Effects of Arbuscular Mycorrhizal Fungi, NPK Mineral Fertilizer and Poultry Manure on Selected Soil Chemical Properties and Performance of Sweet Potato in Sweet Potato/Soybean Intercropping System

In the rich, humid soils of West Africa, a quiet struggle for survival plays out beneath the surface of every farm. Here, the soil is often tired, stripped of the nutrients needed to feed a growing population. Farmers face a difficult choice: rely on expensive chemical fertilizers that can degrade the land over time, or turn to nature's own helpers. Among these helpers are microscopic fungi that live in partnership with plant roots, acting like an extension of the root system to hunt for water and food. This partnership, known as a symbiosis, allows plants to reach nutrients that would otherwise be out of reach. At the same time, organic matter like chicken manure offers a slow-release buffet of nutrients, improving the soil's structure and health. The question facing agricultural scientists is how to mix these tools—chemicals, organic waste, and microscopic fungi—alongside different farming styles, such as planting crops alone or growing them together, to get the best harvest without exhausting the earth.

Researchers at Ladoke Akintola University of Technology in Ogbomoso, Nigeria, set out to solve this puzzle using the sweet potato, a vital staple crop for millions. They wanted to see how this crop would perform when grown alone versus when planted alongside soybeans, a common practice called intercropping. More importantly, they tested a wide variety of soil treatments to find the perfect recipe for fertility. These treatments included two specific strains of the helpful fungi, different amounts of chemical fertilizer, and poultry manure, as well as various combinations of all three. The team worked across two growing seasons, carefully measuring how much the vines grew, how many leaves the plants produced, and, most critically, how many tubers were harvested. They also dug up the soil after the harvest to see how the different treatments had changed the soil's chemistry, looking at levels of nitrogen, phosphorus, potassium, and organic carbon.

The results painted a clear picture of competition and cooperation. When the sweet potatoes were grown alone, they generally outperformed their intercropped counterparts. Without the soybean plants sharing the space, the sweet potatoes grew longer vines and produced significantly more tubers. In the first year, the solo plants yielded nearly 6,000 kilograms per hectare, while the intercropped ones produced about 4,000 kilograms. The second year showed a similar trend, with solo crops yielding over 9,000 kilograms compared to roughly 7,000 for the mixed plots. The researchers found that the soybean plants, while valuable in their own right, competed for light and nutrients, which limited the sweet potato's ability to bulk up its storage roots. This competition meant that the solo crops also absorbed more nitrogen, phosphorus, and potassium from the soil, leaving less behind for the next season.

However, the story of the soil amendments was where the real breakthrough emerged. While the solo crops won on yield, the way the soil was treated mattered just as much. The researchers discovered that mixing organic manure with a reduced amount of chemical fertilizer and a specific strain of the beneficial fungi produced the most impressive results. One particular combination, which included poultry manure, half the standard dose of chemical fertilizer, and the Rhizophagus irregularis strain of fungi, consistently produced the highest yields. In the second year, this specific mix helped the sweet potatoes reach a staggering 12,396 kilograms per hectare. This was far superior to using chemical fertilizer alone or relying on the fungi without the other inputs. The fungi acted as a bridge, helping the plants access nutrients more efficiently, while the manure improved the soil's ability to hold onto those nutrients, and the chemical fertilizer provided an immediate boost.

The study also revealed how these treatments changed the soil itself. The intercropped plots tended to leave the soil with higher levels of potassium and, in some cases, higher pH levels, likely because the sweet potatoes in those plots were less vigorous and removed fewer nutrients. In contrast, the plots with the best yields removed more nutrients, which is expected when a crop is thriving. Yet, the soil health indicators told a positive story for the integrated treatments. The combination of manure, fungi, and reduced fertilizer led to the highest levels of organic carbon and available phosphorus in the soil. This suggests that this method not only feeds the current crop but also builds a richer, more fertile foundation for future harvests. The fungi, in particular, seemed to thrive when paired with the organic manure, colonizing the plant roots more effectively than when they were used with chemical fertilizers alone.

Ultimately, the research points to a balanced approach for farmers in the region. While growing sweet potatoes alone offers the highest potential yield, the most sustainable and productive path forward involves a smart mix of inputs. The study confirms that relying on a single method, whether it is just chemical fertilizer or just organic manure, is less effective than combining them. By integrating poultry manure, a specific beneficial fungus, and a reduced dose of mineral fertilizer, farmers can maximize their harvest while simultaneously improving the long-term health of their soil. This approach offers a practical solution to the challenge of feeding a growing population on land that is often nutrient-poor, proving that the best way forward is often a partnership between nature's own tools and human ingenuity.