The crops, the soils and the residue

This blog has traced two crops whose yields have stopped rising on soils that are running down. India's cotton lint yield peaked at 566 kg a hectare in 2013–14 and was estimated at under 450 kg in 2024–25; pink bollworm resistance did much of the damage, but the cotton tracts of Malwa and Nimar are also zinc-deficient in about 60% of soils and short of humus and nitrogen, and Vidarbha's rain-fed black soils carry decades of near-continuous cotton. The rice–wheat system of the Indo-Gangetic plains is the older case: a widely cited 2003 review of long-term experiments across Asia, led by J.K. Ladha in Field Crops Research, found rice and wheat yields stagnating in most of the region's rice–wheat systems and falling in several experiments even where recommended doses of nitrogen, phosphorus and potassium were applied.

What links the two is soil organic carbon. A 2019 global meta-analysis in SOIL by Oldfield and co-authors found maize and wheat yields rise with soil organic carbon up to about 2%, after which the gains level off — and that about two-thirds of the world's maize and wheat land sits below that level. Rattan Lal puts the level soils must regain before they start to recover at 1.0–1.5%. Where Indian cropland sits against those marks is harder to pin down than headlines suggest: as this blog has noted, the national soil-testing series changed its sampling method in April 2023, so recent trends in soil carbon need care.

Both crops also leave a large residue behind. A 2014 inventory by Jain and co-authors put India's crop residue at about 620 million tonnes in 2008–09, of which about 16% was burnt in the field, mostly rice straw, wheat straw and sugarcane trash. A 2025 review of compressed biogas (CBG) from agricultural residues by Kulkarni and Shastri estimates about 234 million tonnes of surplus residue, including 29.74 million tonnes of cotton stalk, and puts the CBG potential of all residues at about 20 million tonnes a year. Cotton stalk is woody, rarely ploughed back, and usually burnt or used as household fuel.

Why cotton stalk is hard to digest

Biogas plants run on microbes that break down cellulose; lignin, the woody glue in stalks, gets in their way. Cotton stalk is among the most lignified crop residues, and the laboratory evidence shows it. A 2018 study in Scientific Reports tested seven pretreatments on cotton stalk: the best, 3% alkaline hydrogen peroxide, produced 192.4 millilitres of methane per gram of volatile solids, 254% more than untreated stalk — implying about 54 millilitres untreated. Grinding alone helps less: a 2021 study in Carbon Resources Conversion found that cutting particles to 3 mm or below raised methane yield by 20–26% and shortened digestion from about 31 days to 25, but warned that the grinding energy has to be paid for.

This blog's estimates. Cotton rows convert the lab methane yields above assuming the stalk is 90% dry matter and 90% of that organic, methane at 0.717 kg per cubic metre and CBG at 95% methane; lab yields are usually higher than a working plant achieves. The paddy-straw row divides Verbio's stated 33 tonnes a day of CBG by its stated 1.0–1.1 lakh tonnes of straw a year, at 330–365 operating days. Revenue is at the ₹98/kg administered price, before pretreatment, collection and operating costs.

The gap is the whole business case. Untreated, a tonne of cotton stalk earns a plant about a third of what a tonne of paddy straw does at Verbio's Sangrur plant, the country's largest residue-fed CBG unit; pretreated to laboratory standard, it would earn about the same — but alkaline peroxide costs money, and no Indian plant has published what it achieves on cotton stalk at scale. Cotton stalk also has a high carbon-to-nitrogen ratio and needs to be digested alongside dung or another nitrogen-rich feed. A January 2025 study of eleven north Indian CBG plants by TERI for the Global Methane Initiative found that none had a laboratory to measure that ratio before loading feedstock.

How much gas each crop's residue gives

Laboratory tests measure a residue's biochemical methane potential: the methane released per gram of volatile solids, the part of the material that can rot. The table collects published results for the residues of India's main field crops, untreated and after the most effective pretreatment each study tested.

