Final answer:
Stoniness or rockiness becomes a serious limitation for soil when it contains 20% or more of rock, affecting root growth and overall soil health.
Step-by-step explanation:
A serious limitation occurs due to stoniness/rockiness when the soil contains 20% or more of rock. Soil should ideally contain 50 percent solid material and 50 percent pore space, with about half of the pore space containing water and the other half containing air. The composition of soil, including organic matter, inorganic mineral matter, and the balance of water and air, is crucial to support plant growth and maintain soil health. Excessive stoniness restricts root growth, reduces the available water and nutrient supply to plants, and can greatly hinder agricultural activities.