106 EXPERIMENTS WITH PLANTS 



without the mirror, giving a dark background: use the 

 low power of the microscope. 



CLAY. Clay is mostly made up of particles less than 

 .00004 inch in diameter ; the particles have a tendency 

 to cling together and become cemented with sand-parti- 

 cles so as to form aggregate particles, or soil-crumbs, 

 which may be of much larger size than the particles of 

 sand. A clay soil, therefore, varies greatly in charac- 

 ter, depending upon the degree of aggregation of its 

 particles and its content of sand. In drying, clay tends 

 to form a hard, compact mass which plant-roots cannot 

 penetrate. When it becomes w T et it is apt to assume a 

 greasy, sticky texture, 1 impervious to water and im- 

 penetrable to plant- roots. In this state it may be 

 further compacted by chemical agents, by kneading or 

 by pressure, to form hardpan, which is really soil re- 

 verting to rock. Hardpan may also be formed of com- 

 pacted gravel. Clay lands are usually said to be heavy, 

 stiff and cold : the first two terms refer to the fact that 

 they are difficult to work, the last to the fact that they 

 contain more water than sandy soils. Clay is usually 

 the part of the soil richest in plant- food : it results 

 from the decomposition of rocks containing feldspar. 

 Examine some dry powdered clay under the microscope: 

 do you find aggregate particles? are the particles of 

 different colors? Find out what you can about them. 



1 How do the properties of burnt clay differ from those of unburnt clay ? 

 Why ? Why does burning clay land improve it ? How is the operation carried 

 out? 



