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Testing soil.

posted Feb 2, 2015, 3:35 AM by Ranmini Perera   [ updated Dec 3, 2015, 8:08 PM by Upali Salpadoru ]

Is garden soil a Chemical?

Yes of course!  But it is neither an element nor a compound.
 It is rather a mixture of many compounds; and there may even be elements. Look at the two soil samples in this picture.

Fig.1 They have two soil samples. 

It is well known that soil has air and water in between sand and clay. There can also be decaying organic matter from plants and animals which is called 'humus'. More over two samples

of soil collected from two places may not have the same colour and texture This can be
clearly seen by the samples of Ali and Nelly. But soil is generally spoken of as a
substance. . but it is not a pure substance such as pure water or oxygen.






Size  2mm. to 0.05mm 0.05mm to .002 mm Less than 0.002 mm


Fig. 2  Sand, silt and clay in soil.
The texture of a soil sample may be  observed through a  magnifying glass. It will be clearly seen that all soil samples consist of particles of varying sizes. There are two basic methods to separate these particles.   The first method is to pass the dry sample through strainers of different sieve sizes.  Strainers can easily be made from large tin cans or plastic bottles.

 Fig.3. Separating the constituents of soil by sieving.

Ali made holes, using a 2mm wide nail and added the dry soil sample. He kept a tray below and sifted the contents. Silt and clay dropped into the tray no.2.  Nelly took this mixture and sieved through a large, old kitchen sieve.

Ratio of sand and clay
Mix the soil sample with water in a bottle. Then allow it to settle. It will take a day or two(longer you keep it better it is).  Then you will find sand, clay and gravel in different layers. Separating lines may not be as clear as in the diagram. This process of separating the constituents of a mixture is called ‘sedimentation’.

Fig.4. Sedimentation of garden soil. 

Fig. 5.  A page from Ali's Note book.

Measure the height of each layer and mark them in your note book as shown here.


Finding the soluble matter

Do you think the clear water above the soil layers  pure water?   If sugar or salt is dissolved in water the solid substance (solute) does not show. Whenever a white substance is fully dissolved in water , the water remains clear. So it may be the clear water obtained by mixing soil and sedimentation may also have dissolved solid substances.

Ali used a very simple experiment to find this out. He  decanted some water above the layers of soil and took a drop of that with a straw and carefully placed it on a clean sheet of glass. He put a drop of rain water also by the side as a control. Then he exposed the slide with the two drops to bright sunlight. If there are any soluble substances in the soil the drop of water taken from the soil washings would leave a mark. This can be compared with the marking from the rain water.  Purest form of water (distilled water) should not leave a mark.

Fig.6 Evaporating the drops of decanted water after sedimentation. 

More about soil

Soil is definitely a mixture, a mixture of several compounds. Elements are not likely to be present in soil, unless some discarded metals or charcoal (Carbon). Some soils may contain mica and black sand. Top layers of soil usually contain decaying plant and animal matter. Living animals may also be present.
Clay itself is not a pure substance. There are many kinds of clay which are really mixed together.  All clay samples will be  compounds similar to Aluminium silicate.
These are an oxide of silicon called silica. ( silicon dioxide)  Sand  are the fragments of hard crystalline stones called quarts.
  1.    Try to find out the volume of air in a sample of soil..
  2.    Carry out a project to get a sample of the dissolved substances in a sample of soil.