Soil Composition

Clay Particles

Clay particles have the smallest size of other inorganic materials in the soil with diameters of less than 2 x 10-3 mm. They have a very large surface area relative to their volume and are essential to the delivery of nutrients and water to plant roots. This is found in the A and B levels in the diagram at right.

Minerals and Inorganic Materials

Other minerals in the soil include particles of carbonate rock, silicate rock, sand, and iron oxides. Weathering and chemical condensation reactions (in the C Horizon in the diagram at right) in the silicate rock produce the clay. Beneath the soil is the solid bedrock (R Horizon in the diagram at right).

Organic Materials

Plant litter and a variety of decomposing plant materials are in the top layer of the soil. As plant tissues decompose, they are converted to humic materials in the soil.

They complex metal ions, released from the clay minerals, and deliver them directly to plant roots.

There are 3 types of humic materials:
  • Humic acids - the fraction of humic substances that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH values. They can be extracted from soil by various reagents and which is insoluble in dilute acid. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in color.

  • Fulvic acids - the fraction of humic substances that is soluble in water under all pH conditions. They remains in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in color.

  • Humin - the fraction of humic substances that is not soluble in water at any pH value and in alkali. Humins are black in color.

Professor Patricia Shapley, University of Illinois, 2010