1.FELDSPARS. Feldspars are anhydrous alumino silicates of potassium, sodium and calcium. 2.QUARTZ. Quartz or silica is silicon dioxide found in crystalline rocks. It also occurs as free silica or sand. 3.MICA. Mica is layer of alumino silicate of potassium; iron, magnesium and sodium may be present in varying proportions. 4.LIMESTONE. It is largely calcium carbonate; with magnesium carbonate it occurs as dolomite. 5. HORNBLENDE AND AUGITE. They are the ferro magnesium minerals consisting of silicates of calcium, magnesium, iron and sodium in varying proportions. 6.OLIVINE AND SERPENTINE. Olivine is an olive-green ferro-magnesium silicate. Serpentine is a hydrated silicate of magnesium. 7.CLAYS. Clays are secondary minerals. They are hydrated alumino silicates. 8.OTHER MINERALS. Other minerals occurring in the soil include tourmaline- a boro alumino silicate with alkali metals and iron or magnesium; rutile-titanium oxide, zircon-ziconium silicate, glauconite-hydrated silicate of iron and potassium , apatite-calcium phosphate, sulphur-bearing minerals , etc. Allophane and the hydrous oxides of aluminium, iron and titanium also commonly occur in soils and more so in highly weathered and leached soils.

Inorganic components. From what has been stated in the preceding section it is evident that compounds of silicon, aluminium, calcium, magnesium, iron, potassium and sodium form the principal chemical constituents of the mineral matter in the soils. Besides, the soil contains small amounts of a large number of other mineral elements, e.g. phosphorus, boron, manganese, copper, sulphur, zinc and cobalt. Thus the soil supplies all the following essential mineral elements required by the plants.

1.MACROS. Phosphorus, potassium, calcium, magnesium and sulphur. 2.MICROS. iron.manganese, zinc, copper, molybdenum, boron and chlorine. Plants obtain carbon, hydrogen and oxygen from carbon dioxide and water. Soil is also the source of nitrogen for plants, but the ultimate source of nitrogen is traceable to atmosphere from where nitrogen fixation takes place physico-chemically and biologically.

The total amount of elements contained in the soils depends partly on the nature of the parent material from which they are formed and partly on their age and extent to which soluble products have been leached down. The chemical composition of different horizons of a soil also show a good deal of variation. Generally, 'A' horizon is richer in soluble components than the 'B' horizon. Usually some of the elements that are commonly leached out are also ones that are required by the plants. tables 4 and 5 give the percentage inorganic composition of eight representative Indian soils and os the successive horizons of the soils.

*Ion exchange. It is a reversible process by which cations and anions are exchanged between solid and liquid phases which are in close contact with each other. The exchange of cations and anions is termed cation exchange and anion exchange respectively. Ion exchange is the most important of all the processes occurring in the soil. Soil colloids are the seats of ion exchange. Chemical and physical processes connected with ion exchange include the weathering of minerals, nutrient absorption by plants, the swelling and shrinkage of clay, leaching of soluble salts, etc. The capacity of soils to adsorb and exchange cations and anions varies greatly with the nature and amount of clay, and the organic matter. The cation exchange capacity (CEC) is defined as the amount of a cation species bound at pH 7 (neytral pH) and is expressed as milliequivalents per 100 grammes. The CEC values of reference clay minerals are:

Kaolinite	3-10 m.e./100 g
Illite	10-30 m.e./100 g
Montmorillonite	80-150 m.e./100 g

Organic colloids may have CEC of 200 m.e./100 g or more.
Base saturation The portion of CEC accounted for by the basic ions (calcium, magnesium, potassium and sodium) and expressed as percentage of the CEC is the base saturation percentage. A soil saturated with calcium and magnesium is considered normal and fertile. If a soil has more than 15% exchangeable sodium, it is an alkali soil. If the soils are base-unsaturated, i.e. the proprtion of exchangeable hydrogen is more, the soil tends to be acidic.

Acidic, saline and alkali soils are seperately dealt with. Similiarly other items, viz. pH and conductivity are discussed in the section 'Soil-Testing'.