What can happen if heavy clay is piled on top of roots?

When heavy clay is piled on top of roots, low oxygen levels in the roots can occur. This is primarily because compacted clay can create a barrier that limits air movement into the soil. Roots require oxygen for respiration, and when the availability of oxygen is reduced, it can lead to stress for the root system. Insufficient oxygen can impair the plant's ability to uptake water and nutrients, ultimately affecting overall plant health.

In contrast, the other possibilities do not accurately reflect the effects of covering roots with heavy clay. For instance, improved drainage is unlikely because clay is dense and water tends to pool rather than drain effectively. Nutrient absorption generally depends on root health and function, which can be compromised in low-oxygen conditions. Lastly, heavy clay can inhibit both root growth and extension, rather than enhancing it, due to the restricted movement of roots through the compacted soil.