Lesson 1 - Page 1


The soil that we walk on is an underground city. In this city, every organism does its part to make sure the city works together. The actions of the animals, insects, and microorganisms that live in the soil not only influence what happens in the soil but also what happens above the soil. Soil is made of inorganic matter (mainly rocks and minerals) mixed with organic matter. Soil is formed from the weathering of minerals derived from bedrock and contains living organisms and the products of their decay. Soil can be considered a mixture of mineral materials, organic matter, water, and air in varying proportions. Topsoil, the A horizon, is usually the upper ten inches of a soil in a well-developed soil profile. Plant roots, bacteria, fungi, and small animals are abundant in this area along with plants who thrive in this type of environment. Topsoil has less organic matter than in the O horizon (the surface) which is the reason that topsoil is lighter than surface soil. Topsoil is one of our most valued commodities since it provides the nutrients and environment for the growth of plants. 
O horizon
organic material dead plants, animal material

A horizon
plant roots, bacteria, fungi, small animal

B horizon
Fewer organisms less topsoil; plants don't grow well

C horizon
Altered Parent Material:
Weathered, less living matter layers above were formed form it

Subsoil or the B horizon, is the middle soil layer. It has fewer organisms and less organic materials than both the A and O horizons. Consequently, the B horizon cannot support the growth of plants very well. If subsoils are clayey, they usually are harder when dry and sticky when wet than the surrounding soil layers.

The C horizon is the lowest layer and is partially weathered parent material from which the other horizons are formed. It is less altered and weathered than the layers above and has less living matter. The parent material is sometimes named the D horizon. 


The constituents of soil are extremely variable in size, shape and chemical composition. The size of particles is one of the most significant characteristics. Water absorption, air movement, rate of solution and ease of tillage are a few things that are affected by particle size. 

The texture of soil refers to particle sizes and is classified on an arbitrary scale. It can be coarse, sandy, or clayey. Sand would be about the size of sand, coarse would refer to soil that is larger and clayey would be smaller. You can also describe the structure of soil by how the soil particles tick together. When particles are rather porous and small, the soil is considered to have a granular or crumby structure, which is characteristic of many soils high in organic matter. Soil that is lumpy stick together. Sometimes soil has magnetite in it, a magnetic mineral that is attracted to a magnet.

Humus, the partially decayed organic matter accumulated in soils, is a dark-colored structure less material. 

Soil horizons can be different for high productive areas versus low productive areas. The ideal soil horizon as shown in the Pre Lab, may not be present in all areas. You can use the following to help guide you with your students. 


1. contains more organic matter in most areas, most weathered and leached at all levels, loose, easily tilled, fertile
2. Yellow layer containing small quantities of clay and easily penetrated by air, water, and plant roots
3. slightly weathered, permeable, calcareous


1. light gray layer, low in fertility and difficult to till
2. heavy clay layer impermeable to air, water, and plant roots, massive stable aggregates of small particles
3. heavy clay parent matter

An ecosystem is a community of plants and animals, which consists of many individuals and populations. Ecosystems can be any size from a small puddle containing algae and protozoa to the Amazonian rain forest. An ecosystem is made of two components: the physical environment and the biological community. 

In soil, animals, insects, and microorganisms help to maintain a cycle that is very important to the survival of life, the nutrient cycle. Animals such as rabbits, gophers, and badgers use the soil for shelter and food. 

These animals burrow into the ground and cause large pieces of soil to be loosened. Once the larger pieces of soil are loosened, it is easier for insects and worms to travel and move about in the soil. The moving action of these insects and worms causes the smaller particles of soil to be loosened and mixed with air and water that has penetrated the soil. The air and water can easily enter the soil once it is loosened. The air and water mix with nutrients and creep down into the soil and provide the necessary growing conditions for plant roots. These roots absorb the air, water, and nutrients, and provide food for humans and animals.

Soil microorganisms such as bacteria break down organic materials and rock and release nutrients. Without this breakdown, the soil would not have the nutrients for use by plants. These organisms that break down organic material are called decomposers and are responsible for the fertility of the soil. Although your students may not see all this occurring, they can start to understand the importance of soil. 

Define pH on the board by telling students that the more reactive hydrogen in a liquid the more acidic it is; the more hydroxide ions (hydrogen + oxygen) the more basic it is. Water is neutral because it has one H+ ion and one OH- ion, so they balance out and are neutral. It is not important that the students really understand what pH is, but that they understand what the numbers refer to. Advertisers refer to pH all the time, especially in soap and shampoo products.

Measuring whether a substance is an acid or a base is not difficult using litmus paper. However, this is not quantitative, chemists use what is called a pH scale from 0-14 to measure the intensity of being an acid or base. Water, which is neutral is a 7, 0 is a strong acid and 14 a strong base. Lye is 13, bleach is 12, ammonia is 11, milk of magnesia is 10, borax is 9, baking soda is 8, blood and milk are 7, orange juice is 4, vinegar is 3, lemon juice is 2, and battery acid is 1. These numbers refer to the concentration of hydrogen ions in a solution. In this lab, students will use the pH indicator or litmus paper on solutions of soil to try to figure if the acidity or basicity of a soil will affect the plant growth on that soil. 

The pH of mineral soils varies from values of 3 or less in very acidic soil near the coastlines to more than 10 in alkali soils of some arid and semiarid areas. In order for a soil to be productive in humid areas it must have a pH between 5-7 and arid regions must have a soil pH between 7-9. An indicator is a substance that can determine the presence of an acid or base. Indicators change color when they come in contact with an acid or base. Litmus is red (pink) in acid and blue in base. Citrus fruits and vinegar are examples of acids and bleach and ammonia are examples of bases. In this lab, the students will determine the actual pH of a soil by making a solutions and comparing the color of pH paper. Litmus paper can also be used but the samples must be cleaner than what is needed for an indicator solution.

The soil specimens that the students have looked at come from two sides of the San Andreas fault in California. The San Andreas fault has moved one side relative to the another and has allowed different types of rocks to abut right next to each other. On the worksheet you can see that west of the San Andreas and Pilarcitos Faults, there are granitic rocks. East of the Pilarcitos Fault you have rocks composed of serpentinite.

The parent rock material weathers, releasing the elements that compose the minerals of the rocks. It is these elements that provide nutrients for the plants to grow. Students will see in LIFE CYCLE - NATURAL ENVIRONMENT (6B) that the plants are very different. The rock serpentinite is composed mainly of the mineral serpentine. The principle minerals of the serpentine group all have the approximate composition H4Mg3Si2O9, The principle minerals in a granitic soil are quartz, feldspar, hornblende, and mica. 

The factors that influence soil formation include climate (particularly temperature and precipitation), living organisms (especially native vegetation and human beings), the nature of parent material (including texture and structure, and chemical and mineralogical composition), topography of area, and time that the parent materials were subjected to soil formation. 

The process of disintegration of solid rock makes possible a foothold for living organisms. Decomposing minerals release nutrients that nourish simple plant and animal forms. Rocks are broken down by mechanical or chemical mechanisms. Mechanical weathering or disintegration is effected by temperature (differential expansion of minerals, frost action), erosion and deposition (mainly by water, ice and wind), and plant and animal influences. Chemical weathering or decomposition can be accomplished by the chemical processes of hydrolysis, hydration, acidification, oxidation, and dissolution. It is the chemical weathering that frees elements into an ionic state that makes them available for plants to use.



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