Drainage Design Manual - the City of San Luis Obispo

11.4 Planting Site Considerations11.4.1 SoilsConsideration of the properties and condition of site soils is very important to determinethe potential for establishing revegetation plants. Soils vary in fertility, and incharacteristics that affect site drainage and root development. The fertility of most soilsoccurring in the SLO Creek watershed is generally adequate for establishing revegetationplant materials. Where fill is present, or where the upper layers of soil have been highlydisturbed or removed from a site, testing for fertility may be warranted. In situationswhere soils have not been disturbed, or only slightly so, fertility will generally not be asignificant issue.Soil characteristics that affect drainage and root development often present morerevegetation problems than do fertility issues. This soil movement can shear and damagethe roots of revegetation plant materials. If large cracks develop on a site, effectiveirrigation of young revegetation plant materials may not be possible. Furthermore, claysoils subjected to construction activities can become severely compacted and difficult tocultivate. Because of these physical properties, establishing revegetation plants on heavyclay soils can be difficult. Special consideration should be given to selecting appropriateplant materials, irrigation methods and plant establishment procedures for insuringrevegetation success on these soils. Plant losses can be minimized by planting largercontainer sizes and/or providing enough irrigation to prevent excessive soil shrinkage andcracking during the dry season.Soils at construction sites such as bank stabilization and channel modification sites areoften highly compacted as a result of grading and structural requirements. In manysituations, soil preparation is necessary to facilitate successful reestablishment ofvegetation. Typical soil preparation methods include ripping, disking and/or rototilling toimprove soil porosity for greater gas exchange and water infiltration. Often organic soilamendments and fertilizers should be incorporated into soils disturbed duringconstruction. The need to "open-up" or "loosen" soils in channel environments must bebalanced against the potential for erosion.Revegetation problems may also occur that are related to the stratification characteristicsof alluvial sediments in natural, undisturbed soils. If stratification is pronounced, withlayers of clays or other textures differing significantly from those above it, root and waterpenetration may be restricted. Such restrictions may limit the downward penetration ofroots as well as the capillary rise of water into soil layers above the groundwater table. Ifsandy or gravelly soils are encountered, their low water holding capacities can makeestablishing revegetation species especially challenging.Soil testing is seldom required for revegetation projects where the goal is to establishnative vegetation that will be sustained by the available resources at a project site. Insome situations certain tests may be necessary to determine the presence of toxins in thesoil. If soil contaminants are suspected, testing should be conducted early enough in theSLO Creek Drainage Design Manual 138 February 2003