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Agriculture & Rural India22 Concepts & Facts

What Is Green Manure? Agronomic Types, Biological Nitrogen Fixation & Soil Benefits

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Green manure refers to the agricultural practice of growing specific fast-growing plants—primarily leguminous species—and ploughing them directly back into the soil while they are still green and succulent, typically right before flowering. Unlike animal farmyard manure or decomposed compost, green manure is cultivated on-site or gathered from nearby perennial plants to decompose directly within the topsoil horizon. As this fresh plant biomass decomposes, it releases essential plant nutrients, accumulates active organic carbon, improves physical soil aeration, and stimulates underground biological microbial communities, offering an eco-friendly alternative to chemical synthetic fertilizers.

Agronomists categorize green manuring into two primary modalities: in-situ green manuring and green leaf manuring. In in-situ green manuring, crops are grown in the main field and subsequently incorporated into the soil using mouldboard or disc ploughs around 45 to 60 days after sowing, just as flower buds begin to appear. The plants are incorporated at this tender growth stage because their low carbon-to-nitrogen (C:N) ratio facilitates rapid microbial breakdown without immobilizing existing soil nitrogen. Leading in-situ green manure crops in India include Dhaincha (Sesbania aculeata), Sunnhemp (Crotalaria juncea), Cluster bean (Cyamopsis tetragonoloba), Cowpea (Vigna unguiculata), and Sesbania rostrata, a unique stem-nodulating species capable of thriving in waterlogged lowland paddies.

In contrast, green leaf manuring involves collecting tender green twigs, foliage, and loppings from perennial trees, shrubs, and border hedges growing on bunds, wastelands, or forest margins, and incorporating them directly into agricultural fields during preparatory tillage. Prominent green leaf species include Pongamia pinnata (Karanja), Gliricidia sepium (Madre de cacao), Azadirachta indica (Neem), and Calotropis gigantea.

Beyond adding approximately 60 to 120 kilograms of biologically fixed nitrogen per hectare, green manuring exerts distinct physical and chemical benefits: it reclaims saline and sodic soils by generating organic acids that solubilize native calcium carbonate to displace harmful exchangeable sodium, enhances water-holding capacity in light sandy soils, and creates a dense living canopy that outcompetes weeds before planting primary staple crops.

Key Concepts & Self-Assessment22 Key Facts

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#1
Green manure consists of living, succulent plant biomass ploughed into the soil to improve soil fertility and organic matter.
#2
The practice is divided into two types: in-situ green manuring (grown on-field) and green leaf manuring (transported from outside).
#3
Green manure crops are ploughed under at 45 to 60 days of growth, specifically at the initiation of the flowering stage.
#4
Incorporating crops prior to woody lignification ensures a narrow Carbon-to-Nitrogen (C:N) ratio, enabling rapid microbial decomposition.
#5
If plants mature and turn fibrous, microbes consume existing soil nitrogen to break down carbon, causing temporary nitrogen starvation in subsequent crops.
#6
Most green manure species belong to the family Fabaceae (Leguminosae) and harbor symbiotic Rhizobium bacteria in their roots.
#7
Dhaincha (Sesbania aculeata) is a drought- and salinity-tolerant green manure crop widely planted prior to wetland rice cultivation.
#8
Sesbania rostrata is an exceptional green manure plant that develops biological nitrogen-fixing nodules on both its roots and its stems.
#9
Sunnhemp (Crotalaria juncea) is a fast-growing in-situ green manure crop valued for producing high organic dry matter within 45 days.
#10
Green leaf manuring utilizes foliage cut from perennial trees such as Pongamia pinnata (Karanja) and Gliricidia sepium.
#11
Neem (Azadirachta indica) leaves used in green leaf manuring release triterpenoids that deter soil nematodes and suppress pathogenic fungi.
#12
A well-grown green manure crop incorporates between 15 and 25 tonnes of fresh green biomass per hectare into the soil.
#13
Green manuring adds an estimated 60 to 120 kg of biologically fixed atmospheric nitrogen per hectare to the cropping system.
#14
Organic acids released during green manure decomposition lower soil pH in alkaline soils, solubilizing locked phosphorus and micronutrients.
#15
Green manuring with Dhaincha assists in reclaiming sodic soils by displacing toxic exchangeable sodium with solubilized calcium ions.
#16
Incorporating organic biomass increases soil humus, boosting the cation exchange capacity (CEC) and moisture retention of sandy soils.
#17
Dense green manure canopies shade the ground completely, suppressing noxious weed emergence and conserving residual moisture.
#18
The decomposing foliage supplies energy and carbon substrates to earthworms and beneficial heterotrophic soil bacteria.
#19
Green manuring improves the physical crumb structure of clay soils by preventing surface crusting and compaction.
#20
Incorporating fresh succulent biomass requires adequate soil moisture to ensure rapid enzymatic hydrolysis by decomposing microbes.
#21
Farmers in irrigated rice-wheat belts frequently sow summer moong or Sesbania during the fallow period between wheat harvest and paddy transplanting.
#22
Green manuring is a core pillar of organic farming standards (NPOP) and regenerative agriculture to eliminate synthetic urea dependency.

Subject Specialist Commentary

Analytical perspective & practical exam advice from the Master10 academic board

Educator's Insight
Green manure refers to the farming practice of growing succulent, fast-growing plants—primarily legumes like Dhaincha or Sunnhemp—and ploughing them directly into the soil while still green. As this buried green matter decomposes, it enriches the ground with organic humus and biological nitrogen fixed by root bacteria. This natural conditioning technique improves soil aeration, loosens dense clay, retains soil moisture, and substantially reduces a farmer's reliance on expensive synthetic urea.
In UPSC Agriculture, State PSC, and SSC exams, questions often focus on the precise timing and species used in green manuring. Remember that crops must be ploughed under during the flowering stage around 45 to 60 days before stems turn woody. A favorite test trap targets this plant maturity: if plants become fibrous and lignified, decomposing microbes steal existing soil nitrogen, temporarily starving the next crop instead of nourishing it.

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