Samuel Rigu is coy about the secret potion he adds to his fertiliser, revealing only that the brown liquid is called "Safi Proprietary Enhancement Formula". He's less shy about the results.

At his processing plant in Mwea, central Kenya, Rigu has found a way of turning agricultural waste into an organic fertiliser that retails at less than half the price of its inorganic alternative.

It's also more effective and kinder to the soil, his customers and scientists say.

Globally, there is renewed interest in organic fertilizer as global prices for chemical inputs skyrocket because of the war in Ukraine.

Demand has spiked since the onset of the Ukraine crisis, he said, prompting him to double the size of his team, who are working 18-hour days and producing 35-45 tonnes of fertiliser per week, double the output before the conflict started.

For now, Rigu's target market is small-holder farmers who have been using synthetic fertilisers for years, but seen their yields decline over time as their land becomes increasingly degraded.

"What has happened actually is that their soils have been slightly turning acidic and when these soils become acidic they become infertile," he said.

"We are here to rejuvenate that, or reverse that."

Rigu decided to enter the fertiliser business in 2013 when he saw mountains of rice husks being burnt, polluting the environment and contributing to greenhouse gas emissions.

First his team sets fire to mounds of rice husks, letting them smoulder for around eight hours, employing a similar low-oxygen burn as used in charcoal production.

When the carbonised husks, known as biochar, have cooled, the team adds the mystery "Enhancement Formula", mixing and turning the black substance until the right level of moisture is achieved.

Biochar such as Rigu's not only adds nutrients to the soil, but has many other benefits such as helping it retain moisture, combat erosion, encourage micro-organisms, and restore acidity levels, said Leigh Ann Winowiecki, a soil scientist at World Agroforestry.

For Rigu, that translates into happy customers like local tomato farmer David Irungu.

"From what I see, my crops are growing well and during harvest my fruits are even bigger than before," Irungu said.