Air is not a major component of soil, as it occupies the pore spaces between soil particles.

Weathering is the process by which parent material is broken down into smaller particles, making it suitable for plant growth.

Sandy soil has the highest percentage of sand, followed by sandy loam, loam, silt loam, silty clay loam, clay loam, and clay.

Soil water-holding capacity is the ability of soil to hold water and nutrients for plant growth.

Massive is not a type of soil structure. The other options, granular, blocky, and prismatic, are all types of soil structure.

The USDA soil classification system is based on soil texture, which is determined by the relative proportions of sand, silt, and clay in the soil.

D horizon is not a soil horizon. The other options, O horizon, A horizon, B horizon, and C horizon, are all soil horizons.

Humification is the process of converting organic matter into humus, which is a stable organic material that is essential for soil fertility.

Stream erosion is not a type of soil erosion. The other options, sheet erosion, rill erosion, gully erosion, and wind erosion, are all types of soil erosion.

Mulching is the process of adding organic matter to soil, such as straw, leaves, or compost, to improve soil structure and fertility.

Physical models are not a type of soil model. The other options, empirical models, mechanistic models, conceptual models, and statistical models, are all types of soil models.

Soil modeling is the process of using soil models to predict soil behavior.

Reduced cost of soil management is not a benefit of using soil models. The other options, improved understanding of soil processes, better prediction of soil behavior, and increased accuracy of soil management practices, are all benefits of using soil models.

Ease of use is not a challenge associated with soil modeling. The other options, lack of data, complexity of soil processes, uncertainty in model parameters, and high computational cost, are all challenges associated with soil modeling.

The future of soil modeling includes all of the above: development of more accurate and reliable models, integration of soil models with other environmental models, and increased use of soil models in decision-making.