Soils are an essential resource for agriculture and humanity (FAO, 2015). The soil nourishes the plants with water and nutrients. It is a non-renewable resource on a human scale and is under pressure from urban and industrial activities that consume more and more agricultural land.
The soil also fulfills other functions than the healthy function:
- Degradation of organic matter and nutrient recycling
- Quantitative regulation of the water cycle
- Purification and filtration ensuring the quality of water
- Storage of carbon and nitrogen in humus constituting soil organic matter
Also, the soil is home to an intense biological activity marked by high biodiversity of organisms that live there and interact with the growth of plants. Soil fertility is the result of human action through agricultural practices interacting with the climate. It refers to the capacity of the soil to sustainably produce quality crops for future generations.
At. Organic matter, the biodiversity of a living soil
We speak of the three states of soil fertility: the physical state (the structure, the aggregates, the aeration, the drainage Definition: Evacuation of the excess water in the soil by gravity (natural drainage) or using drains or ditches…
), chemical (acidity measured by pH Definition: Notation that accounts for the concentration of H + ions in the medium and thus denotes the highly acidic (pH 4 to 5.5), acidic (5, 5-6.8), neutral (6.8-7.2) or alkaline (greater than 7.2) of soil…
, nutrients) and biological (organic matter, wildlife, microorganisms), but there are strong interactions between these states to arrive at a diagnosis.
Plants leave roots, stems and leaves on the ground after harvest, which is a source of organic matter (OM) for living soil organisms that feed on them. The OM is partly decomposed and then mineralized while the other part is transformed into stable humus, an essential constituent of soil fertility. MO is food for soil life. It promotes a more substantial and aerated structure and increases the useful water reserve. Finally, the MO expands the cation exchange capacity Definition: Conventional chemical measurement carried out on a sample of soil, intended to approach the capacity of a soil to store exchangeable mineral elements including cations reversibly. ..
and improves the storage of nutrients such as calcium, potassium and magnesium.
Do mineral fertilizers cause a decrease in soil MO stock? This is a widespread misconception that is confused with the role of the crop system. A system that does not return enough organic matter from photosynthesis to the soil Definition: Plant photosynthesis involves reducing carbon dioxide from the atmosphere by the water absorbed by the roots using solar energy captured by the leaves, in the presence of mineral salts, with the release of oxygen, to produce carbohydrates…
leads to a decrease in the stock of organic matter. Fertilizers help to increase plant production, but the soil must be left with enough residue to feed the soil. Thus, in cultivation successions with grain maize, rapeseed or cereals with straw left on the ground, the amount of MO can increase as in the long-term trial in Boigneville, Arvalis (91) even with exclusively mineral fertilization.
The favorable effect of essential organic and inorganic amendments on soil fertility and life can be measured by indicators such as the amount of live microbial biomass or the activity of microorganisms (respiration, mineralization, mineralization Definition: Transformation of organic matter which leads to the formation of mineral salts where the nutrients become soluble and accessible to plants ….
, nitrification …) measured after a contribution. Regular use of some forms of fertilizer may influence the structure of microorganism populations, but interactions are complex with crop plants and tillage and require further investigation.
Four sets of practices have a decisive influence on the biological component of soil fertility: tillage that creates or disrupts habitat used by wildlife and microorganisms, including fertilization organic amendments that provide food for living organisms soil, plant protection products that have an impact on soil life and plant covers that avoid leaving the land bare and also contribute to feeding the ecosystem.
This diagnosis is broader than the soil analysis, which allows us to assess the nutrient potential of the soil but only takes into account the role of the soil layer worked by the farmer in the behavior of the crop.
Earth analysis can serve different purposes:
- define the type of soil (physical and chemical analysis)
- to estimate the potentialities of a recently renewed plot
- evaluate the evolution of the acid-base status acid-base status Definition: Expression of the acidity of soil by considering, in particular, the value of its pHwater and its saturation rate in exchangeable cations…
( pH Definition: Notation which accounts for the concentration of H + ions in the medium and thus denotes the highly acidic (pH 4 to 5.5), acid (5.5 to 6.8), neutral (6.8 to 7.2) or alkaline character (greater than 7.2) of a soil …
- adjust the multi-year fertilization plan (chemical analysis)
Other observations made on the plot should be taken into consideration:
- the stoniness which diminishes the mass of beautiful earth,
- the depth which also reduces the mass of fine soil and which limits the rooting,
- Soil structure that influences density Definition: The density or relative density of a body is the ratio of its density to the mass of a collection taken as a reference. The reference body is pure water at 4° C for liquids and solids. Density is size without dimension and its value is expressed without a unit of measurement. Very often, the term density is misused instead of mass
Other methods such as respirometry, enzymatic measurements can also give information on the development of microorganisms and the evolution of soil organic matter. These are not conventional methods.
