CHAPTER IV 



CHEMISTRY OF SOILS 



CHEMICAL COMPOSITION. 



THE chemical composition of soils is determined by 

 chemical analysis, and the subject may be conveniently 

 considered from that point of view. Apart from the ques- 

 tion of methods, the investigation falls naturally into five 

 different sections, vi. : 1. Water and the substances dis- 

 solved in it or capable o ( f being extracted from the soil in 

 aqueous solution ; 2. Substances soluble in dilute acids, 

 commonly regarded as capable o,f being absorbed by plants, 

 and often called " available plant foods " ; 3. Substances 

 insoluble in dilute acids but soluble in concentrated acids 

 these are not immediately available to plants but are 

 more or less easily changed into the available state, and 

 constitute a store from which the supply of available plant 

 foods is replenished ; 4. Organic matter ; 5. The insoluble 

 mineral residue, consisting of mere fragments of rock and 

 minerals in the fresh condition or only very slightly 

 weathered. 



Soil Water. The presence of water in the soil, the con- 

 ditions which determine the quantity of it and its influence 

 on the physical properties of the soil, have already been 

 fully discussed. The proportion of water is estimated by 

 exposing the soil to dry air, first at the ordinary tempera- 

 ture until it is air dried, and afterwards at a temperature of 

 105 C. till it ceases to lose weight. The soil is then said to 

 be dry, but it may, and usually does, still contain combined 