Millilitres of methane per gram of volatile solids, laboratory batch tests; pretreatment conditions as given in the text below. "~" marks untreated values this blog derived from the percentage gain each study reports. Sources: cotton, Zhang et al. (2018); rice, Song et al. (2013); wheat, Victorin et al. (2020); maize stover, You et al. (2013); maize straw, Song et al. (2014); sugarcane trash, the range compiled by Formann et al. (2020) and Janke et al. (2017). Each row comes from a different laboratory, inoculum and test length, so compare within a row more than across rows.

Two cautions before reading it as a league table. The same crop can test very differently in two laboratories: the two maize studies differ by a factor of more than two before any pretreatment. And laboratory potentials overstate what a working digester gets. As a rule of thumb, at 90% dry matter and 85% of that organic, every 100 millilitres per gram is worth about 58 kg of CBG per tonne of residue, or about ₹5,700 at the new administered price. On that basis untreated rice straw would give about 89 kg a tonne; Punjab's operating paddy-straw plants, at about 107 tonnes a day of CBG from about 3.5 lakh tonnes of straw a year, are getting about 110 kg — between the untreated and pretreated laboratory figures, as one would expect from plants that chop the straw but do not soak it in peroxide.

Sugarcane trash and maize stover, the two residues added to this piece in its second revision, are covered in their own sections below.

Which pretreatments are worth it

Pretreatment is where residue biogas succeeds or fails economically: a 2017 review in AMB Express by Amin and co-authors put it at 20% or more of total production cost. The options sort into five families.

• Mechanical — chopping, milling, extrusion. Every residue plant does some of this, because bales have to be broken up. It gives moderate gains: 20–26% for cotton stalk ground to 3 mm or below, 21% for roll-milled wheat straw, with extrusion giving the fastest digestion. It produces no chemicals that inhibit digestion, but fine milling uses a lot of electricity; the AMB Express review judged milling alone "not economically viable".
• Alkali — caustic soda, lime, soda ash. The most consistent performers on straw. Caustic soda raised rice-straw biogas by 27–65% in a 2008 study and wheat-straw methane by 37% in a classic 1986 one, and cut maize stover's digestion time from 18 days to 12–13. Lime (calcium hydroxide) gave rice straw 225 mL per gram. At Punjab Agricultural University, Kaur and Phutela found that 4% soda ash with microwave heating stripped 85% of the silica from paddy straw and raised biogas from 217 to 335 litres per kg of straw, a 54% gain — silica being the particular obstacle in rice straw.
• Oxidative — hydrogen peroxide, alkaline peroxide. The biggest laboratory gains: 254% for cotton stalk, 88% for rice straw, 115% for maize straw. Hydrogen peroxide and lime were judged the most cost-effective of seven chemical treatments in the maize study; peroxide is still a purchased chemical at a scale no Indian plant has published.
• Heat and steam. Steam explosion and thermal hydrolysis are proven industrially but energy-hungry, and above about 100°C they can form compounds that slow digestion; the 1986 wheat-straw study found alkaline pretreatment above that temperature lowered methane yield.
• Biological — fungi, microbial consortia, ensiling. Cheap but slow. A microbial consortium has been used on cotton stalk, and Vietnamese household digesters ensile chopped rice straw for about five days before co-feeding it with pig dung.

Whatever the pretreatment, straw and stalk need a nitrogen-rich partner. Digesters run best at a carbon-to-nitrogen ratio of 20–30, according to the rice-straw chapter of IRRI's 2019 Sustainable Rice Straw Management; straw and stalk sit well above that, so they are co-digested with dung or poultry litter.

One consideration the laboratory literature rarely raises matters for the soils this piece is about. Sodium-based pretreatments leave sodium in the digestate, and that digestate is meant to go back on the land — including the canal-irrigated Deccan black soils where this blog has reported sodicity is already a problem. Lime adds calcium instead, which those soils need. This is this blog's inference from soil chemistry, not a finding from a field trial; it is a reason to test lime-based pretreatment before soda-based ones where the manure will be spread on sodic land.

