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

    ·       wand magnets

    ·       hand lens

    ·       beakers

    ·       measuring spoons

    ·       Powerpoint Soil  Formation


    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.


    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.  Go over the Soil Formation powerpoint.

    Pass out the worksheet that will be used for both parts of the lab.

    2.  Activity--TESTING DENSITY BETWEEN MINERALS, COMPOST AND TOPSOIL. Tell students "first we are going to test the density of minerals, soil and compost."  Have a beaker of water ready for teams of students.  They will also need measuring spoons and three soil samples (can do 3 students together so two beakers per table)

    ·       Students will add 5 ml of sand to the water and observe whether it sinks or floats  and record it on their worksheet (it should sink).  They can stir the mixture after they make their first observation. 

    ·       They should then add 5ml of compost (can be taken from the science garden) and record whether it floats (it should mostly float).

    ·        Lastly they should add 5m of topsoil (can be taken from the garden) and record whether it sinks or floats (it should do both).

    ·       Have them tell you their results

    3.  Activity:  DETERMINING THE SOIL LAYER BY AMOUNT OF ORGANICS. Remove the water from the tables and give the students soil samples in bags.  Student must leave the soil samples in the bags.  Just observe through the plastic.   They can use hand lens to make observations.

    For each soil sample the students should

    ·       describe the sample (does it have plant debris or animal remains in it? what color is it?)

    ·       based on their observations and the key on the worksheet, hypothesize what soil layer the sample corresponds to (O, A, B or C)

    ·       Have them tell you their results

    They can then look at the soil from the school (in trays rather than bags).  They can touch this soil.  What layer do they think it is from and why?


    4.  Activity --  Students use a wand  magnet over the outside of the soil sample bags to see if there is any that have magnetite, a magnetic mineral.  If the soils have magnetite they will see small bits respond to the magnet.  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

    final wrap up:

    Ask students if they remember the four main components of soil.  (Minerals, water, air and organic material). 

    What is compost (only organic material).  Does decay happen naturally (yes just usually takes longer). 

    Remember, Soil formation is complex!  It takes a long time to form so we need to take care of soil.  Adding compost helps keep the soils healthy.  They will get to put the class compost in the garden in the next lab.


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