202 



ELEMENTS OF GENERAL SCIENCE 



We may therefore have many gradations in soil : from finest 

 clay to coarse clay, from finest sand to coarse sand, and on to 

 coarse gravel and even to rocky soil (fig. 99) ; from sandy 

 or clayey soil with but a slight amount of decayed organic 

 matter to peat in which there is little or no clay or sand. In 

 nature these soils are found in all possible mixtures. A careful 

 study of the table giving the structural analysis of different 

 types of soils will enable you to learn a good deal about how 

 soils may be mixed, and also about the size and percentages 

 of the different component parts. 



223. Size of soil particles. Careful measurements have been 

 made of the actual size of the particles of different kinds of 

 soils; also of the percentage of air space in these different 

 kinds of soils. The following table presents these data for 

 a few types: 



224. Plants and animals as soil formers. In sand upon 

 which plants have grown for a year or more, magnification 

 will usually show, in addition to the sand, some of the remains 

 of plant roots, stems, or leaves, although these may have de- 

 cayed to an extent that makes it difficult to recognize their 

 origin. 



When peat is examined under magnification, little or no 

 sand is seen, but this soil consists of the more or less divided 

 particles resulting from dead plant and animal bodies. Peat 



1 King, F. H., Physics of Agriculture. 



