Technical Abstract: Differences in ecological sites, and sometimes ecological states, are ultimately due to differences in soil properties and processes within a climatic zone. Soil properties are features of a soil, such as soil texture or soil depth. Soil processes are a series of actions in the soil that bring about a result, for example, water percolating into the soil that determines soil water amounts. Measured relationships between soil properties and soil processes allow us to estimate soil processes given information on a set of soil properties and other variables, such as rainfall amounts. For example, we can predict the rate at which water will percolate into a soil profile if we have data on soil properties such as texture, soil structure, bulk density, and organic matter. How do we determine which soil properties and processes differ among ecological sites or ecological states? This is typically done by observing relationships between soils and the plant communities occurring on them. We inventory plant communities and soil properties within a climatic zone and look for statistical relationships among them. We then draw on research to infer the soil processes that occur and develop hypotheses about how those processes explain plant community patterns. Although soil and landform properties are used to describe the characteristics of ecological sites, it is the soil processes that are controlled by the properties that actually cause differences between ecological sites. Soil processes explain why ecological sites (and sometimes states) differ. The goal of this article is to equip developers and users of ecological site descriptions with a basic understanding of how differences in soils arise, the relationship between soil maps and ecological sites, and how soil properties affect soil processes to create differences among ecological sites and states. Our hope is that this understanding can be used to guide data collection and help develop narratives to explain the properties of ecological sites and states.