Des arbres et des sols :key-fertility-elements

Agroforestry Water management Fertility management in arable crops Soil regeneration NBSOIL

Soil biological activity – mostly invisible, and its economy – by definition underground, remains today one of the most mysterious and promising fields of research. The "soil capital" is at the very foundation of life on the earth’s surface; this makes it one of the most precious heritages that should be a constant concern and the focus of all our attention when planning territories and cultivating the land. Especially since if erosion, its main enemy, is a natural phenomenon, it is human activities that are the primary cause.

Often guilty and always the first victim of this situation (yield decline, abandonment of sterile lands, bankrupt farms...), agriculture is nevertheless the surest solution to reclaim fertility and "produce [[Soil and fertilization|living soils]", more productive and sustainable...

Erosion : problem number 1

The threats weighing on soil are numerous : erosion, salinization, acidification, compaction, loss of biodiversity, etc. Whether caused by water or wind, erosion is the number one problem in France. Strictly speaking, erosion due to tool passage is low in France (pers. comm. Claire Chenu). However, it greatly promotes water erosion. Various measures are implemented to curb it but remain insufficient, and it is urgent to tackle the problem head-on. Water erosion takes many forms: networks of rills, gullies, ravines, combinations of linear and sheet erosion. Landslides, for example, are triggered by a set of factors, two of which are decisive :

the cohesion force,
the composition of the terrain.

High organic matter content makes soils more resistant to erosion.

Restoring soil fertility : a virtuous circle

Soils are a limited natural resource, renewing very slowly. Improving it (the opposite of degrading it) will involve plants. By covering soil with plants as much as possible and returning a lot of dead organic matter, it is possible that a virtuous circle sets in and that quickly (in a few years), the soil naturally becomes more fertile.

A naturally more fertile soil :

will produce more biomass,
will shelter more biodiversity,
will store more carbon and water,
will favor the development of mycorrhizal networks,
will have a greater thickness...

Wealth begets wealth : this is the aggradation spiral.

Soil-plant couple at the origin of fertility

Release of mineral elements assimilated by plants

During the process of degradation and transformation of organic matter into mineral matter by digestion, each has a very specific task :

the macro-fauna (earthworms and other "large" surface animals) fragment coarse pieces into smaller particles,
the fungi secrete enzymes that break down the toughest organic materials (such as lignin) throughout the process,
the bacteria (highly specialized for some) attack the bonds of the various molecules constituting the organic matter.

All these "attacks" allow the release of mineral elements assimilated by plants.

A plant needs, in addition to 'nitrogen, about twenty mineral elements. On a fungus-poor soil, a plant will grow but less well. It is little said but chemical fertilizers applied in high doses strongly destabilize soil life by changing its chemical properties (mainly pH). Phosphates and nitrates inhibit symbioses and pesticides (fungicides, herbicides, insecticides) kill life.

Biological nitrogen fixation

Biological fixation of nitrogen allows transforming inert atmospheric nitrogen, unusable by plants yet inexhaustible, into assimilable nitrogen. Most of the nitrogen accumulated in soils comes from fixation by bacteria :

for 80% of them in symbiosis with plants (such as in legumes),
for 20% free bacteria, whose contribution to fertility could be essential, provided they are on living soils (Orr et al., 2011).

Natural fertility

Natural fertility is the ability of a soil to produce a lot of plant matter by rapidly recycling its elements. It is essential to conduct a thorough reflection on long-term fertility. Fertility also and above all plays out in fields, at all scales of time and space and first at the square meter scale, thanks to inputs :

of permanent cover crops,
of trees.

The two can ideally be combined because they are complementary, to maximize fertility. Fertility ultimately occurs in a couple relationship (fertility can indeed only be defined relative to the plant) and has proven itself for several million years : this is the "soil-plant couple".

The natural fertility of a forest results from maintaining the natural environment under conditions that allow matter to degrade at an optimal speed, so that the nutrients contained in plant debris are made available and taken up by root systems. Forest soil allows recycling all the nutrients necessary for its growth. Thus, cultivating in 3 dimensions thanks to trees can be the solution for virtuous agriculture...

Wood and its lignin, origin of humus

Wood is a sustainable way to store carbon dioxide from the air and convert it into "fertile matter". Dead branches and roots, like other organic amendments, help to :

increase biological activity,
improve structure,
maintain or even increase soil humus.

Lignin, found mainly in trees and shrubs (woody plants), is the characteristic molecule of wood. It is very energy-rich organic matter but difficult to digest by microorganisms, except for some fungi (such as white rot fungi) that degrade it in the presence of oxygen. Once dead, plants containing lignin decompose very slowly : a twig or a leaf of oak can take several years. This slow decomposition of woody materials stabilizes the carbon they bring to the soil in an organic form. The organic carbon thus stored in the soil mineralizes over the years, so humus can be considered a reserve of nutrients for the plant.

Ramial Wood Mulch (RWM), an organic input, facilitates soil restoration by feeding living organisms. But it is not a short-term fertility source, unlike soil amendments such as composts and manures.

Soil cover and agroforestry

Crop associations

Associations of cultivated species occupying different soil layers allow plants to be complementary in accessing soil resources without competing. This is the case between crops and agroforestry trees. Likewise, among cover plants in the upper soil horizon. Moreover, it is proven that some crop associations produce more, through the facilitation phenomenon.

