Cover crops stimulate the biological activity of the soil after harvest and their return as mulch or compost material. They stimulate micro-biotic activity, the proliferation of earthworms, and stabilise the soil. They also make nutrients available in large quantities and in more available forms.
Cover crops can also improve the in-depth structure of the soil, thanks to their hair nets and their taproot. They control the spreading of nitrates and soil erosion; they can be all the more effective if the plants are well-developed before the autumn rains. They compete with pioneer plants, due to their rapid growth or the alelopathic effect (limiting germination), and fight against certain pests and diseases thanks to their disinfectant effect.
Below, I will try to delve into the multiple functions of cover cropping:
To recover impoverished or polluted land
Cover crops are a staple in the process of bringing the soil back in balance. There are a few situations from which recovery might be needed, starting with the most common: broad-scale agricultural soil impoverished by heavy cultivation. At this scale, cover crops aim to replenish the soil mainly with Carbon and Nitrogen, and different combinations can be used (Cereals and plant legumes, for example); after a few seasons of cultivation, other nutrients might also be addressed: Calcium, Iron, Magnesium, etc. More sophisticated plants might be needed, such as radishes, mustard, broad beans, tansy, purple tansy, etc. After cultivation, those plants are then cycled through the soil.
Where humans are reckless with handling materials, soils might be affected, either holding high levels of pollutants, or even excess nutrients. In that case, the plants used aim at extracting certain elements from the soil, and are then removed or composted: brown mustard, comfrey, and nettles.
After earthworks, to bring the soil into stability
Earthworks are a wonderful opportunity to set up long-term polyculture systems. The soil disturbance is meant to create better conditions for more productive plants to grow in, but it also creates conditions for invasive pioneers to move into the “niche”. Therefore, fast-sprouting, and hairnet root plants are needed to control the soil as quickly as possible. Options such as mustard, rye, and wheat are efficient in establishing vegetation that not only performs important functions but also is easy to work with.
Depending on the climate, other options are available depending on the season. In the tropics, due to the very fast growth rate of all vegetation, vigorous plants that will shade the ground are needed, such as cowpea; in arid regions, the need is different: the soil needs to be covered with vegetation as quickly as possible, to avoid erosion. Options: vetiver, lemon grass.
To encourage succession
As we’re moving in the direction of more sophisticated techniques, more questions need to be asked, in order to find specific solutions. What climate, what time of year, what soil conditions, and also what objective?
Are we aiming to prepare a perennial vegetable garden, or to plant fruit trees? Or is it a combination of both?
The purpose is to “control” the ground, we have to look at how easy it is to work with the chosen plant, to transition toward our objective. In the case of perennial gardens, nettles and clover are options. For trees, comfrey can be an option in temperate climates or sweet potatoes in more tropical areas.
If animals are to be rotated through the land, cows for example, you might find that lucerne is a suitable option.
Learn more about cover-cropping for different purposes, in a subtropical context, with Geoff Lawton see the video below:
For time stacking
Time stacking in Permaculture takes an example after ancient Asian practices, that people such as Masanobu Fukuoka perfected. His technique centred around the cultivation of wheat and rice, in successive fashion, while also including the rotation of ducks into cycles. Keeping in mind that the main function of cover crops is to control the type of vegetation that grows in the given conditions, he would harvest the wheat from the fields, without removing the stems; at the same time, rice would be sown. As the rice grains sprout, the stalks of the wheat provide partial cover from the sun, wind, and birds,
Once the rainy season arrives and the paddock is left inundated, ducklings are introduced, and forage on the remaining vegetation. Their manure fertilises the soil for the rice sprouts, and the ducks are removed once they approach maturity; by that time, the rice stems are mature enough to handle the competition from other plants.
For the purpose of cover cropping for other climax species (trees) or for value crops (corn, for example), other combinations can be used that follow the same principles, and are appropriate to the climate.
For mineral balance
The repeated cultivation of certain value crops, or of plants that are necessary for subsistence, can bring the soil out of mineral balance. Over time, techniques have been developed that allow the soil to “rest”; that means it won’t be planted to a crop, but it will instead be left fallow for a season, or for a year so that it can naturally come back into balance.
The so-called weeds are actually also indicators of possible mineral deficiencies, so it’s important to research what each plant “mines” for, and use it to our advantage. As a complement to these pioneers, plants like nettles, comfrey, and yarrow accumulate a wide array of minerals, from different depths of the soil or the subsoil; their leaves or stems can be harvested regularly, and be returned to the soil, providing the microbiota with organic matter.
Where animals can be involved, through scheduled rotation, this sequence can be even more efficient; cover crops needed for mineral balance can be cycled through the guts of the animals, and are added to the soil in the form of manure. In this way, the elements needed for soil balance actually can be absorbed quicker.
To conclude, cover crops are like the “cheat codes” that accelerate the transition to working with Nature, and add to our tasks in becoming stewards of the land. For best results, practical experience is necessary; given the current situation worldwide, cover crops need to be included in our practices, for building regenerative systems that also support our human needs.