry organic matter per year to the soil. This figure excludes hay exports from grassland and wood production stored in trunks.

To reach this figure of 20t/ha of dry organic matter per year in our agricultural system, it is useful to know what thickness of organic matter must be added. For example, this corresponds to 1cm of RAM.

Inputs are indeed necessary for vegetable fertility which is not autonomous. Indeed, if one tried to supply dry matter from vegetables left in place, as roots or tops, only 2 t/ha of dry matter would be obtained.

By supplying organic matter such as RAM or straw, the soil's self-fertility cycle is maintained or accelerated, especially in vegetable production which leaves too few residues in the soil (1 to 5 t/ha dry matter while grassland consumes 20t/ha annually).

Cultivating grasslands involves the destocking of one ton of carbon per hectare each year, whereas it will take much longer to reverse this phenomenon : No-till techniques generate storage of 0.2 tons per hectare per year.

Carbon cycle to place before the OM cycle

Carbon is an atom. It is a basic constituent of organic matter. In agriculture, organic matter and carbon are considered proportional : dry OM = 1.72 x C

Biomolecules (sugars including cellulose, lipids, amino acids and proteins, lignins, tannins, essential oils, charcoals...) are carbon chains that provide every living organism with the chemical energy needed to function: it is the fuel of life.

Present in the air as CO2 (inorganic carbon), atmospheric carbon is captured by plants to form carbon chains as carbohydrates through photosynthesis. They are indeed the only organisms capable of converting light energy into chemical energy by creating simple carbon chains (sugars). This is called autotrophy.

However, photosynthesis is not the only way plants obtain carbon. Plants can also absorb sugars and amino acids directly from litter decomposition. Plants are thus also capable of heterotrophy, i.e., feeding on pre-existing organic constituents, like animals and all non-photosynthetic organisms.

Nitrogen cycle

Nitrogen is a key element of proteins and amino acids that make cells function. It represents 78.08% of air but plants cannot assimilate it directly from the air. Nitrogen is absorbed by roots from nitrogen dissolved in precipitation water, especially storm rain and snow, at about 10%. Indeed, 90% of nitrogen used by plants comes from soil organic matter.

Of this 90% : 1/3 comes from the recycling activity of organic matter by soil fauna, which recovers nitrogen in waste and dead organisms for feeding, and redistributes it to the soil via excrement or death. This is mineralization.

The remaining 2/3 enter the ecosystem thanks to certain bacteria, free-living but mostly symbiotic, capable of using atmospheric nitrogen and converting it into a form assimilable by plants : this is biological nitrogen fixation.

It is also possible to supply nitrogen to plants using synthetic nitrates but these often negatively impact soil life, which they destroy. Moreover, nitrogen doping has many adverse effects such as plant obesity, loss of immune capacity, increased water needs, and loss of autonomy, the plant's inability to naturally find nitrogen supply.

In agriculture, natural nitrogen sources are varied : animal manure, compost, urea, guano, green manure nitrogen fixers, … In these cases, soil microorganisms again transform available nitrogen into assimilable nitrate.

Sources

Structure:MSV Normandie

Cette technique s'applique aux cultures suivantes

Vegetable farming

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