Maximizing plant cover

Herbaceous cover crops are used by farmers to maximize :

the level of soil protection,
its level of solar energy capture.

The use of cover crops includes practices of plant associations, sowing of green manure in intercrops, nitrate trap crops, catch crops… The general idea is to never leave the soil bare, because a bare soil produces nothing and degrades.

One key is to intensify, throughout the year, the plant cover of soils by playing not only on horizontal complementarity (species association within the same layer) but also vertical (superposition of herbaceous, shrub, and tree layers). Trees represent a perennial "crop", stable in crop rotations and effectively participating in solar energy capture. This 3D agriculture allows benefiting from all the effects of plant covers :

biodegradable bioclimatizers,
nutrient pumps at the base of fertility recycling,
greatly increased water infiltration, etc.

Sowing under cover

No-till sowing under plant cover are soil conservation techniques consisting of sowing directly into a living crop (flattened by the roller in front of the tractor) or into mulch from an intermediate crop. They offer significant agronomic and environmental benefits.

In permaculture, the soil is always covered and climate-controlled. In common language, permaculture refers to "natural" and sustainable agricultural systems, most often in market gardening or horticulture. It is a concept inspired by nature’s functioning, which never leaves soil bare.

A concrete case of crop succession over 365 days

The rotation of a summer cover crop (sunflower + sorghum), a mixed crop (field bean, vetch, pea, oat) and a spring crop (corn) can produce up to 56 tons of dry matter/ha/year. Both aerial and root biomass are accounted for.
Corn alone can produce about 35 tons of DM/ha/year (14 tons of grain, 11 tons of stalks, and 10 tons of roots and root deposits).

At the same time and on the same area, this farmer now produces almost 2.5 times more than 14 years ago... (figures from the French Agroforestry Association - AFAF, 2014).

A large part of the biomass can thus return to the soil, thereby feeding the living organisms that inhabit it and maintaining fertility. The time scales to consider for rebuilding carbon stocks in soils are significant, but the Gers farmer practicing the described rotation saw his organic matter content rise from 1.5 to 2.6% in barely 10 years.

Agroforestry : 1+1=3

Agroforestry, which best valorizes natural resources in time and space, allows building resilient and autonomous agricultural systems. Annual crops from population varieties growing under trees require fewer inputs, have greater mycorrhization capacities, and are more shade tolerant, allowing to make the most of local potential.

The goal is to make the use of natural production factors (nitrogen and other minerals, water, light, ground surface...) as efficient as possible. For example, a 100 ha agroforestry farm produces as much biomass as a 140 ha farm where trees and crops are separated.

Conservation agriculture, forestry, and agroforestry share :

a living soil,
good use of natural environmental resources,
good organic matter levels.

More specifically, agroforestry is interesting for :

increased carbon storage potential (lignin and maximum production),
strong reduction of losses by erosion and leaching,
soil stabilization,
increased sharing of nutrients and water at multiple time scales,
multiple productions (food, energy...).

Thinking yield at the square meter scale

The notion of "agricultural yield" as commonly used today reflects a mistaken conception of agriculture’s objectives and the production factors on which it depends. All productions must be taken into account :

foodstuffs,
fibers,
energy.

At what production cost?

In arable crops, for example, switching to no-till under cover can save up to 40% of costs related to agricultural mechanization.

By relocating fertilization at the scale of a small agricultural region, a farm, or even a plot, sustainable soil fertility can be guaranteed. Organic matter is the key. The essential question is thus : how many tons of organic matter per hectare must be returned to the soil for it to regain and then maintain its natural fertility and production performance?

While the answer is complex, the general principle is simple : it is necessary to produce a maximum of biomass per square meter over a year.

Soil and plant for water available in quantity and quality

Hedges, trees, and plant covers help better manage rainwater flow, from its arrival to its exit into watercourses and aquifers. At the plot scale as well as at the scale of a watershed, the objective is threefold : to fix, slow down and filter water : the rule of the "3 Fs". A production model change through plant ingenuity can address challenges related to the water resource, to cope with both drought and excess water.

By its functions as a nitrate trap and infiltration agent, the tree combined with permanent plant cover on agricultural soils helps achieve good chemical and ecological status of water bodies. The chronic erosion phenomena affecting our tilled agricultural lands dangerously degrade their production potential as well as surface and groundwater quality. The color of our rivers’ water testifies to the impact of our agricultural practices on water quality.

Organic matter plays a major role in soils’ capacity to retain water. By promoting greater micro- and macroporosity, the plant tissue that originates it regulates and increases water infiltration, thus reducing runoff caused by impermeability. In addition to preventing erosion phenomena, this water volume will be stored longer in the field and available for cultivated plants.

High wind speeds also increase plant transpiration and soil evaporation, reducing water accessible to crops. Hedges as windbreaks, like herbaceous plant cover, limit these phenomena and will be particularly useful in water-scarce areas, in addition to all other services provided.

Sources

SCHEERCOUSSE P. 2015. Trees and soils, key elements of fertility. Arbre&Paysage 32 [22/09/2022]. https://ap32.fr/wp-content/uploads/2019/12/livretAP32_arbres_sols.pdf