The onset of spring brings relief in more ways than one to Delhi. The air is cool and crisp, and with the milder weather come light showers that make the vegetation more lush. Around April, south-westerly winds sweep through the region, and the blanket of acrid smog that covers the city in the autumn and winter months begins to disperse. But it never really goes away.

For the last 10 years, Shaheen Khokhar has witnessed this annual cycle as a resident of Gurugram, south-west of Delhi in the Northern Indian state of Haryana. Around October, when she drives into the city, the unnaturally grey, seemingly overcast skies creep up without warning. "One minute, there's sunshine, and the next, you're engulfed in this dark, smoky haze," she says. "Every day, we see a deeply distressing, visual reminder of the pollution that we're forced to live with."

The effects of that pollution range from skin and eye irritation to severe neurological, cardiovascular and respiratory diseases, asthma, chronic obstructive pulmonary disease, bronchitis, lung capacity loss, emphysema, cancer, and increased mortality rates. Globally, outdoor air pollution kills around 4.2 million people each year.

In October and November, school children across Northern India, especially in India's capital city of New Delhi (which lies within the National Capital Territory of Delhi) and in the vicinity around Gurugram, have to contend with frequent disruptions. As the pollution worsens, schools shut for around two weeks every year. "Our children have worn masks to school long before the Covid crisis," says Khokhar.

Twenty-one of the world's 30 cities with the worst levels of air pollution are in India, according to data compiled in the 2021 World Air Quality Report. Six Indian cities are in the top 10. New Delhi has the highest exposure to toxic air in the country. People in India had the fifth highest annual recordings of fine particulate matter (PM2.5), a particularly harmful form of air pollution. The year-round average for PM2.5 pollution in New Delhi was the worst of any capital city in the world by a large margin.

Industrial pollution and vehicular emissions are some of the greatest factors accounting for toxic air the whole year round, according to a report by the think tank Observer Research Foundation. But in the months of October and November, the pollution grows still more intense because of farmland fires. In 2019, Nasa's Earth observing satellites detected these fires from space.

Exactly how much of the annual peak in air pollution is down to crop burning is uncertain – official figures put it at around 10%, while other research suggests it could be higher. In Delhi, crop burning is thought to contribute as much as 42% of all particulate matter in the air. In the state of Haryana, observed PM2.5 and PM10 (a larger but also harmful form of particulate matter) rise to 2-3 times higher than National Ambient Air Quality Standard limits during the autumn burning season. People from all age groups experience increased respiratory illnesses during this time of year.

In 2015, crop burning was made illegal in Delhi and the states of Rajasthan, Punjab, Uttar Pradesh and Haryana – but the ban has proven hard to enforce. In January 2019, the Indian government launched the National Clean Air Program, a five-year plan aiming to curb worsening levels of pollution across the country and to improve monitoring and awareness. Today, the country may be on the cusp of change. The answer has involved addressing age-old agricultural practices that are worsening pollution, says Ashok Kumar Singh, director of the Indian Agricultural Research Institute.

No fire without smoke

On a pleasant afternoon in September 2021, Dhruv Sawhney, an engineer and COO of nurture.farm, a digital platform for sustainable agricultural solutions, was addressing an audience of 200 men and women – all farmers in a village near Karnal in Haryana. In the open courtyard of a farmer's home, filled with rickety chairs stools and lightweight cots, and surrounded by ripe paddy fields, Sawhney explained how switching to a new method of clearing agricultural residue could help the farmer earn more, and in the long-term, would improve the health of the soil.

As is commonplace in these gatherings, Sawhney was met with some disbelief. One burly farmer in particular wasn't impressed. "Are you sure this will work?" he asked sceptically. "I'd rather set my fields alight and be done with it."

Sawhney paused mid-speech. "If it doesn't work, I'll set your fields alight myself," he joked.

The solution he was urging farmers to try was a new organic microbial spray developed by the Indian Agricultural Research Institute in Delhi. Sawhney's platform, nurture.farm, was one of 12 Indian companies to whom the institute had licensed the use of this technology in August 2021.

Called the Pusa Decomposer, it is composed of seven different species of fungus naturally present in the soil, says Singh. After many lab trials, these species of fungi were found to be extremely effective in decomposing the stubble for energy and nutrients. This microbial spray would completely and rapidly decompose the stubble still left in the fields after the paddy was harvested. Within three weeks, the old stubble would integrate with the soil, acting as compost for the next growing season.

But field tests alone wouldn't be enough to see the spray rolled out among farmers. Understanding why farmers set their fields alight in the first place was critical to developing a solution, says Singh.

"Rice and wheat are predominant crops in India. These are crops that require substantial ground water for good growth," he says. "Ten years ago, rice paddy was cultivated in the beginning of April, during the hot summer months and harvested in September."

However, because of groundwater depletion, the government decided to shift the sowing season to mid-June (when groundwater would be replenished by India's monsoons). The crop would then be harvested in the first week of November.

November is also the ideal time for farmers to grow wheat. "When you delay sowing wheat past 20 November, the yield declines drastically," says Singh. "So now, the farmer has a very narrow window to clear the fields of paddy stalks [residue from the rice harvest] and to prepare the field for sowing wheat. Burning this residue allows them to clear their fields quickly."