Paddy straw: the residue India is actually building for

Rice straw is the residue at the centre of India's policy, for reasons that have little to do with its gas yield. India grows rice on about 43.8 million hectares, producing an estimated 165.8 million tonnes of straw a year, and burns about 50 million tonnes of it, nearly half in the north-west, according to a 2022 review in Agronomy for Sustainable Development. Farmers burn it because the window between the rice harvest and wheat sowing is short, combines leave loose straw behind, and cattle will not eat much of it — its high silica content makes it poor fodder.

Punjab is where the plants are going up. The Punjab Energy Development Agency has allotted 57 CBG projects with about 816 tonnes a day of capacity; six, with about 107 tonnes a day, were operating in 2025 and using about 3.5 lakh tonnes of straw a year, according to trade-press reporting of PEDA's figures. If all 57 are built they would use about 3 million tonnes a year — against about 20 million tonnes of paddy straw the state produces and about 10 million tonnes reported as available for energy. Storage is the hidden cost: a CEEW survey found companies other than Verbio losing 15–25% of their stored bales a year to decay.

Wheat straw: good gas, but cattle got there first

On the laboratory evidence wheat straw is the best of the cereal straws untreated, and a little milling gets most of the rest. The problem is that it already has a buyer. Across north India wheat straw is chopped into bhusa and sold as dry fodder; in a scarce year dairies in Haryana's Bhiwani and Rohtak paid ₹700–1,000 a quintal, or ₹7,000–10,000 a tonne, The Tribune reported. That is about what a tonne would earn as CBG at the new administered price before any processing cost, so a biogas plant cannot outbid a dairy for it. Some wheat stubble is still burnt — wheat straw was about a fifth of the residue burnt in the 2008–09 inventory — but wheat straw is not the surplus that residue biogas should plan around. Paddy straw and cotton stalk, which cattle do not want, are.

Sugarcane trash: mulch first, gas second

Sugarcane is the crop in this piece where the soil case and the gas case pull hardest against each other. Ratoon crops — regrowth from the stubble of the previous harvest — make up more than half of India's cane area and cut cultivation costs by 25–30%, but farmers seldom keep a field beyond one or two ratoons because yields fall in each successive one, on soils that become compacted and less fertile, according to a five-year experiment at the Sugarcane Research Institute in Muzaffarnagar, Uttar Pradesh, published in 2011 in the Journal of Agricultural Science. That makes cane, alongside cotton and the rice–wheat system, a crop whose yields are held back by its soil.

The trash — dry leaves and green tops stripped at harvest — is large: about 10–15 tonnes a hectare, or 10–12% of the weight of cane harvested, by commonly cited estimates. National totals vary too widely to quote with confidence, from about 18 million to 65 million tonnes a year in published and trade estimates. What is clear is that much of it is burnt: sugarcane trash was a fifth of all residue burnt in India in the 2008–09 inventory, and a review of trash burning puts the nutrients lost at about 41 kg of nitrogen, 5 kg of phosphorus and 31 kg of potassium per hectare.

The best use of trash for the soil is to leave it on the field. In the Muzaffarnagar trial, the best package — trash mulching at 8 tonnes a hectare with stubble shaving, sub-soiling and ridge dismantling — produced 23%, 27% and 29% more cane than trash burning alone in the first, second and third ratoons, and higher soil organic carbon. A Haryana trial reported in the Journal of the Indian Society of Soil Science found a 6-tonne trash mulch raised soil organic carbon by about 20% and cane yield by about 12%; a Pantnagar study found trash and green-manure treatments beat both trash removal and burning on soil carbon and available nitrogen.

As a biogas feedstock, trash is middling. A compilation by Formann and co-authors in Frontiers in Energy Research (2020) puts sugarcane straw at 79–234 millilitres of methane per gram of volatile solids, against 160–281 for filter cake (press mud) and 226–326 for bagasse, and notes its very low nitrogen — 0.5–1.1% of dry matter — and 13–19% lignin. Janke and co-authors found that milling plus caustic soda raised the methane yield of the fibrous fraction from 260 to 291 millilitres per gram, and by up to 28% when the pretreatment liquor was digested too. Collection is the cost: the same review cites about US$50 a tonne of dry matter just to recover straw from the field in Brazil.

