A widely used practice for the natural fertilization of the vegetable garden is the green manure of legumes (or Leguminous cover crops). When we talk about green manure, we refer to an agronomic technique typical of organic farming that enhances soil fertility through the specific properties of a family of plants, in this case, legumes.
Let’s explore the working mechanism of this totally natural agricultural practice, which can be easily implemented in a home garden, especially during the autumn season.
Natural fertilization and the importance of soil fertility
Throughout history, one of the fundamental challenges for farmers has been achieving fertile soil to grow their crops vigorously and healthily. Before the introduction of chemical agriculture, soil fertility was achieved through correct agronomic practices, including crop rotation, intercropping, proper and non-invasive soil cultivation, and natural fertilization through the addition of organic matter.
However, with the rise of chemical farming in the last century, many good agricultural practices were neglected. Organic and eco-sustainable agriculture dwindled as synthetic chemical fertilizers became the go-to solution. While these fertilizers increased yields, they also led to gradual soil degradation, as they failed to replenish the soil with essential organic matter.
Organic agriculture and the elimination of chemical fertilizers
In response, organic agriculture emerged as a way to return to more sustainable practices, aiming not only to produce food but also to protect the environment and the land for the long term, preserving the future for generations to come. In 2002, the European Union adopted the communication COM(2002) 179 – Towards a Thematic Strategy for Soil Protection, which emphasized the role that agriculture should play in conserving soil quality, preserving the vegetation layer, and preventing desertification. All agricultural activities must, therefore, focus on maintaining and improving soil fertility, the foundation of life.
Now, let’s explore in practical terms how to achieve these important goals, starting with the natural fertilization of the soil using an ancient agricultural practice involving a specific family of plants, the legumes.
The legume plant family
Legumes, scientifically known as Leguminosae or Fabaceae, represent a large botanical family that includes both plants primarily used for fresh or dry consumption (such as broad beans, peas, beans, chickpeas, soybeans, lentils, peanuts, etc.) and plants utilized for natural soil fertilization through the green manure technique (such as field beans, alfalfa, clover, lupins, melilot, vetch, crotalaria, etc).
In this article, we will focus on the properties of legumes that contribute to significant improvements in the soil.
Action of legumes as green manure
The action of legume green manure involves a somewhat technical process. In organic farming, legumes are known for their ability to fix atmospheric nitrogen. This process is facilitated by a symbiotic relationship between the plant roots and certain bacteria naturally present in the soil, known as rhizobia.
These rhizobia capture nitrogen from the atmosphere and fix it in the soil through a specific endogenous enzyme called nitrogenase. The fixed nitrogen is converted into compounds that the plants use to synthesize proteins.
This symbiotic relationship is especially pronounced in the roots of legumes, where the bacteria activate the nitrogen fixation process. The plants benefit from the fixed nitrogen for their robust growth, while the carbon compounds produced during photosynthesis are used by the bacteria as an energy source for nitrogen fixation. This creates a virtuous, symbiotic cycle that benefits both the plants and the soil.
Legumes are self-sufficient plants in terms of nitrogen requirements during their vegetative growth. Moreover, they can fix and contribute significant amounts of organic nitrogen to the soil, aiding in its natural fertilization. This organic nitrogen becomes available for subsequent crops.
Another advantage of legume green manure is the significant input of organic matter when the plants are incorporated into the soil after their growth period.
It’s essential to highlight that all legumes have these characteristics, which are especially enhanced in species not destined for human consumption. These non-consumption varieties are easier to manage in terms of cultivation in the field.
Legume green manure
Previously, we discussed green manure with brassicaceae, a technique useful in controlling soil parasites like nematodes.
In general, legume green manure is an agricultural practice widely employed in organic farming. It involves sowing and incorporating non-harvestable herbaceous crops into the soil to improve it.
The specific goal of legume green manure is natural fertilization through nitrogen fixation, also known as green manuring. Simultaneously, this technique enhances the physical structure of the soil by adding organic matter.
As we have seen, the legume family encompasses various species that can be utilized for this purpose. Depending on the species chosen, different quantities of fixed nitrogen and biomass (organic matter) will be contributed to the soil. One commonly used species is field bean (Vicia faba minor), which can fix over 150 kg of nitrogen per hectare of land.
Field bean green manure
Field bean green manure is usually sown at the beginning of autumn, with a crop cycle lasting about 28 to 32 weeks. The incorporation of the crop into the soil, or green manuring, occurs in spring. The sowing of field beans should be dense, with approximately 40-60 plants per square meter of soil; the seeds are smaller than those of regular broad beans, with 1,000 field bean seeds weighing about 700 g. Thus, you can calculate the necessary seed quantity accordingly.
In spring, when the crop cycle is completed, and the field beans have grown, the crop needs to be incorporated into the soil. This can be achieved using a plow (if a tractor is available) or even a simple Rototiller or Motocultivator (available here), burying the organic mass at a depth of at least 15 cm.
After a further resting period of about 15-20 days, your soil will be ready for spring and summer crops. These crops, such as tomatoes, greatly benefit from the presence of nitrogen and organic matter in the soil.
Legume green manure, particularly field beans and others, is crucial, especially when you lack access to manure or organic compost for natural fertilization. It is a practice that should ideally be repeated every two or three years, depending on the soil conditions and its ongoing evolution – the most precious resource we have at hand: the soil!
- University of Minnesota: “Cover crops and green manures in home gardens” – This resource discusses how green manures like vetch, clover, beans, and peas can be used in home gardens.
- University of California: “Green-Manure Cover Crops” – This resource discusses how green manures can replace and hold nutrients in the soil, increasing organic material that feeds microorganisms and improves soil structure.
- University of Tennessee: “Cover Crops and Green Manures” – This resource discusses how green manures and living mulches can be used in agriculture, with a focus on leguminous cover crops like purple vetch.
- University of Vermont: “Cover Crops and Green Manures” – This resource discusses how legume cover crops can add “free” symbiotically-fixed nitrogen to the farming system.
- Colorado State University: “Cover Crops and Green Manure Crops” – This resource discusses how when the cover crop is tilled into the soil, it is referred to as a green manure crop.