Guide to Bioindicator Plants Inspired by Didier de la Porte
150 million years ago, the seed, a crucial phenomenon in the evolution of the plant world, did not exist. It is very recent in the evolution of the planet. At the moment when pollen fertilizes the ovule, the seed enters dormancy. Therefore, when the plant has produced its seed, it has not yet reproduced.
Which brings us to the following question : what is the real reproduction of plants? It is the breaking of dormancy!
Another interesting question is to understand what makes plants grow? It is the breaking of dormancy! And so, over 150 million years of evolution, the criteria for breaking dormancy have greatly diversified and become more complex. This is why today in France there are 7,000 plant species and 7,000 combinations of dormancy-breaking criteria. By decoding these criteria, it is possible to understand what is happening in our soil. The conditions that break plant dormancy in the soil indicate the activity of the plant in the soil.
This guide, written by MSV Normandy within the framework of the GIEE "Zero Weeding", uses a diagram created by Didier de la Porte from the Farm of the Castle. The objective is to provide you with a list, easily usable in the field, of bioindicator plants to be able to establish a diagnosis of the state of your soil. However, it is necessary to follow a methodology that is easy to handle but allows ensuring that your diagnosis is representative of your bed/block of cultivation.
Methodology
To carry out botanical inventories, you must have a 0.5 m² quadrat, the guide of bioindicator plants which you can find in this article, and ideally a smartphone to use Pl@ntNet if you have difficulty identifying some spontaneous plants.
For a bed or a block of cultivation, it is recommended to carry out three inventories at different locations to have a good representation. Regarding the location of the botanical inventories, you have two options :
- Randomly choose the sampling locations. To do this, you can throw your quadrat randomly on your bed and carry out the inventory where the quadrat lands.
- Define homogeneous zones on your bed. A homogeneous zone corresponds to a soil area with physico-chemical characteristics distinct from the rest of the bed. For example, if you observe that a part of the bed is invaded by a particular plant, you can define it as a homogeneous zone. Alternatively, a homogeneous zone can be defined if you have observed that your vegetables grow less well in that part. If you observe that your bed has a homogeneous community of spontaneous flora, it is preferable to randomly choose the quadrat locations.
A seed germinates because in its immediate environment, the dormancy-breaking conditions correspond to what is described in its genetics but that does not mean that if you see a nettle plant in the middle of a 10-hectare plot of permanent meadows that the nettle is indicative of everything happening on the plot. Generally, if you have one square meter of nettle in the middle of a 10-hectare plot of permanent meadows or hay meadows, you will find a broken machine part buried there, because nettle germinates to absorb excess iron. So this diagnosis is only valid where the plant grows.
Therefore, if you find several indicator species of the same conditions, there is a good chance your soil has the characteristics mentioned in the diagram.
To establish a good diagnosis, you must consider not only the species but also the density of the species. The table below will help you assign an abundance value :
How to interpret the diagram and establish a diagnosis of my soil?
The diagram is divided into 5 zones :
1) At the top, these are acidic soils.
2) At the bottom, these are calcareous soils.
3) In the middle, outlined by an oval and dashes, corresponds to a balanced soil.
4) On the left, these are soils containing a high proportion of nitrogenous organic matter.
5) On the right, these are soils containing a high proportion of carbonaceous organic matter.
Legend & symbols
If the species name is accompanied by a number (x), it means the species is present x times on the diagram. For example, the fumitory is preceded by the number three. This means that fumitory is present in 3 distinct zones : in nitrate excess, with good nitrate richness, and in balanced soils. How to interpret the presence of fumitory? If your bed is heavily covered by fumitory then your soil is in nitrate excess whereas if you find only a few plants within a diverse community then your soil is balanced.
The indications in parentheses provide additional information about the state of your soil and the conditions necessary for the germination of spontaneous plants. For example, fumitory is followed by a (b+), which means that fumitory is both rich in nitrates and trace elements (Magnesium, Calcium, etc.).
The zodiac signs represent seeds whose dormancy breaking is favored by the mentioned zodiac signs.
From nitrogenous organic matter (C/N < 10) to carbonaceous organic matter (C/N > 30) : an inevitable evolution from disturbed soil to forest
Ecological successions or the evolution of the vegetative community towards forest is represented in the guide : when moving to the right, the species recorded are indicative of forest soils. Thus, you find species present on soils with a high C/N > 30, typical of forest soils. Conversely, species present on the left side of the sheet are typical of disturbed soils with a C/N around 10.
Note that forest lands are not vegetable lands because they are too rich in carbon. In forests, trees require fewer soluble elements than in cultivation. They obtain their trace elements thanks to their deep rooting.
Additionally :
- The degradation of litter, i.e. the mineralization of nitrogen, is much slower.
- There is almost no export of organic matter but many woody plant restitutions.
- There are no annuals, not due to lack of light but for food reasons. Indeed, annual plants live on soils with a high rate of organic nitrogen mineralization.
Which plants allow evaluating the type of organic matter and how to modify the C/N?
Blackberries, easily recognizable plants, are a good way to identify soils with C/N greater than 40.
