94 SOIL CONDITIONS AND PLANT GRO WTH 



of protein-like compounds, whilst others can assimilate carbon 

 dioxide. 



Thus the complex that we speak of as the soil consists of 

 four parts : — 



1. The mineral matter derived from rock material, which 

 constitutes the frame-work of the soil and is in the main 

 unalterable, but it contains some reactive decomposition 

 products. 



2. The calcium carbonate and phosphate (the latter being 

 usually in much smaller amount), and organic matter derived 

 from marine or other organisms deposited simultaneously with 

 the soil. 



3. The soil water, a dilute solution of carbonic acid con- 

 taining small quantities of any soluble soil constituent. 



4. The residues of plants that have grown since the soil 

 occupied its present position, consisting of the mineral plant 

 food taken up from the soil water and of part of the complex 

 organic matter synthesised during their life period. As the 

 source of energy for the soil population this may be regarded 

 as the distinguishing characteristic of soils. 



These four constituents are invariably present, but not in 

 the same proportion ; their relative abundance affords the 

 basis on which soils are classified. From the agricultural 

 point of view we thus have : {a) mineral soils consisting mainly 

 of rock material, subdivided into sands, loams, and clays ; 

 {b) calcareous soils containing notable amounts of chalk or 

 limestone ; (c) alkali soils rich in soluble, saline matter ; (d) 

 acid humus or peat soils where much organic matter has ac- 

 cumulated in absence of calcium carbonate ; {e) neutral humus 

 soils where much organic matter has also accumulated, but in 

 presence of sufficient calcium carbonate to prevent acidity. 



By far the greater proportion of agricultural soils belong to 

 the first group. 



The Mineral Portion of the Soil. 



It is usual to divide the soil into a number of fractions by 

 a sedimentation process, and this method has been used for a 



