Once the solid rock materials were weathered or the alluvial and colluvial materials and aeolian dunes were deposited, plants began to grow. Nitrogen was added to soil from rainfall and the nitrogen-fixing activity of lichens, algae and legumes. Soil organic matter began to accumulate, and the soils developed a distinct dark-colored A horizon. Rain water began to dissolve and leach the calcium and magnesium carbonates. Some soils were so high in lime the leaching has not depleted the supply and these soils remain calcareous. Generally, the lime inhibits further soil profile development by reducing downward leaching of clay particles.
On the soils formed from dunes and non-calcareous sandstone and shale materials, the leaching process has removed some of the exchangeable basic cations (Ca2+, Mg2+, K+, Na+) resulting in a slightly acidic soil (pH 6 to 7). On the more permeable materials, the rain water has had sufficient time to translocate silicate clay minerals from the A into the B horizon. The clay accumulation filled the pore spaces and decreased the water permeability of the B horizon. This zone of illuvial clay accumulation is defined as an argillic horizon.
In poorly drained soils, little leaching occurs. These soils have suffered from a lack of oxygen resulting in reduction of iron. When this process has occurred periodically, the soil exhibits mottles, while flooded soils exhibit olive and gray colors indicative of gleying.
Because extensive erosional processes have occurred, the youngest soils may overlie the oldest geologic formations (Table 1). Young soils have developed on the stabilized sand dunes, on the flood plains, and on the steep mountain uplands. Some of the oldest soils have developed on alluvial terraces and alluvial fans of the Paso Robles Formation in California.
|10 to 1,000
|1,000 to 10,000
|10,000 to 100,000
Carbonates leachEluviation dominates
|Low to moderate