Green Manuring: Types, Benefits, and Challenges in Agriculture

Farmers have been turning green plants back into the soil for thousands of years – and for good reason. Green manuring is the practice of growing specific crops and incorporating them into the soil while still green, with the primary goal of enriching organic matter and restoring nutrients for the next crop. It’s a low-cost, ecologically sound strategy that works particularly well in rainfed farming systems, where farmers depend entirely on rainfall and cannot afford expensive chemical inputs. But like any practice, it comes with its own set of challenges that require careful planning to get right.

What is green manuring? 🔗

At its core, green manuring is straightforward. You grow a fast-growing crop – usually during a fallow period between main crops – and then plow it back into the soil before it reaches full maturity. As these plants decompose, they release nutrients, stimulate soil microbial life, and build organic matter. The un-decomposed plant material gradually prepares the ground for cash crops by slowly releasing nitrogen and other nutrients into the soil. These crops are not harvested and removed from the land, because doing so would defeat the purpose – the nutrients stay right where they’re needed.

The history of green manuring stretches back to antiquity. According to ATTRA’s Sustainable Agriculture program, by around 300 BC the Greeks and Romans were already plowing legumes into the soil for their benefits to soil fertility. Early North American colonists used buckwheat, oats, and rye to add organic matter. The practice declined with the rise of synthetic fertilizers in the 20th century, but interest has surged again as farmers seek sustainable alternatives.

Types of green manure crops 🔗

Not every plant qualifies as an effective green manure. The best candidates grow quickly, produce substantial biomass, and offer specific benefits to the soil. Green manure crops broadly fall into two categories: leguminous and non-leguminous.

Leguminous green manures 🔗

These are the most valued for soil fertility. Leguminous crops have a symbiotic relationship with nitrogen-fixing bacteria that live in nodules on their roots. These bacteria capture atmospheric nitrogen and convert it into a plant-available form – essentially providing free fertilizer. According to Wikipedia’s entry on green manures, depending on the species grown, the nitrogen released into the soil can range from 45 to 224 kg per hectare. Common leguminous green manures used in tropical and subtropical rainfed farming include:

• Cowpea (Vigna unguiculata): Drought-tolerant and fast-growing, cowpea can fix the equivalent of 40-60 kg of nitrogen per hectare, making it especially suited to rainfed conditions.
• Sunhemp (Crotalaria juncea): Produces large amounts of biomass quickly, and its deep roots help break up compacted soil layers. It has been cultivated since the early days of agriculture in India.
• Dhaincha (Sesbania bispinosa) and Sesbania: Woody legumes that tolerate waterlogged conditions, making them particularly useful in rice-growing regions.
• Clover, alfalfa, and vetch: Widely used in temperate regions, these are excellent for nitrogen fixation and soil structure improvement.

Non-leguminous green manures 🔗

While they don’t fix nitrogen from the air, non-leguminous crops offer other important benefits. As Greentumble explains, certain crops like buckwheat and oilradish enrich soils with phosphorus, while mustard and radish are valuable for breaking pest cycles and suppressing soil-borne diseases. Cereals like oats and rye act as “nitrogen lifters” – they scavenge excess nitrogen from the soil profile and prevent it from leaching away before the next crop can use it. Cotswold Seeds notes that some winter non-legume green manures can capture up to 90% of the nitrate in the soil that would otherwise be lost to rainfall leaching.

In-situ vs. ex-situ green manuring 🔗

Research published at Preprints.org distinguishes two methods of green manuring. In-situ green manuring involves growing and incorporating the crop within the same field – the most common approach. Ex-situ green manuring involves collecting plant material from another location (such as tree or shrub clippings) and incorporating it into the target field. Both approaches deliver organic matter and nutrients, but in-situ is generally more practical and cost-effective for most farms.

Benefits of green manuring 🔗

Nitrogen addition and nutrient cycling 🔗

The most significant benefit of leguminous green manuring is nitrogen enrichment. A well-managed green manure crop can deliver 50-150 kg of nitrogen per hectare – equivalent to several bags of costly chemical fertilizer, at a fraction of the expense. But the nutrient contribution goes beyond nitrogen alone. Green manures also cycle back potassium, phosphorus, calcium, magnesium, and sulfur – nutrients that get re-incorporated as the plant material decomposes. The root systems of many green manure crops penetrate deep into the soil, drawing up nutrients from layers that shallower-rooted cash crops cannot reach.

Improved soil structure and water retention 🔗

As green manures decompose, microbial activity produces mycelium and other sticky substances that bind soil particles together into aggregates. According to the University of New Hampshire’s College of Life Sciences and Agriculture, this improves soil aeration, water infiltration, and water retention – creating a more robust root environment for subsequent crops. This is particularly important in rainfed agriculture, where the ability of the soil to hold moisture between rainfall events directly affects crop survival.

Weed suppression 🔗

Dense, fast-growing green manure crops shade out weeds by forming a closed canopy that blocks sunlight. Non-leguminous crops like buckwheat are especially effective at this, developing extensive root systems that outcompete weeds for water and nutrients. Cultivation Ag reports that on many organic farms, incorporating green manures significantly reduces weed pressure, lowering the need for herbicides and manual weeding labor.

Soil microbial health 🔗

When green plant material is incorporated into the soil, it provides a burst of fresh organic matter for soil microorganisms. A study published in PMC found that green manure incorporation significantly enhances the activities of key soil enzymes – including β-glucosidase and urease – which are associated with organic matter degradation and nitrogen cycling. This stimulated microbial activity accelerates nutrient release and improves what researchers call “soil efficient fertility and ecosystem functioning.”

