Correlation Between Integrated Soil Fertility Management (ISFM) Induced Selected Soil Chemical Properties, Crop Yields and Nutrients Uptake in Sweet potato/Soybean Intercrop

In the warm, humid regions of the tropics, farmers face a constant struggle to keep their soil productive. The ground often lacks the essential nutrients plants need to thrive, and the cost of buying chemical fertilizers can be prohibitive for many. To solve this, scientists look toward a strategy called Integrated Soil Fertility Management. This approach does not rely on a single solution but instead blends three types of inputs: organic matter like compost or manure, which feeds the soil slowly; mineral fertilizers, which provide an immediate burst of nutrients; and biological helpers, such as beneficial fungi that live in the soil and help plant roots reach further. The goal is to create a system where these different elements work together, improving the soil's health while boosting the amount of food harvested. This balance is particularly critical for intercropping, a practice where two different crops are grown side-by-side in the same field. One crop might be a heavy feeder that demands lots of nutrients, while the other is a nitrogen-fixer that pulls gas from the air and turns it into food for itself and its neighbor. Understanding how to manage these relationships is key to feeding growing populations without degrading the land.

Researchers in southwestern Nigeria recently put this concept to the test in a field experiment involving two vital crops: sweet potato and soybean. The study took place over two growing seasons, 2024 and 2025, at a university farm in Ogbomoso. The team set out to see how different combinations of soil treatments would affect the chemical makeup of the soil and the final harvest. They established twelve distinct treatment groups, ranging from doing nothing at all to applying specific mixes of poultry manure, mineral fertilizer, and two different strains of beneficial fungi known as arbuscular mycorrhizal fungi. These fungi act as extensions of the plant's root system, helping the plant absorb water and nutrients more efficiently. The researchers applied these treatments to plots of land where sweet potato vines and soybean seeds were planted together, carefully measuring everything from the soil's acidity and nutrient levels to the weight of the harvested tubers and seeds.

The results painted a clear picture of how these different inputs interacted. When the researchers looked at the soil after the harvest, they found that the untreated plots, where nothing was added, actually ended up with the highest soil pH, a measure of acidity. In contrast, the plots that received mineral fertilizers, especially when mixed with the fungi, became slightly more acidic, a common reaction when plants actively take up nutrients. The most significant changes appeared in the soil's organic carbon and nutrient content. The combination of poultry manure and one specific strain of fungi, Rhizophagus irregularis, proved particularly effective at boosting the amount of available phosphorus in the soil, a nutrient that is often scarce and difficult for plants to find. In 2024, this mix raised phosphorus levels to 20.3 milligrams per kilogram, and in 2025, it reached 37.3 milligrams per kilogram, far outperforming the other treatments.

When it came to the actual harvest, the benefits of these combined treatments were undeniable. The sweet potato tubers, which are the primary food source in this pairing, responded dramatically to the integrated approach. The highest yield came from a plot that received poultry manure, the Rhizophagus irregularis fungi, and the full dose of the mineral fertilizer blend. In 2024, this treatment produced 7,250 kilograms of sweet potatoes per hectare, and in 2025, the yield surged to 12,542 kilograms per hectare. By comparison, the untreated control plots produced only 2,167 kilograms in 2024 and 3,083 kilograms in 2025. This massive difference highlights that the crops were not just surviving but thriving when the soil was managed with a mix of organic and biological inputs. The study also tracked how much nitrogen, phosphorus, and potassium the plants absorbed. The data showed a strong link between the amount of nutrients the sweet potatoes took up and the size of their harvest; the more nutrients the plants could access, the more tubers they produced.

The soybean component of the intercrop also showed interesting responses, though the results varied slightly between the two years. In 2024, the combination of poultry manure and the Rhizophagus irregularis fungi produced the highest seed yield at 1,332 kilograms per hectare. In 2025, the highest seed yield was observed with the sole application of mineral fertilizer (163 kg/ha), slightly edging out the poultry manure and fungi combination (159 kg/ha). This suggests that while the biological and organic mix is powerful, the specific needs of the soybean can shift depending on the season. The study also measured how much nitrogen the soybeans pulled from the air, a process known as nitrogen fixation. The researchers found that when the soil already had high levels of nitrogen from fertilizer, the soybeans relied less on fixing their own nitrogen from the air. This confirms a well-known biological trade-off: if the plant can easily find nitrogen in the soil, it stops working as hard to create it from the atmosphere.

Ultimately, the research demonstrates that a balanced approach to soil management works better than relying on a single type of input. The study found that mixing poultry manure with specific beneficial fungi and a reduced dose of mineral fertilizer created the best conditions for both crops. This combination improved the soil's ability to hold nutrients, increased the organic matter, and led to significantly higher yields of sweet potatoes. The findings suggest that farmers in similar tropical environments can achieve better harvests and healthier soil by moving away from heavy reliance on chemical fertilizers alone. Instead, integrating organic matter and biological helpers with a modest amount of mineral fertilizer offers a sustainable path forward, turning the soil into a more productive and resilient foundation for food production.