The research published in Scientific Reports by Michael O. Alaka, Joseph K. Ogunjobi, Osaretin E. Omoruyi, Olugbenga O. Oluwasina, and Labunmi Lajide explores a sustainable method for repurposing secondary agricultural waste. The study focuses on turning the husks left over from cashew nut oil extraction into a carbon-rich material known as biochar. This material was then tested as a soil improver for cultivating jute mallow, a staple leafy vegetable in Nigeria and many other African countries. The researchers found that this waste-to-wealth approach not only addresses environmental challenges associated with cashew processing but also provides a highly effective, low-cost strategy for improving agricultural productivity. The findings highlight a dual benefit: cleaning up industrial waste while simultaneously boosting the nutritional and physical growth of important food crops.

One of the most significant results from the experiments was the dramatic improvement in plant growth when cashew nut shell biochar was added to the soil. For the jute mallow plants, the application of biochar alone resulted in a leaf number increase of approximately 50% compared to plants grown in untreated soil. When the researchers combined the biochar with poultry manure, the results were even more pronounced. Specifically, the co-application of biochar and manure increased the number of leaves by up to 222.8% in some treatments. This proliferation is crucial because the leaves are the primary harvestable part of the plant. The biochar helps create a favorable environment for the roots, improving soil air flow and water retention, which allows the plants to grow much more vigorously.

The study also revealed that this treatment significantly enriches the soil’s nutrient profile long after the harvest is finished. Post-harvest analysis showed that soil amended with a combination of biochar and manure experienced a 217.7% increase in phosphorus and a 160.6% increase in nitrogen compared to regular soil. This suggests that the biochar acts as a reservoir, catching the nutrients released from the manure and holding onto them so they do not wash away during watering or rain. This improved nutrient retention means that farmers could potentially grow multiple cycles of crops on the same plot without the soil losing its fertility. Furthermore, the soil’s organic matter increased by as much as 148.4%, which helps keep the soil structured and healthy for long-term use.

In addition to physical growth, the researchers discovered that the nutritional quality of the vegetables improved significantly. Jute mallow harvested from biochar-amended soil showed a marked increase in protein content, rising by about 28.4% with biochar alone and up to 67.6% when combined with manure. Healthy fat levels in the leaves also increased by 43% in the best-performing treatments. These changes are particularly important for communities that rely on leafy greens as a primary source of vitamins and minerals. The biochar helps the plant absorb essential nutrients like potassium and zinc more efficiently, leading to a more nutrient-dense food source. Interestingly, while the “good” nutrients went up, the researchers observed a decrease in carbohydrates and ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More, suggesting a shift toward a more protein-rich vegetable.

Another key finding involved the reduction of “antinutrients,” which are natural compounds in plants that can interfere with how the human body absorbs vitamins and minerals. The study found that the combination of cashew shell biochar and manure reduced saponin levels by 81% and phytate levels by 82.4%. By lowering these compounds, the biochar makes the nutrients already present in the jute mallow more “bioavailable,” meaning the human body can actually use more of the vitamins it consumes. This effectively increases the health value of the crop without needing any chemical additives. The researchers believe this reduction happens because the biochar either sucks up these compounds or encourages soil bacteria to break them down before the plant can store them.

Ultimately, the study proves that cashew nut shell biochar is a viable and environmentally friendly alternative to expensive chemical fertilizers. Because it stays in the soil for hundreds of years, it provides a permanent improvement to land quality that traditional fertilizers cannot match. The researchers conclude that this method supports a circular economy by taking a difficult waste product and turning it into a tool that helps small-scale farmers grow more food. By using biochar produced at moderate temperatures, farmers can achieve the best balance of plant growth and soil health. This research offers a clear roadmap for using agricultural leftovers to solve food security and waste management problems simultaneously.

Source: Alaka, M. O., Ogunjobi, J. K., Omoruyi, O. E., Oluwasina, O. O., & Lajide, L. (2026). Transforming extracted cashew nut shell into biochar and its application as soil amender for jute mallow (Corchorus olitorius L.) cultivation. Scientific Reports.