2.2.5 Green manure

What is green manure?

A green manure can be defined as any plant grown to increase soil fertility and not to be harvested. The cultivation of green manure is a well-known ancestral practice; it is used in many cropping systems (cereal-based rotations, market gardening, etc.). However, it is a technically-complex procedure and must be carefully planned if the desired positive effects on the soil are to be obtained. Green manure is widely used in Californian viticulture but has not received as much attention in France. It can be useful if applied when the vine is dormant or to manage the recovery of soil after vines have been pulled up.

Why plant a green manure?

Planting green manure can have several advantages:

• Mineral fertility improvement: some green manure species use insoluble mineral elements which cannot be assimilated in this form by the vine (e.g. crucifers and potash). When the green manure is ploughed back into the soil, these elements become available to the vine in a usable form. They also store these minerals throughout the winter, thereby reducing nutrient run-off during periods of rain. Leguminous plants contribute to soil nitrogen enrichment through the symbiotic fixation of atmospheric nitrogen if the cultivation period is longer than 50 days.

• Contribution of organic matter and improvement of biological activity: when they are growing and especially after they have been buried, green manures rapidly stimulate and intensify biological activity in the soil. The humus formed can maintain the level of organic matter in the soil but is not usually sufficient to increase it.

• Improvement of soil structure: via the mechanical action of the green manure's roots, the soil between rows is loosened to a depth of 1.5 m, thereby improving water and air penetration. Root exudates and microorganisms in the plant cover's rhizosphere also contribute to stabilizing soil particles.

• Protection against erosion and run-off: green manures such as permanent grassing or living mulch have a dual effect, protecting the soil and improving water penetration. They also reduce runoff and combat erosion.

Are there any risks associated with green manure?

Green manure has many advantages but can also, if poorly maintained, present some disadvantages for the vineyard:

• The presence of green manure can increase the risk of spring frost.
• Some ill-suited green manures can alter the vines' nitrogen supply and create a nitrogen deficiency.

What are the possible choices of species?

In a few rare cases, such as with wild mustard, spontaneous vegetation growing on a plot can play the role of green manure. However, in the majority of cases, the growing of green manure requires sowing. As the cultivation of green manures is still not widely practised in viticulture, the seeds available on the market usually come from cereal farming or market gardening.

When should the manure be sown? What tools are required?

The first step is to prepare the soil, it must be sufficiently broken up to enable good germination. A single pass with a rotary hoe or one or two passes with a vibrating tiller may be sufficient. Sowing is then carried out by scattering the seed by hand, or with a sowing machine, ideally during the harvest period to take advantage of the mild temperatures and the rain at the end of summer or autumn, or during October / early November, choosing an appropriate species. It is advisable to roll the soil or carry out a light harrowing after the sowing.

When and how should the green manure be incorporated?

The destruction of the manure depends on the objectives. Mowing and shredding in the spring will destroy the above-ground parts and allow them to dry out. The presence of green manure increases the risk of spring frost. This parameter should be considered when determining the destruction date.
Mowing is worthwhile when the vegetation is not fully developed or to make mulch and maintain the soil without chemical weeding or tillage. The advantage of mulching compared to shredding is that it slows down the mineralization of organic matter, reducing the risk of nitrogen deficiencies and avoiding setbacks for the vines. An alternative solution consists in using a specific piece of equipment called a rolofaca which enables the plant cover to be laid on the ground and be pinched so as to stop the rise of the sap. Unlike shredding or mowing, the cover's growth is stopped and the "pinched" part breaks down very slowly. This mulch allows nutrients to be returned to the soil. It protects the soil, has a good water retention capacity and improves the soil structure. On a sloping plot, this type of practice can make it difficult for tractors to get a good grip.

Burying the plant cover is optional and depends on objectives: it allows a rapid release of nitrogen which is then available for the current season. Burial of plant cover should be carried out taking into account how damp it is, and should never be done on wet soil. If it is done one to two days after mowing or shredding, decomposition will be rapid with significant mineralization. It may be advisable to let it dry for 30 to 60 days. Burying is then done in one or two stages depending on the species either with a toothed or disc tool.

Is it necessary to apply fertilizer or other inputs with green manure?

No fertilization is generally required as fertilization tends to favour the development of the above-ground parts to the detriment of the roots. If sown late, crucifers may need some nitrogen and a small amount of manure may therefore be beneficial.

This chapter is an excerpt from Vigne Vin Occitanie.