The urban watershed balance involves sources and sinks that are not always evident in urban areas. The dynamics of phosphorus (P) additions, or sources, include management decisions within the densely populated area and in outlying areas of different landuses that drain through urban areas. These sources of excess “P” can cause problems when they outweigh the sinks for “P” in the same watershed. Urban soil studies to measure and monitor transformations of phosphorus are needed. Urban sources, urban sinks, non-urban sources, and non-urban sinks all contribute to the balance, and all must be included in planning at the watershed level for long-term urban sustainability.
A working knowledge of soil physics, chemistry, and biology is needed for management of phosphorus transformation in urban watersheds. A detailed scientific understanding of the processes and interactions involving phosphorus helps to clarify the limitations of the estimates of phosphorus transport and adsorption. Urban soil survey can guide users in defining needed soil information, measuring soil properties, and estimating the behavior of different soils in a watershed for phosphorus management.
An ongoing debate concerns the choice of on-site versus landscape models for urban surveys to deal with phosphorus and other issues. The next section considers some recent developments in soil survey that can facilitate links between traditional soil survey and the need for more intensive surveys for high-risk urban areas. A series of intensive surveys across a watershed provides data on soil processes, but the landscape model of traditional surveys provides the linkages and pathways for water and phosphorus flow that lead to understanding of long-term sustainability for the ecosystem in the watershed.