4.3: California State Soil

Many states have a designated state bird, flower, fossil, mineral, etc. In California the state bird is the California Valley Quail, the state flower is the Golden Poppy, the state fossil is the Saber- toothed Cat, and the state mineral is Native Gold. Many states also have a state soil – one that has significance or is important to the state. The San Joaquin is the official state soil of California. Let’s explore how the San Joaquin is important to California and even the entire world.

The Formation of Soil

Weathering is a key part of the process of the soil formation, and the soil is critical to our existence on Earth. In other words, we owe our existence to weathering, and we need to take care of our soil.

Soil forms through the accumulation and decay of organic matter and both mechanical and chemical weathering processes described below. The factors that affect the nature of the soil and the rate of its formation include climate (average temperature and precipitation), the consequent types of vegetation the type of parent rock material, the slope of the surface, and the amount of time available.

Climate

Soils develop because of the weathering of materials on Earth’s surface, including the mechanical breakup of rocks, and the chemical weathering of minerals. Soil development is facilitated by the downward percolation of water. Soil forms most readily under warm temperate to tropical conditions (not cold) and where precipitation amounts are moderate (not dry, but not too wet). Chemical weathering reactions (especially the formation of clay minerals) and biochemical reactions proceed fastest under warm conditions, and plant growth is enhanced in warm climates. Too much water (e.g., in rainforests) can lead to the leaching of important chemical nutrients and hence to acidic soils. In humid and poorly drained regions, swampy conditions may prevail, producing soil that is dominated by organic matter. Too little water (e.g., in deserts and semi-deserts), results in very limited downward chemical transportation and the accumulation of salts and carbonate minerals (e.g., calcite) from upward-moving water. Soils in dry regions also suffer from a lack of organic material.

Parent Material

Soil parent materials can include all different types of bedrock and any type of unconsolidated sediments, such as glacial deposits and stream deposits. Soils are described as residual soils if they develop on bedrock, and transported soils if they develop on transported material such as glacial sediments. But the term “transported soil” is misleading because it implies that the soil itself has been transported, which is not the case. When referring to such soil, it is better to be specific and say, “soil developed on unconsolidated material,” because that distinguishes it from soil developed on bedrock.

Soil Horizons

The process of soil formation generally involves the downward movement of clay, water, and dissolved ions, and a common result of that is the development of chemically and texturally different layers known as soil horizons (figure 4.6). The typically developed soil horizons are:

• O: the layer of organic matter
• A: the layer of partially decayed organic matter mixed with mineral material
• E: the eluviated (leached) layer from which some of the clay and iron have been removed to create a pale layer that may be sandier than the other layers
• B: the layer of accumulation of clay, iron, and other elements from the overlying soil
• C: the layer of incomplete weathering
• R: the parent material or bedrock

Another type of layer that develops in hot arid regions (such as in the Mojave Desert) is known as caliche (pronounced ca-lee-chee). It forms from the downward (or in some cases upward) movement of calcium ions, and the precipitation of calcite within the soil. Well-developed caliche may cement the surrounding material together to form a layer that has the consistency of concrete.

Figure 4.6: Soil Horizons. The R Horizon Would be Below the C Horizon. Image by United States Department of Agriculture is in the public domain.

Like all geological materials, the soil is subject to erosion, although, under natural conditions on gentle slopes, the rate of soil formation either balances or exceeds the rate of erosion. Human practices related to home building, forestry and agriculture have significantly upset this balance.

Soils are held in place by vegetation. When vegetation is removed, either through cutting trees or routinely harvesting crops and tilling the soil, that protection is either temporarily or permanently lost. The primary agents of the erosion of unprotected soil are water and wind.

Case Study - The San Joaquin Series

The San Joaquin soil was initially documented and officially established in California in 1900 and therefore is the oldest, continuously recognized soil series within the state. The process of establishing the San Joaquin as the official state soil in California began with the Professional Soil Scientists Association of California (PSSAC).

Inspired by a visit to the USDA offices in Madera and Hanford, California, science teacher Alex Lehman envisioned a project to connect students and the public with soil conservation. This project, titled "Proposing a California State Soil—Preserving a Legacy and a Commitment to Future Generations," sparked a unique learning experience for students at Martin Luther King, Jr. Middle School in Madera.

Students embraced a cross-disciplinary approach, integrating science, math, English, social studies, and history with conservation education. Social studies classes delved into the lawmaking process, English classes saw students crafting proposals and poems about soil, science and art classes collaborated on science fair displays, and music classes even produced an official state soil song.

The San Joaquin Soil Series became the official state soil of California on August 20, 1997. It was the culmination of the efforts by students and teachers, natural resources professionals, the PSSAC, state legislators, and various state universities.

What is the San Joaquin Soil?

Every soil can be separated into three separate particle size fractions called sand, silt, and clay, which makes up the soil texture. In addition, the arrangement of soil horizons can tell you a lot about the age of a soil and how it developed over time. Some common horizon designations are A, E, B, and C or combinations of those.

Figure 4.7: Soil Profile of the San Joaquin Soil. Image by United States Department of Agriculture is in the public domain.

Typically, San Joaquin soils have a brown to reddish brown surface or surface soil horizon with a loam (a combination of sand, silt, and clay) texture that has an accumulation of organic matter. The next underlying horizon is similar in soil texture to the surface horizons; however, it does not have an accumulation of organic matter. Iron oxides released in this horizon are not masked so much by the organic matter and impart a redder hue to the soil when moist. This horizon rests on a brown or reddish-brown clay or clay loam horizon that has a distinctive prismatic structure (soil broken up into pillar like structures) with cracks between the prisms. This horizon, with a dramatic increase in clay which restricts root and water penetration, is about 15 to 30 inches 38 to 76 cm (38 to 76 cm) from the surface of the soil.

The first characteristic in a San Joaquin soil that people notice is a distinctive soil horizon (layer) known as a duripan to soil scientists and generally as “hardpan” to many people. It is extremely hard, and it can be chipped with mechanical means or through use of a pick or very strong, heavy shovel. However, there is much more to the San Joaquin soil than just the duripan.

Finally, the fourth horizon is the brown to reddish brown silica cemented duripan. The duripan has an abrupt upper boundary at a depth of 50 to 101 cm (20 to 40 inches) and is impervious to roots and water. The duripan often continues to a depth of 152 cm (60 inches) or more.