I.] A SPIES OF PLANTS. 17 



can be accurately determined. The knowledge we 

 thus obtain enables us to supplement the results 

 obtained by the mechanical analysis (19), and thus 

 we can extend the classification of soils. The me- 

 chanical analysis enables us to determine whether" 

 a soil is a sand, sandy loam, loam, clay loam, or clay 

 soil ; and the chemical analysis enables us to determine 

 whether they are calcareous or peaty. If there should 

 be any large quantity of stone, gravel, or rock, or 

 other exceptional matter mixed with the soil, these 

 would add an additional character ; as for example, 

 a calcareous loam with much or little stone, gravel, 

 or rock; or a sand with a large quantity of iron; 

 or a loam with much organic matter, &c., &c. The 

 term marl has been proposed for soils which contain 

 from 5 to 20 per cent, of lime, but this is a term which 

 should only be used for describing those beds of earth 

 commonly known as marls. 



CHAPTER II. 

 COMPOSITION OF CULTIVATED CROPS* 



29. By the aid of chemistry we are enabled to 

 learn what is the composition of our cultivated crops, 

 and the sources from which plants obtain the materials 

 of which they are made. We find that every plant 

 has two distinct groups of bodies within its structure, 

 and that these may be distinguished as organic and 

 inorganic matter. 



30. If any vegetable matter be carefully burnt, by 

 far the greater portion disappears in the form of 

 smoke, but a portion remains behind in the form of 

 ash. This ash consists of mineral matter, and it is 

 known as the inorganic matter of plants. It is 

 sometimes described as "the ashes of plants/' but in 

 each case the mineral matter of the plant is referred to. 

 When this ash is analysed it is found to consist of a 

 large number of different substances, which are present 



