Clay minerals, due to both their very large surface area and to their chemical structure, can retain the nutrients from decomposed plant material and slowly release these nutrients to growing plants. Larger, quartz particles in soil do not do this. Soil particle sizes are clay (smallest), silt and sand (largest). The distribution of particle sizes in soil is called soil texture. A sandy soil consists of mostly sand size particles, whereas a loam will have a mixture of clay, silt and sand size particles. Blue Sky soils have a lot of sand size particles in them because they are derived from granitic rocks, but as they have been weathering for many thousands of years, they have some silt and clay size particles as well. The texture of all Blue Sky soils is sandy loam, which mean they contain some clay and silt size particles, but are predominantly sandy. Sandy loam soils are often favored by gardeners. The high percentage of sand means that the soil will generally have good drainage and good aeration. However, sand size particles don’t retain or supply the nutrients that plants need as well as silt and clay size particles do.
The four soil series at blue sky
The soils in Blue Sky are derived from similar material and have similar texture; the main factor that distinguishes them is their position in the landscape. This will give the soils different characteristics, primarily soil depth and organic matter, that are then used to assign them to different soil series, each with its own name. Soil depth is very important to plants, because it means there is a greater volume of soil for their roots to take up nutrients and water. Landscape position also influences the erosion of soil and the movement of water, which in turn affects the soil, decreasing its depth and organic matter content, and then the vegetation. In consequence, the soil series at Blue Sky are generally associated with one of the 4 habitats.
The Visalia soil is found at the bottom, flat part of the landscape, called the toeslope, near the stream. It is formed from particles that have been carried by water from further upslope and deposited at the bottom of the slope. This material is called alluvium. These soils can be fairly deep (60 inches) and are also in a position where the most water is available in the landscape. This is where the riparian habitat and parts of the oak forest habitat at Blue Sky are found. This biome includes trees such as sycamore and willow, which need water near the surface to survive. Because of the lush vegetation found in this biome, the soil has a lot of organic matter (2.5%) in its surface layer (0-12 inches). This soil is usually dark grayish-brown.
The other 3 soils at Blue Sky are located on the footslopes and backslopes. They differ in depth, with the Cieneba soil being the shallowest soil (4 to 20 inches), followed by the deeper Vista soils (20 – 40 inches) and then the deepest Fallbrook soil (40 - 60 inches) and the degree to which weathering has occurred, with Cieneba being the least weathered and the Fallbrook being the most weathered. Due to landscape position, the Cieneba soil is the most subject to erosion and consequently is younger than the Vista or Fallbrook soil series. You can see Cieneba soils along road cuts as you walk up to the Ramona dam. They are young soils that form on slopes (9-85%) from weathered granite. This soil is primarily associated with the coastal sage scrub habitat within Blue Sky but it is also found on the upper north facing slopes that you can see from the Toretto and Green Valley Truck trails. These soils are often rocky (up to 35% rock fragments). They have a surface layer (A) a bit enriched in organic matter (<1%) on top of weathered granitic material that still has some rock structure (Cr) and may have some cracks in it that roots can grow through.