Physical quality indicators
From a soil sampling, the environmental analysis studies the soil type and the carbon stocks through:
- Granulometry, which defines the distribution of mineral particles of beautiful earth, between clay, sands, silts -fin and coarse-
- The organic matter, calculated from the organic carbon content measured in the previously dried soil and sieved to 2 mm.
- Other measures such as soil moisture characteristics (water retention capacity), soil structural stability (the type of crops / organic inputs). These are not conventional methods.
Soil sampling consists of mixing 12 to 14 individual samples taken at a depth of 25 to 30 cm in arable land. Plants are considered to find most of their diet in this volume of soil;
For a sample to be representative, it is necessary to:
- take in a homogeneous area
- always collect in the same season
- adopt a method and keep it for the following analyzes
- wait at least 2 to 4 months after adding manure or fertilizer.
Chemical quality indicators
The chemical analysis makes it possible to study the physicochemical conditions of the parcel as well as the quantitative evaluation of the mineral nutritive elements present and available for the cultures.
It conventionally concerns the following measures:
- PHP Definition: A notation that accounts for the concentration of H + ions in the medium and thus denotes the highly acidic (pH 4 to 5.5), acid (5.5 to 6.8), neutral (6.8 to, 2) or alkaline (greater than 7.2) of a soil…
- cation exchange capacity (CEC)
- total limestone
- exchangeable cations: Ca 2+ , Mg 2+ , K + , Na +
- extractable phosphorus
- sulfur, SO 3 (sulfate and organic sulfur available)
- trace elements, Fe, Mn, Cu, Zn, B, Mo
Biological quality indicators
The biological analysis makes it possible to evaluate the evolution of the organic status of the soil. A first indicator is the C / N ratio (total carbon over total soil nitrogen).
A C / N <10 ratio shows an intense microbial life which does not go in the direction of the formation of stable organic compounds.
Also, there is the determination of the mineralization mineralization
coefficient Definition: Transformation of the organic matter which leads to the formation of mineral salts where the nutrients become soluble and accessible to the plants ….K 2 (analysis not available in routine ), it makes it possible to develop a diagnosis of the conditions of the evolution of the organic matter.
Finally, three compartments can be measured in soil organic matter:
- Living Organic Matter (MOV) or microbial biomass that groups soil micro-organisms (bacteria, fungi, etc.). The amount of carbon generated by chloroform vapors (fumigation-extraction technique) applied to live organisms is calculated. The organic carbon emitted is then compared to that of a control sample soil, not fumigated. The extractable carbon supplement is directly proportional to the microbial biomass present.
- Labile Organic Matter (MOL), which is the non-living organic matter, extracted with hot water under pressure
- Stable Organic Matter (MOS), which results from the difference between Total Organic Matter and the sum of Living Organic Matter and Labile Organic Matter. This compartment is the largest (75% of the total).
Photosynthesis photosynthesis carbon
Definition: Plant photosynthesis is the reduction of carbon dioxide from the atmosphere by the water absorbed by the roots using solar energy captured by the leaves, in the presence of mineral salts, with the release of oxygen, to produce carbohydrates ….plants are the energy source of living organisms in the soil. Aeration of land, its humidity, its pH pH Definition: Notation which accounts for the concentration of H + ions of the medium and thus designates the very acid (pH 4 to 5.5), acid (5.5 to 6, 8), neutral (6.8 to 7.2) or alkaline (greater than 7.2) of a soil.
And its temperature is the main conditions of their development.
Indicators of biological activity
The analyzes make it possible to measure either the quantity or the action of the microorganisms.
- The amount of microbial biomass is assessed by the determination of organic carbon in the living matter of the sample. The extraction of microbial DNA from the soil is another method used in research by INRA.
- The activity of microorganisms is assessed either by their respiration corresponding to the CO 2 released by the soil sample, or by the result of a specific function such as the nitrification of ammoniacal nitrogen, for example.
Laboratories provide the method of sample collection and cold storage of the sample to be sent to them.