That points to a narrow role. Trash that would otherwise be burnt is better digested than burnt, and the natural place to digest it is at the mill alongside press mud, whose nitrogen (1.5–1.9% of dry matter) offsets the trash's lack of it, with the manure going back to ratoon fields. Trash that a farmer would mulch should stay on the field.

Maize stover: more of it every year, because of ethanol

Maize does not fit this piece's pattern of falling yields: India harvested a record of about 43.4 million tonnes in 2024–25, up from 37.9 million the year before. It belongs here for a different reason. Close to 29% of the crop went to ethanol distilleries in 2024–25, and maize became the largest single source of fuel ethanol, supplying about 431 crore litres of the 837 crore litres allocated, according to industry and press reporting. More maize means more stover — and more burning: a 2023 study in Science of the Total Environment found maize's share of India's residue burning roughly doubled from 4% (0.30 million tonnes) in 2011 to 8% (1.60 million tonnes) in 2020.

Maize stover is the easiest of the field residues to digest, and its parts differ sharply. A 2020 Polish study in Energy found methane yield fell with lignin content (correlation −0.86), with stalks the poorest fraction at about 185 cubic metres per tonne of dry matter and needing about 40 days in the digester; leaves and husks gave more, faster. Pretreatment gains are moderate and well documented: 6% caustic soda in the solid state raised biogas by 48.5% and cut digestion time by about a quarter in a 2008 study; 6% caustic soda at 35°C for three hours took methane from 276 to 350 millilitres per gram and digestion time from 18 days to 12–13; mild steam explosion at 160°C for two minutes raised methane by 22% in a 2017 Austrian study, with harsher conditions lowering it.

The soil warning is clearest for maize because it has been studied most, mostly in the United States. A 2009 synthesis in GCB Bioenergy found that harvesting 25–100% of corn residue consistently lowered soil organic carbon by 3–8 tonnes a hectare in the top 30 cm. For India's expanding maize belt the lesson is to take part of the stover, not all of it — leaving the stalks, which digest worst, on the field and sending leaves and husks to the digester — and to return the manure.

Pulses and soybean: better in the manger and the mix than on their own

Pulses and soybean are the legumes in India's cropping systems, and they come into this story twice: as crops with weak yields of their own, and as the rotation partners that the cereal and cotton soils above need. Soybean is the clearest case of a stuck yield. India grew it on about 118 lakh hectares in 2024–25 at about 1,063 kg a hectare, up from 1,002 the year before, by the Soybean Processors Association's estimates; Madhya Pradesh, the largest grower, produced about 55 lakh tonnes from about 52 lakh hectares. SOPA's figures for 2025–26 show Madhya Pradesh's area down by over 6% and its yield down by nearly 14% to 916 kg. A 2013 review by scientists at ICAR's Directorate of Soybean Research put India's average at about 1.1–1.2 tonnes a hectare against a world average of 2.2 and a rainfed potential of about 2.1 — but attributed the gap mainly to farmers not adopting improved practices, not to the soil. The soil does play a part in the soybean–wheat system of the central Indian Vertisols: a long-term study there found that keeping or ploughing in wheat residue, instead of the usual practice of burning it, raised soil organic carbon and available phosphorus, especially with farmyard manure.

As biogas feedstocks, legume residues split three ways.

• Chickpea and lentil straw are fodder, not waste. Pulse straws carry 40–100% more digestible crude protein than cereal straws, according to Indian animal-nutrition guidance, and chickpea and lentil straw are more palatable to livestock than wheat straw. Like wheat straw, they already have a buyer.
• Soybean straw digests poorly alone but helps in a mix. The literature is thin — a 2017 review of harvest residues for biogas noted there was little information on soybean straw — and one recent study found almost no methane from soybean straw alone after 30 days. Its carbon-to-nitrogen ratio, about 37 in that study, is closer to a digester's optimum of 20–30 than cereal straws, which makes it a partner rather than a main feed: a 2017 study that co-digested rice straw with soybean straw after three days of pretreatment with the fungus Trichoderma reesei got 318% more biogas than the untreated mix, and reached 90% of its potential in 17 days instead of 28.
• Pigeon pea (arhar) stalk is woody, like cotton stalk. A 2024 study found that heating pigeon pea stalks to 200°C for two hours produced 88% more biogas than mechanically pretreated stalks — an energy-hungry pretreatment that puts the stalk in the same difficult class as cotton.

