The layer of soil from which plants get their nutrients is the topsoil.

Explanation:
The topsoil is the uppermost layer of soil, usually ranging from 2 to 8 inches in depth. It is the layer where most of the plant roots are located and where the majority of soil organic matter is found. The topsoil is rich in nutrients that are essential for plant growth and development.

Importance of Topsoil:
1. Nutrient Availability: The topsoil contains a variety of essential nutrients such as nitrogen, phosphorus, potassium, calcium, and magnesium. These nutrients are released through the decomposition of organic matter and are readily available for plant uptake.

2. Water Retention: The topsoil has a high water-holding capacity, allowing it to retain moisture for plant use. This is important for sustaining plant growth during dry periods.

3. Aeration: The topsoil is well-aerated, allowing oxygen to reach the plant roots. Oxygen is necessary for root respiration and the uptake of nutrients.

4. Microbial Activity: The topsoil is teeming with microorganisms such as bacteria, fungi, and earthworms. These organisms play a crucial role in decomposing organic matter and releasing nutrients in forms that plants can absorb.

5. Root Development: The topsoil has a loose, friable texture that allows plant roots to penetrate easily. This promotes healthy root development and enables plants to access the nutrients they need.

Conclusion:
Plants obtain their nutrients primarily from the topsoil, which is the layer of soil where most of the roots are located. The topsoil is rich in essential nutrients, has good water retention and aeration properties, supports microbial activity, and provides an ideal environment for root development. It is therefore crucial for maintaining healthy and productive plant growth.