The practice flourished from the 1980s, particularly after Indian farmers started using mechanised harvesting techniques that left plenty of paddy stalks stuck in the soil. Prior to this, traditional labour meant harvesting paddy by hand. While this may have been time-consuming, it didn't leave the fields studded with stalks, Singh says. However, as farmers scaled up operations, 23 million tonnes of paddy residue is now burnt every year in Northern India. If you could package up all those stalks into 20kg (44lb) bales and stack them on top of one another, the tower would reach further than the Moon.

From waste to wealth

The fungal spray was not the first solution put forward to deal with the astronomical scale of the problem. In 2014, farmers were given the option to sow a drought tolerant hybrid rice variety that could be harvested in 120 days – that would give them a month to plough their fields manually and get rid of paddy stalk instead of burning the residue. However, hybrid varieties of rice weren't as popular with farmers as they remained unconvinced of their economic viability.

In 2006, The Happy Seeder – a machine devised for sowing could also remove the stubble, mulching and scattering it across the field. And though the government offers it at a 50% subsidy for small farmers, it is still an expensive proposition, especially if you are farming smaller parcels of land. The Indian Agricultural Research Institute noted the machine didn't distribute seeds uniformly and it caused issues with germination. Many farmers saw it as unviable investment.

The fungal spray, too, got off to a bumpy start. Initially, farmers were required to ferment and prepare the microbial solution themselves. Each farmer was given five capsules containing the fungi. They were instructed to add five litres of water to each capsule, 150g of jaggery (a type of cane sugar that acted as a food source for the fungi) and 15g of chickpea (a source of protein). Each capsule was fermented for three days, and 25 litres of this solution was manually sprayed onto the fields over a period of two weeks. Each capsule cost the farmer 60-70 rupees (61-71p/80-93 cents) and could be used over one hectare (2.5 acres).

However, media reports indicated that farmers weren't able to execute the solution effectively and authorities surmised that there were bound to be irregularities in preparing the capsules. The Pusa decomposer is now available in powder form; 300g of the formula is enough to spray on roughly half a hectare of land. Adapting the process has ensured that machines, made freely available to farmers, spray the fields in a more uniform way. The decomposed stubble enriches the soil, reducing dependence on chemical fertilisers by as much as 25%, says Singh. "When farmers burn the crop residue, the temperature of the top layer of soil rises to 42C and ends up killing all the beneficial microbes in the soil. The microbial spray, however, enriches the soil," he says.

The solution tackles not just air pollution, but poor soil health that can compromise our quality of food and water, according to a report led by Natalia Rodríguez Eugenio of the Food and Agriculture Organization of the United Nations. Poor soil health has the ability to impact biodiversity as well, says Rattan Lal, a distinguished professor of soil science at Ohio State University, Columbus.

"Soil in India is increasingly depleted of its organic matter stock," says Lal, who is not connected with the project. "In north-western states such as Punjab, Haryana, Rajasthan and Uttar Pradesh, soil organic carbon content in the surface 30cm (1ft) layer of soil is less than 0.25% and often as little as 0.1%." The optimal range for soil organic carbon should at least be 1-1.5%. He sees the fungal spray as a promising way to address the gap. "This way of using agricultural residue won't have adverse effects on the health of the soil," he says. "However, it's important to keep an eye on the cost and to ensure that the farmer is compensated for adapting a process that protects the planet."

Earlier, some amount of crop residue was used as cattle feed before farmers burnt the rest. Using the Pusa spray could mean that there may be less available to feed India's 553 million livestock, says Lal. "Ensuring that doesn't happen and keeping an eye on the domino effect is important too. A judicious management of crop residue is critical to strengthening the 'one health' concept – the health of soil, plants, animals, people and ecosystems – it's all inter-related," he says.

Krishna Kumar, 48, has been farming for the last 30 years in the village of Bhanan Khera in the Hisar district of Haryana, where he owns five acres of land and has leased another 15. "I was intrigued by the decomposer spray, ever since a relative recommended I try it out," he says.

He used it last year and the crop residue on his lands decomposed completely. It enriched the soil and helped him save around 1,000-1,500 rupees (£10-15) per acre in fertiliser costs. He believes the long-term health of the soil looks promising. "Burning our fields, dealing with all that smoke isn't easy for the farmer either. I'm glad there's another viable option now," he says.

Companies like nurture.farm have stepped in to make the process of deploying the microbial spray much easier for farmers, offering an app where the spraying can be booked for free, as well as offering other paid agricultural services such as equipment hire. Sawhney hopes eventually that the app could become a platform for the sale of carbon credits, due to the emissions saved by fungal decomposition.

Out of 3 million hectares (7.4 million acres) in Punjab under paddy, roughly half was burnt in the last cropping season across North India, says Singh. So far, the Pusa decomposer has been employed across 500,000 hectares (1.2 million acres) in the four states where the majority of the crop burning takes place: Punjab, Haryana, Delhi and Uttar Pradesh.

After being used in this pilot project over a single season (the next phase will span a bigger area), it's still too early to exactly quantify the spray's impact on curbing overall pollution. Stubble burning remains just one part of India's air pollution challenge, alongside industry and transport. But if the spray is adopted on a wider scale, with more farmers and companies signing on, the difference could be quite significant, says Singh. Especially when pollution peaks in October and November.

As the relative respite of spring and changing winds help to thin the smog in the region of Delhi, residents like Shaheen Khokhar can only hope that interventions like this microbial spray might make the onset of the next smog a little more manageable than the season just past. (BBC)

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