Farmer uses bioslurry to cure soil acidity, enhance yields

Friday March 20 2020
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Uasin Gishu farmer, Joseph Tanui, poses next to a vegetable plant during a tour of his farm in Kapseret. He is one of the farmers in the region who are finding bio-digesters useful in the production of bioslurry which they use in their farms. PHOTO | DENNIS LUBANGA | NATION MEDIA GROUP


In the country’s rural villages, tens of farmers are purposely embracing bioslurry – a liquid discharge from bio-digesters after gas has been tapped for energy – as a soil nourishing gem.

Rising soil acidity across many farms is dealing a heavy blow to crop yields that keep dwindling by the season.

Soil analysis conducted across farms in 25 counties in 2014 revealed high acidity levels far beyond the acceptable limit.

The research found the soil pH in breadbasket counties such as Uasin Gishu, Nandi, Trans Nzoia, and Bungoma to be quite low, hence acidic.

Soil from Uasin Gishu for instance, measured between extremely acidic (4.35 pH) to slightly acidic (6.36 pH) on the chemical pH scale. Soils with such levels of acidity are quite unsuitable for farming.

Kenya Agricultural and Livestock Research Organisation (Kalro) that conducted the analysis, linked the acidity to too much use of inorganic fertilisers, particularly Diammonium Phosphate or DAP. It recommended soil specific fertilisers to mitigate the crisis.


Also recommended was the use of liming materials to neutralise the acidity. Unfortunately, only a few smallholder farmers can afford the process, as a tonne of lime costs over Sh4,000.

Some farmers are now using the slurry from bio-digesters as an alternative method of enhancing their farms productivity.

Joseph Tanui, a farmer in Kapreset in Uasin Gishu County, is one such farmer who is finding bio-digesters useful. Tanui grows Boma Rhodes grass on a two-acre plot and vegetables on a quarter acre.

He explained that he collects fresh cow dung from his neighbours which he supplements with those from his own two cows to feed his biogas plant. To make bioslurry, water and cow dung are mixed in a 1:1 ratio then left to ferment.

During the fermentation natural microorganisms break down the manure to produce biogas (in the form of methane gas). Another by-product is, compost, which adds humus in the soil.

Researchers say that if the methane gas is not captured through biogas production, a lot of it ends up in the atmosphere where it damages the ozone layer leading to global warming.


Kevin Kinisu, Kenya Biogas Programme (KPS) country manager, points out that more than 22,000 households in the country have adopted the technology, sequestering over 417,000 tonnes equivalent of carbon emissions per year and contributing to prevention of deforestation of over 34,000 hectares of forest –about 34 times the size of Karura forest in Nairobi.

KPS estimates that there are more than 100,000 people in the country that have installed bio-digesters as biogas plants in their homes.

Among the main uses are energy production and bioslurry fertiliser production. Others, like Tanui, also use the sludge as natural pesticide against farm pests like aphids.

The technology is gaining ground faster in Kenya than in any other African country. The country’s biogas sector has attracted six companies that are all successfully deploying prefabricated systems.

Over 140 entrepreneurs are at work constructing fixed dome masonry type bio-digesters. The technology’s biggest challenge is affordability.

Tanui, for instance, bought his digester in 2013 at Sh150, 000 using a bank loan. Luckily, he was able to repay the loan within a few months from proceeds from the Boma Rhodes sale.

“It has not been easy to reach a large number of farmers since it requires huge investments to educate, train and sensitise households on the business case for having a bio-digester in their homes,” Mr Kinisu observed.

With proper extension services and training of women and youth, he believes it is possible for Kenya to reach 2.3 million households.

A biomass and bioenergy study published in the Elsevier scientific journal in 2014 reported that bioslurry can provide a great amount of nutrients to crops when applied in the soil.

“Typically 5–10 percent of the nitrogen is lost during anaerobic digestion, but bioslurry provides immediately available nutrients that can be applied as needed, reducing risks of nutrient loss,” the authors of the study said.

The study, that was conducted in Sub-Saharan Africa, further noted that while application of untreated manure provides high input of available nutrients, which results in an initial flush in crop growth, there are high risks of losing nutrients due to phytotoxicity, resulting in increased losses by leaching.


Mr Kinisu observed that when passed through a bio-digester unit, cow dung yields 30-40 percent more net energy and about 35-45 percent more nitrogen in the manure as compared to that obtained by burning dung cakes and ordinarily prepared compost, respectively.

He says bioslurry also replenishes organic matter in the soil which chemical fertilisers do not. Organic matter also improves the soil structure, aeration and water retention. In the process, most microorganisms in the soil also thrive.

Experts explain that soils become acidic due to high concentration of hydrogen ions which can be reversed through liming or inorganic manure.

Bioslurry, for example, contains both micro and macro nutrients that are required by crops to thrive as opposed to chemical fertilisers which are prescriptive and thus too expensive if they were to be applied sufficiently.

Dr Abigael Otinga, a soil scientist at the University of Eldoret, explains that high acidic conditions inhibit plants from taking up essential elements from the soil. This leads to poor yields.

She says that while liming materials produce negative ions such as hydroxyl, bicarbonate or carbonates to neutralise soil acidity, inorganic materials such as bioslurry produce negative charges that bind and remove the ions causing the acidity.

“If the soil does not have a negative charge it will not hold water or nutrients,” the soil expert explains. This means that soils will not be productive, even if a farmer applies fertiliser.

Dr Otinga advises farmers to conduct soil tests on their farm to know which kind of fertiliser to apply.

“I don’t buy fertiliser, but use slurry instead. I apply it on my farm before I plant. In fact, my soil is currently very fertile and most of my slurry is used by my neighbours,” Tanui points out.

He grows cabbages, pumpkins and managu (African nightshade). Ever since he began using the bio-digester some seven years ago, Tanui notes that his vegetables have been teeming with health.

It is not only his produce that is flourishing. Tanui believes the use of manure instead of fertiliser might also hold health benefits for his clients.

“In the past my customers used to complain of stomach acidity but ever since I started using animal manure all has been well,” he says.