164 THE SOIL: INORGANIC MATTER 



roots due to the solvent action of the carbon dioxide excreted. 

 Carbon dioxide is soluble in pure water to the extent of 

 about 1 part in 600 parts of water. The presence of soluble 

 salts reduces its solubility. 



i/ 



Oxygen is very generally present in the moisture of well 

 aerated soils, attacking minerals containing ferrous iron, 

 like hornblende, and breaking them down with water and 

 carbon dioxide. It is soluble to the extent of about 1 part 

 in 20,000 parts of water. 



Organic Acids like acetic, butyric, and others of a more 

 complex nature, all formed by the bacterial decay of organic 

 matter, particularly carbohydrates, dissolve in soil water 

 to a greater or less extent and act on many minerals. 



Inorganic Acids like sulphuric and nitric, formed from 

 sulphur and nitrogen in organic matter, serve as very active 

 reagents. They are present, however, to a very small 

 extent at any one time. 



Soluble Salts, derived from various minerals, such as 

 chlorides, nitrates, and sulphates, all have a greater or less 

 effect on minerals. 



135. Soil Minerals. — In considering the chemical changes 

 by which inorganic plant food becomes available, it is 

 necessary to know some of the principal soil minerals which 

 are the source of these elements. They will be taken up and 

 discussed in groups according to the element or elements 

 which they supply. In addition there will be discussed 

 the minerals of a few elements which are thought not to be 

 essential and yet which occur very commonly in plants, 

 or which have some effect in the soil. 



(a) Phosphorus Minerals. — The principal mineral con- 

 taining phosphorus is apatite, Ca 5 (P0 4 )3Cl.F. To show its 

 chemical structure better it may be written graphically: 



/0 >Ca 

 0=P— Cr ^ 



>g>Ca 



0=P— O— Ca— C1(F) 



>o> Ca 



\0 >Ca 



