Green manure in olive growing: a technical guide for autumn and winter

In an era when agriculture must meet the challenges of sustainability, green manure remains a highly valuable agronomic practice, especially in olive growing. It's not just an ancient technique, but a modern tool for regenerating soil, reducing the use of chemical fertilizers, and improving the overall health of the agroecosystem.

We had already addressed this topic ten months ago, but the experience of many olive growers and the climatic conditions of the last year make it useful to return to itAutumn is expected to be mild, while winter 2025–2026 will be more dynamic and variable: an ideal context to reflect on two key aspects: How often should green manure be repeated and how much does it really affect soil fertility?.

How often should you use green manure?

Il green manure It should not be considered a mandatory annual intervention, but a practice to be planned based on the characteristics of the soil, the climate and the management of the olive grove. in poor or tired soils, with less than 1,5% organic matter, is It is useful to do it for 2–3 consecutive years, to re-establish a stable biological balance. In the already fertile olive groves, every 3–4 years may be sufficient, alternating it with permanent grassing or organic inputs (compost, mature manure). biennial rotation, executing green manure on half the rows at a time, allows for maintaining coverage and operational readiness. The important thing is not to make it automatic, but rather a strategic choice, calibrated to the soil's response and climate trends.

What is green manure?

It consists of the sowing of herbaceous plants between the rows, subsequently buried in the pre-flowering or flowering phaseThis process enriches the soil with organic matter and nitrogen, improves its structure and stimulates microbial biodiversity, creating the ideal conditions for “living” soil.

Legumes: Allies of Fertility

Le legumes are the protagonists of green manure: thanks to bacteria of the genus Rhizobium, they fix atmospheric nitrogen making it available to plants.
Among the most widespread species we find field beans (150–180 kg/ha), vetch (100–120 kg/ha), lupine (150 kg/ha), forage pea (120 kg/ha) and bean (160–200 kg/ha).
In recent years, more and more mixtures with grasses have been used, such as oats (30–40 kg/ha), ryegrass (25–30 kg/ha) or rye (40–50 kg/ha), which increase biomass, protect the soil and reduce the risk of pathogens.

A balanced mixture of legumes and grasses averages 100–120 kg/ha of total seed, depending on the composition and grain size of the seeds.

When and how to sow

The planting period varies with the climate and water availability. For autumn 2025, with mild temperatures and irregular rainfall, we recommend:

• Autumn green manure: sowing between late October and November, slightly earlier in the hilly areas of Northern and Central Italy;
• Spring green manure: sowing between mid-February and mid-March, useful if autumn has been too dry.

Sowing is done by broadcasting or with a fertilizer spreader (80–100 kg/ha of mixture), burying lightly (2–3 cm) before rain or light irrigation.

How to sow in an olive grove already covered in grass

In olive groves with natural grassing it is necessary to:

• shred or mow spontaneous vegetation;
• work superficially (3–5 cm) in compacted soils;
• sow by broadcast or with a fertilizer spreader, possibly mixing the seeds with sand or fine compost;
• roll lightly to improve seed-soil contact;
• sow before the rain to promote germination;
• Annually alternating seeded rows and those maintained as grass allows for the preservation of biodiversity and walkability.

The right time for burial

ICutting and planting are done in pre-flowering or flowering, when nitrogen fixation is at its maximum.After shredding, let the biomass dry for 1–2 days, then bury it with a rotary tiller or subsoiler at a depth of 8–12 cm.

A good green manure can provide up to 180–200 fertilizing units of nitrogen/ha, which are gradually released through microbial activity, making the nutrients available in an absorbable form over the following months.
After 2–3 years of well-managed green manure:
• organic matter grows on average by 0,1–0,2% per year;
• cation exchange capacity (CEC) increases by 10–20%;
• available water capacity improves by up to 25%;
• the organic nitrogen pool increases by 40–60 kg/ha/year, reducing the need for mineral fertilizers over time.

The result is a more fertile, stable and resilient soil, with a better vegetative response of the olive tree.

The role of organic carbon

In addition to providing nitrogen, green manure plays an essential role in restoration of stable soil organic carbon, the component that more than any other determines the soil's ability to regenerate, retain nutrients, and maintain microbial vitality. Organic carbon represents the soil's energy reserve, nourishing microflora and contributing to the formation of humus, a key element of long-term fertility.

Fundamental in this process is the carbon/nitrogen (C/N) ratio of buried biomassA ratio that is too low, typical of green manures composed only of legumes, leads to rapid mineralization with the risk of nitrogen loss; a ratio that is too high, typical of grasses only, slows decomposition and limits nutrient availability.

The choice of balanced mixtures of legumes and grasses, with C/N ratios between 12 and 20, allows for a gradual and continuous release of nutrients, promoting the construction of stable humus and improving the structure, porosity and cation exchange capacity of the soil over time.
In this sense, green manure is not just a fertilizing intervention, but a regenerative practice that transforms the soil into a more resilient and self-sufficient system from a biological and energetic standpoint.

Indicative costs of green manure

Costs vary depending on the type of mixture and the sowing methods:

The agronomic value returned (in nutrients, structure and fertility) is far greater than the cost incurred: a well-managed green manure cycle can pay for itself in 1–2 years with lower fertilization costs and greater vegetative vigor.

Economic and patrimonial value

An increase of 0,2% of organic matter is equivalent, on average, to 50–70 kg/ha of nitrogen, 30 kg/ha of phosphorus and 60–80 kg/ha of potassium more in the following cycle, for an estimated value of €200–300/ha/year. Added to this is a lasting structural effect: on hilly or marginal lands, a documented improvement in fertility can increase land value by 5–10% in the medium term.

In addition to its nutritional benefits, green manure promotes biodiversity, ecological stability, and water retention, counteracting erosion and compaction.
It also improves the resilience of the olive grove against pathogens such as Xylella fastidiosa, helping to keep the plants more robust and vital.

In short, with careful planning and adaptation to new seasonal trends, such as those expected for autumn 2025 and winter 2025/2026, green manure confirms itself as one of the most effective tools for increasing the fertility, biodiversity, and quality of olive production.

AIPO Director
Interregional Association
Olive producers