232 Physics of the Soil 



less firmly. Fig. 70 represents the textural elements of a 

 clay loam in pretty good tilth. There are shown seven 

 sizes of granules large enough to be readily distinguished 

 with the naked eye, and each size is composed of fine soil 

 grains cemented together. All are represented natural 

 size and were carefully drawn from an actual sample taken 

 from a three inch mulch as left after the cultivator. 



The granules were sorted by means of a series of sieves 

 and the relative amount of each size of granules is repre- 

 sented by the shading in the vials where it is seen that the 

 largest size constitutes the smallest part of this soil, and 

 No. 5 the largest portion. The finest grade, No. 8, is also 

 largely composed of compound grains, many large enough 

 to be clearly distinguished by the unaided eye, but many 

 more of the ultimate grains which were rubbed off from 

 the larger grains by cultivating and during the process of 

 screening. 



Just as woolen cloths differ when the threads are 

 of the same size because some are twisted from finer and 

 others from coarser wool, so soils differ in having their 

 granules made of coarser or finer soil particles cemented 

 together. 



Then, too, just as one cloth may differ from another in 

 having its threads loosely twisted, while another is hard 

 twisted, so one soil may differ from another in the degree of 

 firmness with which the soil particles are cemented to- 

 gether. 



Still again, just as one fabric may be loosely woven 

 while another is firm, so one soil may have its granules more 

 strongly cemented together than another, making it hard to 

 work and heavy while the other is light and mellow. 



A sand differs from a soil in being composed of simple 

 separate grains, usually of rather large size, while a clay is 

 composed very largely of extremely fine granules made 

 from the finest of particles. 



A soil is in good tilth when its granules are neither too 

 fine nor too coarse, and when they are not too firmly 

 cemented together. 



