What aspect of soil varies in texture and structure?

The correct choice is composition because it encompasses the various components that contribute to the soil's texture and structure. Soil composition refers to the mixture of organic matter, minerals, water, and air that make up the soil matrix. Different proportions and types of these materials can drastically alter the properties of the soil, including its texture (which is determined by the relative amounts of sand, silt, and clay) and structure (which refers to how soil particles are arranged).

For example, soil with a higher proportion of sand will have a coarse texture with larger particles, allowing for good drainage but lower nutrient-holding capacity. Conversely, a soil with more clay will be finer, leading to a denser texture that retains moisture well but can become compacted easily.

Understanding soil composition is crucial for horticulture because it influences plant growth, moisture retention, nutrient availability, and aeration. While color, density, and pH level are all important soil properties, they do not directly address the variations in texture and structure as comprehensively as composition does.