Fourth Grade NGSS
Biogeology and Heat Generation

Identification and Classification of Soil


  • Investigating natural soil
  • Comparing compost with natural soil

  • soil horizon
  • exothermic reaction
  • topsoil

  • worksheet on soil
  • different soil samples


    Soil is composed of organic matter and broken down rocks.  The organic matter is from other surrounding life that has started to mix with the small rocks.  Many soil dwelling organisms spend their lives breaking down dead animals and plants, releasing nutrients for use by growing plants.  These decomposers, sometimes called reducers, are responsible for the fertility of the soil.  

    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 usually sticks 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.  Making compost would simulate and speed up nature’s way of making humus.     

    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.


    A.  contains more organic matter in most areas, most weathered and leached at all levels, loose,  easily tilled, fertile

    B. Yellow layer containing small quantities of clay and easily penetrated by air, water, and  plant roots

    C. slightly weathered, permeable, calcareous


    A. light gray layer, low in fertility and difficult to till

    B. heavy clay layer impermeable to air, water, and plant roots, massive stable aggregates of  small particles

    C. heavy clay parent matter
    Nutrients in the soil are important to plants in order to survive.  Nutrients can be complex organic molecules like carbohydrates, fats or protein.  They can also be inorganic like zinc or copper.  However all nutrients are composed of elements in a chemical state that can be used by the organisms. 

    In a process called photosynthesis, plants use energy from the sun to change carbon dioxide (CO2 - carbon and oxygen) and water (H2O- hydrogen and oxygen) into starches and sugars. These starches and sugars are the plant's food.  Photosynthesis means "making things with light". Since plants get carbon, hydrogen, and oxygen from the air and water, there is little farmers and gardeners can do to control  how much of these nutrients a plant can use.

    The 13 mineral nutrients, which come from the soil, are dissolved in water and absorbed through a plant's roots. There are not always enough of these nutrients in the soil for a plant to grow healthy. This is why many farmers and gardeners use fertilizers to add the nutrients to the soil. 

    The mineral nutrients are divided into two groups:  macronutrients and micronutrients. 

    Macronutrients can be broken into primary and secondary nutrients.  The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking from the soil first because plants use large amounts for their growth and survival.  The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil so fertilization is not always needed. Also, large amounts of Calcium and Magnesium are added when lime is applied to acidic soils. Sulfur is usually found in sufficient amounts from the slow decomposition of soil organic matter, an important reason for not throwing out grass clippings and leaves.

    Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities . These elements are sometimes called minor elements or trace elements. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micronutrients (as well as macronutrients) to growing plants.

    Notice that all the components are ultimately composed of chemicals.  However, there are inorganic components that are "given" or specific to an area.  The ecosystem has to build itself on soil (organic matter + rock).   Organic matter is carbon based, but rocks can be made of a variety of chemical compounds which add character to a particular soil. Remember, minerals make-up rocks, and minerals can be composed of elements or compounds. 



  • 1.    A soil profile is a slice of earth several feet deep that illustrates the layers of soil.  Most soil profiles have a surface layer of organic material and two or three layers of soil layers with different characteristics.  Students in this lab will look at 4 samples representing ideal O, A, B, and C horizons.  

    2.    Give students soil samples, magnet, microscope, and beaker of water.  It would be ideal to get local samples for this lab and to record where the sample came from. They will only use the water for samples collected by the teacher, the reference collection should only be observed.  It is difficult to dry soil samples. 

    3.    Instruct the students to look at the reference soil samples under the microscope and describe what they see.  They should ask themselves if the sample has broken up rocks or very fine clay particles.  They should also see if there are other distinguishing characteristics like plant debris or animal remains

    4.    If you have time you may want students to go outside and dig a hole to see the soil horizon around the school.  This may be difficult in some areas.  Students could collect some of the samples.  If not enough time is available,  the teacher should pre-collect the samples.

    5.    Instruct students to put a little amount of the soil in some water to see if anything floats.  Plant debris that may not have been obvious under the microscope may float. 

    6.    Use the magnet to see if there is any magnetite, which is a magnetic mineral.  You can use the magnet with the reference collections. The presence of magnetite means that the parent rock may have been granitic.  Magnetite erodes out of the rock and is left in the soil



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