Soil A has a pH of 7.5, which means it is slightly alkaline, while Soil B has a pH of 4.5, indicating that it is quite acidic. To adjust the pH of soil, powdered chalk (which contains calcium carbonate) is typically used to raise the pH of acidic soils.
Therefore, Soil B should be treated with powdered chalk to raise its pH, making it less acidic and more suitable for plant growth.