Most farmers in Niger live below the poverty line and require low-capital intensification to improve agricultural production. The purpose of this study was to evaluate local knowledge on less beneficial weed for future use in managing the soil fertility of the Sahel region. Sida cordifolia was utilized in a scientific procedure to verify how local farmers utilize this species and the nature of the important roles behind this wild plant in terms of soil fertility management. The results of a questionnaire showed that S. cordifolia was less frequently used than other wild plants found in the same area, but that local farmers recognized this plant as one of the indicators of soil fertility in millet fields. The results of soil analysis also revealed a significant coefficient of variance in certain parameters such as total acidity and aluminum saturation of the soil, while multiple regression analysis showed the high influence of total acidity and soil pH on plant height and dry matter weight, respectively. These results indicate that the significant difference in plant growth of S. cordifolia was mainly due to soil acidity rather than soil fertility. The application of organic matter is one effective approach to resolve this problem and it was shown that the poor growth of S. cordifolia is a particularly useful marker to identify patches where organic matter should be applied. Despite its simplicity, this technique is considered particularly beneficial to allocate limited resources efficiently for sustainable, improved production.