Which of the following is NOT a primary component of soil?

Soil is primarily composed of four main components: mineral particles, organic matter, water, and air. Water and air play crucial roles in supporting the biological and chemical processes that occur within the soil, while organic matter, which comes from decomposed plants and animals, enriches the soil and supports diverse microorganisms.

Plastic, on the other hand, is not a natural constituent of soil. While plastics can be found in soil due to pollution and human activity, they do not contribute to the soil's natural functions or its ability to support plant growth and ecosystem health. Therefore, identifying plastic as not being a primary component aligns with the fundamental understanding of soil composition in geography.