It is also possible to evaluate the nature of the organic matter present in your soil via two species of veronicas. The ivy-leaved speedwell is characteristic of soils rich in carbonaceous organic matter and mineralizable where good aerobic microbial life develops. However, care must be taken not to add too much carbonaceous organic matter at the risk of shifting towards a forest soil which is not suitable for vegetable cultivation. Therefore, nitrogenous organic matter should be added, such as young destroyed green manures (before heading for grasses and before flowering for dicots). Conversely, the Persian speedwell indicates that the soil is mainly composed of nitrogenous organic matter. It would be preferable to add carbonaceous organic matter to increase your C/N.
Balanced soil : optimal conditions for vegetable growth
The zone outlined by dashes represents soils :
- Rich in organic matter with a C/N favoring vegetable growth.
- High aerobic microbial activity.
- Soils rich in bases (Magnesium, Potassium, Calcium, Copper, Boron, etc.).
- High porosity.
- Good water retention.
- Neutral pH.
The most fertile and active zone for plants is the meeting zone of the humiferous and mineral horizons, where sometimes clay-humus complexes are formed. Agriculture has always used this meeting zone to establish crops, very rich and biologically active zones. Historically, market gardeners cultivated on drained marshes, which allowed having soil enriched in organic matter. The practices implemented at the Farm of the Castle aim to move towards this zone.
Which plants indicate a balanced soil
In the neutral zone, fewer spontaneous plants can be observed because the soil is balanced and does not bring perennials, such as field bindweed. However, there are several species indicating that the soil is balanced. Starting with the chickweed which breaks dormancy in the presence of stable balanced humus and good aerobic microbial life. The role of chickweed is to reorganize nitrogen into a non-leachable organic form. Ribwort plantain is a good indicator species of high biological activity and associated with porous soils.
The spotted alfalfa grows on soils rich in bases and stable humus. The appearance of alfalfa indicates that the soil is rich in organic matter. Also, the cranesbill geranium indicates stable organic matter with balanced C/N.
This zone is suitable for the majority of vegetables. However, nitrogen-demanding vegetables will have higher yields when the soil is mainly composed of nitrogenous organic matter.
Disturbed soils composed of excess nitrogenous organic matter
In conventional market gardening, soils are recombined because:
1) There are frequent interventions, such as tillage, on the soil all year round.
2) There are deconstructions, oxidations, and strong exportations of trace elements.
3) There are strong exports of organic matter, but also strong imports via large amounts of organic fertilizers or chemical fertilizers.
4) Often it is an environment with high irrigation. Irrigation greatly affects the flora.
Practices used in conventional market gardening affect soil fauna, such as decomposers. It is therefore overall an environment very low in plant diversity and most plants in market gardening systems are plants from extreme environments. These are plants coming from flood zones, riverbanks, or estuarine mudflats, thus dirty zones rich in asphyxiated organic matter. For example, the primary biotope of dock is the Atlantic estuaries.
Indicator plants of soil often left bare creating a high risk of leaching
Indicator plants of soil affected by asphyxia causing the presence of nitrites
One of the dormancy-breaking criteria retained is hydromorphism and soil asphyxia. There are plants characteristic of asphyxiated soil. Dock and broadleaf plantain are perfect examples of this type of soil. Species present in this zone have the ability to correct imbalances by absorbing nitrites to metabolize proteins whereas nitrites are normally toxic to plants. Once nitrites are metabolized in the plant leaves, dock releases nitrogen, benefiting the rest of the vegetation. The role of these plants is to clean the soil of nitrites.
If you find a high abundance of white goosefoot, black nightshade or field bindweed, your soil is at risk of hypermineralization, with excess released elements N, P, K, a strong decrease or even disappearance of fungi and destabilization of the soil complex.
Cutleaf geranium indicates that the organic matter is too nitrogenous, whereas round-leaved geranium indicates the soil is shifting into nitrite excess. It is therefore preferable to increase the C/N of your soil via carbon amendments as soon as you see a strong emergence of geranium cutleaf.
Indicator plants of soil affected by nitrites caused by trampling
Indicator plants of soil compacted by working the soil during wet periods and/or hydromorphic soil
Nitrites are also found in environments asphyxiated by water, such as marshes. If a bed is invaded by creeping buttercup, persicaria knotweed or creeping bentgrass, there is a strong chance the soil is hydromorphic.
It is possible to drain hydromorphic soils with an Actisol, a subsoiler with tines often called "Canadian".
One of the levers used at the Farm of the Castle is to let certain spontaneous plants, like setaria cock's-foot, complete their life cycle to correct the soil. Setaria cock's-foot germinates in soils compacted by soil work and having a majority of nitrogenous organic matter. Letting this spontaneous plant complete its life cycle will bring carbonaceous organic matter to the soil.
Indicator plants of compacted and destructured soil
Species present in this zone germinate to decompact soils and restore porosity. When pulling out a plant from this zone, one observes that the soil has a crumbly texture.
Indicator plants of acidic soil
Representative species of an acid soil are located at the top of the page.
Special cases: nettle, poppy and soft rush
Since it is possible to find these three species in a variety of soils, they have been listed in the table at the bottom right of the diagram. For example, nettle is not an indicator plant of a specific environment. The role of nettle is to regulate organic matter and excess iron.
Diagram for bioindicator plants of meadows
Just like the diagram for spontaneous plants present in market gardening systems, it is possible to diagnose your meadow using the adjacent diagram:
