Mycorrhizae / Microbes
Mycorrhizal fungi are common in all types of soil where plants grow. They make a large root network growth throughout the soil. They associate with the root system to create a greater association with plants.
Mycorrhizal fungi are not planted roots. They only have similar behaviour with the plant roots. Just like roots, they burrow into the soil to collect nutrients and water for the plants. To sustain the fungi, the leaves of the plants send sugars to them as food.