Ploughing is a common agricultural practice that involves turning over the soil to prepare it for planting crops. It is important to understand the various benefits of ploughing to determine which option is NOT true about this practice.

**Loosening the Soil:**
One of the main purposes of ploughing is to loosen the soil. The plough's blades cut through the soil, breaking up compacted layers and clods, making it easier for plant roots to penetrate. This process also enhances soil aeration and water infiltration, allowing roots to access essential nutrients and moisture.

**Preventing Soil Erosion:**
Ploughing is not primarily aimed at preventing soil erosion. In fact, ploughing can sometimes contribute to erosion if not done properly. When the soil is turned, it exposes the bare soil surface, leaving it vulnerable to erosion caused by wind and water. However, certain ploughing techniques, such as contour ploughing or using cover crops, can help minimize erosion by creating barriers and reducing the speed of water runoff.

**Aerating the Soil:**
Ploughing plays a crucial role in aerating the soil. By breaking up compacted soil, ploughing allows for better air circulation within the soil profile. This helps to create a favorable environment for soil organisms, including beneficial bacteria and earthworms, which contribute to the breakdown of organic matter and nutrient availability for plants.

**Easy Penetration of Roots into the Soil:**
Ploughing facilitates the easy penetration of roots into the soil. When the soil is loosened, plant roots can grow more easily, extending deeper into the soil profile. This allows the roots to access water and nutrients that are essential for plant growth and development.

Based on the above explanations, it is clear that option 'B' ("It prevents soil erosion") is NOT true about ploughing. While ploughing may indirectly contribute to erosion if not practiced correctly, its primary purposes are to loosen the soil, aerate it, and promote easy penetration of roots.

Contour plowing/ploughing conserves soil by following the lay of the land and creating ridges that form a water break to prevent soil erosion.