The practical conclusion follows the wheat-straw one. Pulse straws should stay in the manger; soybean straw is worth adding to paddy-straw or cotton-stalk digesters as a nitrogen-richer partner where it would otherwise be burnt; and the wider value of legumes in this story is the rotation itself — the nitrogen they fix and the break they give a cereal or cotton monoculture — which no digester replaces.

Who actually feeds India's CBG plants

Residue is the feedstock policy talks about most and plants use least. Of the eleven plants in the TERI study, ten used cow dung, seven used sugar-mill press mud and four poultry litter; one, IOCL's Gorakhpur plant, used paddy straw alongside press mud and dung, and one inactive Haryana plant had used corn stalks. None used cotton stalk. Four of the eleven had shut down, one after the price of dung doubled during the pandemic. The pattern makes sense: dung and press mud are wet, digest easily and sit in one place — at a dairy or a sugar mill. Straw and stalk are dry, bulky, seasonal and scattered across thousands of fields.

The sugar belt is the working model. Press mud, the filter cake left after cane juice is clarified, is already a CBG feedstock across Uttar Pradesh and Haryana, and a 2016 review of sugar-industry by-products estimated that recycling all of India's press mud through agriculture would return about 32,000 tonnes of nitrogen, 28,000 tonnes of phosphorus and 14,000 tonnes of potassium a year to the soil. Verbio's Lehragaga plant shows that straw can work at scale too: about 1 lakh tonnes of paddy straw a year in, 33 tonnes a day of CBG and 600–650 tonnes a day of wet bio-manure out — roughly two tonnes of manure for every tonne of straw. Matching India's 29.74 million tonnes of surplus cotton stalk would take some 270–300 plants of that size; India has 132 CBG plants of all kinds, producing about 920 tonnes a day between them.

The surplus may also be smaller than the reviews assume. A 2023 field-survey study by Kapoor, Navinya and co-authors from sixteen institutes, covering 43 districts, found that the usual top-down estimates rest on broad assumptions about how much residue is spare, put the bioenergy potential of surplus residue at 1,313 petajoules, and concluded that the shortage of residue and the sustainability concerns call for a rethink of how it is used.

What the manure does for the soil

The objection to burning residue for energy is that it takes carbon out of the field. Digestion takes some out too — that carbon leaves as methane and carbon dioxide — but less of the benefit is lost than that suggests. A 2015 review of the evidence by Kurt Möller in Agronomy for Sustainable Development found that the organic matter left after digestion is more resistant to breakdown than the raw feedstock, so it decomposes more slowly once spread, and that the carbon lost in the digester is mostly offset by that slower decay. Digestate rebuilds soil organic matter about as well as applying the feedstock directly or composting it. Nitrogen, phosphorus and potassium stay in the digestate rather than going up in smoke.

The fair comparison for cotton stalk, then, is not between digestion and ploughing the stalk back in, which few farmers do with a woody stalk, but between digestion and burning. Burning returns ash and loses the carbon and most of the nitrogen; digestion returns a stabilised manure. That is the case for linking residue CBG to the crops with failing soils: the plant takes a residue the field is losing anyway and sends back the part the soil needs.

The evidence base for this in India is thin. The digestate studies Möller reviewed were mostly European, mostly of manure and energy crops. This blog found no Indian long-term trial of residue-based digestate on cotton Vertisols or rice–wheat soils reporting both soil carbon and yield. The claim that CBG manure can help reverse cotton's yield decline is plausible on the mechanism; it is untested on the ground.

The pest argument for moving cotton stalk off the field

Cotton stalk carries one extra problem. Pink bollworm larvae overwinter in unopened bolls left on stalks, and the standard advice — from India's Central Institute for Cotton Research and from pest-management services elsewhere — is to clear and destroy stalks soon after the last picking, shred them, and not store them near fields. A 2019 review of bioenergy from cotton stalk notes that the residue must be buried or burnt for exactly this reason, and Indian research on north India's cotton belt identifies removing leftover stalks and bolls as the key to cutting the pest's carryover between seasons.