Erosion control and soil cover 🔗

A growing green manure crop protects bare soil from the impact of raindrops, which is critical in rainfed farming systems where monsoon rains can be intense. The canopy absorbs the force of rainfall, enabling water to infiltrate rather than run off, reducing topsoil erosion. Winter green manure crops are especially valuable in this respect, protecting fields that would otherwise lie exposed through the off-season.

Challenges of green manuring 🔗

Green manuring offers substantial benefits, but it is not without complications. Understanding the challenges is just as important as appreciating the advantages – particularly for rainfed farmers working with limited resources and narrow planting windows.

Nitrogen immobilization 🔗

One of the most counterintuitive challenges is that incorporating green material can temporarily reduce available nitrogen rather than increase it. This happens when the carbon-to-nitrogen (C:N) ratio of the incorporated material is too high. Soil microbes need nitrogen to break down carbon-rich material, so they compete with the next crop for the available nitrogen in the soil – a process called nitrogen immobilization. Wikipedia notes that incorporating green manures without enough lead time before crop planting can stop the flow of nitrogen, leaving insufficient nutrients for the next planting. Researchers at Preprints.org emphasize that it is essential to incorporate green manures before flowering, since older, more lignified plant tissue has a wider C:N ratio that slows decomposition and worsens temporary immobilization.

Pest and disease carryover 🔗

While green manuring can break certain pest cycles, it can also create new problems if the wrong crop is chosen. If a green manure crop becomes infected with a fungal disease, incorporating it into the soil can spread the pathogen to the next crop. Cultivation Ag points out that mustard family crops (rapeseed, radish) have biofumigation properties that reduce nematodes and soil pathogens – but crops that are closely related to the main cash crop can act as alternate hosts for the same pests and diseases. Careful crop selection – choosing green manures that are not related to the main crop – is the primary way to manage this risk.

Water competition in rainfed systems 🔗

In water-scarce rainfed environments, a growing green manure crop competes for the same soil moisture that will be needed by the main crop. ATTRA’s sustainable agriculture guidance identifies water consumption by green manure crops as a significant concern in areas with limited precipitation. The solution is strategic timing – in many regions, green manure crops are grown during the monsoon season when water is abundant, and incorporated before the post-monsoon crop is planted. However, this requires careful planning to avoid the green manure running too long into the planting window.

Narrow incorporation window and labor costs 🔗

Green manure crops typically need 45-60 days to build sufficient biomass, but they must be incorporated before they mature and their C:N ratio widens. In intensive farming systems with short gaps between crops, this timing window can be frustratingly narrow. The Preprints.org review highlights the hesitancy many farmers feel about dedicating six to eight weeks solely to a non-income-generating green manure crop, especially when seed costs, equipment use, and additional labor for incorporation are factored in. Farmers may also be put off by the slower and less immediately visible returns compared to chemical fertilizers.

Risk of nitrogen leaching 🔗

If green manures are incorporated and there is a delay before the next crop is planted – combined with heavy rainfall – the nitrogen released during decomposition can leach out of the root zone before the crop can absorb it. The Organic Research Centre has studied this extensively, noting that the timing and method of incorporation significantly affect how much of the released nitrogen actually reaches the succeeding crop, rather than being lost to the environment.

Best practices for successful green manuring 🔗

Most of the challenges above can be managed with good planning. A few key principles help maximize the benefits while minimizing the risks:

• Incorporate before flowering: Young, succulent plants decompose faster and are far less likely to cause nitrogen immobilization than mature, lignified tissue.
• Allow adequate decomposition time: Leave at least 2-3 weeks between incorporation and planting the next crop to allow decomposition to progress and nitrogen to become plant-available.
• Choose crops that are unrelated to your main crop: This minimizes the risk of pest and disease carryover.
• Match timing to rainfall: In rainfed systems, grow green manures when moisture is naturally available, and terminate them before the critical dry period or before soil moisture is needed by the cash crop.
• Inoculate legume seeds: Applying the correct Rhizobium bacterial inoculant to legume seeds before sowing ensures effective nitrogen fixation, especially in soils where these bacteria are not naturally present in sufficient numbers.

As Geopard Agriculture notes, genetically fertile soil – built up through practices like green manuring – produces crops that are more resistant to both disease and drought, creating a compounding benefit over multiple seasons. The economics also improve over time: reduced fertilizer costs, better water retention, and higher yields gradually offset the initial investment in seed and labor.

Green manuring is not a silver bullet, but it is one of the most cost-effective tools available to farmers – particularly those in rainfed systems – for building lasting soil health. Used wisely, it reduces dependence on expensive chemical inputs, improves the land’s long-term productive capacity, and contributes to more resilient farming systems overall.

What do you think? Given the narrow planting windows and water stress common in rainfed farming, which type of green manure crop – leguminous or non-leguminous – do you think would be more practical for smallholder farmers in your region? And how might combining green manuring with other organic practices change the long-term fertility of soils that have been heavily dependent on chemical fertilizers?

References

1. https://en.wikipedia.org/wiki/Green_manure
2. https://attra.ncat.org/publication/overview-of-cover-crops-and-green-manures-2/
3. https://greentumble.com/the-role-of-green-manure-in-soil-fertility
4. https://www.cotswoldseeds.com/articles/227/guide-to-green-manures
5. https://www.preprints.org/manuscript/202406.0340
6. https://colsa.unh.edu/blog/2024/08/what-green-manure
7. https://cultivationag.com/green-manure-in-organic-farming/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC10757628/
9. https://www.organicresearchcentre.com/manage/authincludes/article_uploads/iota/technical-leaflets/green-manures-effects-on-soil-nutrient-management-and-soil-physical-and-biological-properties.pdf
10. https://geopard.tech/blog/how-does-green-manure-improve-soil-fertility/