A CBG supply chain could do that sanitation job if stalks are chipped or shredded at collection and digested promptly. It could also make the problem worse if stalks sit in roadside piles for weeks waiting to be hauled, and residue is gathered over weeks after harvest. Whether residue biogas helps or hurts the cotton belt's worst pest depends on how the stalk is handled between field and digester, which no current guideline specifies.

What the new GOBARdhan scheme pays for

The economics changed in August. The Cabinet approved a central GOBARdhan scheme on 6 August 2026 that brings most CBG support under one roof, according to the scheme handbook published by the Ministry of Petroleum and Natural Gas:

• Price. An administered CBG price of ₹2,110 per million British thermal units, about ₹98 a kilogram of CBG at 95% methane, excluding taxes and compression charges, with affordability support of up to ₹10 a kilogram for ten years.
• Capital. ₹1.25 crore per tonne-a-day of plant capacity, plus up to ₹0.75 crore per tonne-a-day for feedstock-aggregation and manure-processing machinery, capped at ₹30 crore a project. MNRE's waste-to-energy support and MoPNG's biomass-aggregation machinery scheme are folded in.
• Manure. The Department of Fertilizers' Market Development Assistance for fermented organic manure is subsumed from 1 September 2026. Instead, the administered price "includes an incentive component for the sale of by-products", and selling the manure becomes the producer's responsibility.
• Districts. A ₹500 crore ecosystem fund for 2026–27 to 2030–31, including an annual District CBG Challenge and money for technology and by-product gaps.

Two features matter for residue from depleted soils. First, the ₹30 crore cap binds early: plant support alone reaches it at 24 tonnes a day, and plant plus machinery at 15, so a Verbio-scale 33-tonne residue plant gets no extra help for the costly collection equipment that straw and stalk need and dung does not. Second, the manure incentive is now paid through the price of gas. A plant earns it for every kilogram of CBG, whether its manure goes to a zinc-deficient cotton field in Nimar, a nearby orchard or a heap. The scheme handbook bars "unscientific" disposal, but it does not measure where the manure ends up.

What would fix this

• The Ministry of Petroleum and Natural Gas should publish the manure component of the administered CBG price separately and pay it per tonne of fermented organic manure verified as applied to farmland, with a higher rate in districts whose Soil Health Card data show low organic carbon, so the subsidy follows the soil rather than the gas.
• MoPNG should use the GOBARdhan ecosystem fund to finance plant-scale pretreatment trials — cotton stalk in Vidarbha or Nimar with ICAR’s Central Institute for Cotton Research and Central Institute of Agricultural Engineering, paddy straw in Punjab with Punjab Agricultural University, and cane trash co-digested with press mud at a sugar mill with the Indian Institute of Sugarcane Research — comparing lime with sodium-based pretreatment and publishing methane per tonne, cost per tonne and the sodium content of the resulting manure, since every cotton-stalk figure and most pretreatment figures available today come from laboratories.
• ICAR-CICR and the agriculture departments of Maharashtra, Madhya Pradesh, Gujarat and Telangana should issue a stalk-handling protocol for CBG supply chains — shredding at collection, maximum storage time and distance from fields — so that removing stalks for biogas breaks the pink bollworm's carryover instead of spreading it.
• ICAR's long-term fertiliser experiments should add residue-digestate treatments on cotton Vertisols and rice–wheat soils, reporting soil organic carbon and yield, because India has no field evidence yet on whether CBG manure from crop residue reverses yield decline.
• MoPNG should raise or remove the ₹30 crore cap on combined capital assistance for plants whose feedstock is mainly crop residue, so that collection and baling machinery is supported at the scale residue plants need.

These are proposals from this blog, not recommendations any agency has adopted.

Sources and caveats

About this article: Researched, written and edited by Umashankar Triplicane Dwarakanathan, with AI research assistance; every figure is meant to trace to the primary source cited. See the Editorial Policy for how sourcing, AI use and corrections work.