ii2 THE BOOK OF NATURE STUDY 



on soil, it must be borne in mind that the soil does not consist 

 of particles of a uniform size, that the particles are coated with 

 films of water, and that the particles are cemented together with 

 a small quantity of a colloidal substance. When, therefore, a 

 mass of soil is exposed to alternations of cold and heat, there 

 are different pressures in different directions, which cause the 

 mass to split, not into its ultimate particles, but into compound 

 particles, and it is the production of these compound particles 

 which constitutes one of the principal objects promoted by 

 tillage. 



Next to the spade, the hoe is probably the most generally 

 important garden tool. There are two main kinds of hoes, the 

 draw hoe, which, as the name indicates, is used by pulling it 

 towards the worker, and the Dutch hoe, in which a pushing 

 motion is adopted. The latter is probably the more generally 

 useful for schoolboys. The most obvious use of the hoe is for 

 severing the roots of weeds. The draw hoe is also used in ridging 

 up rows of plants, and sometimes for making drills for seeds. 

 In a dry season, whether in spring or summer, the production with 

 the hoe of a surface mulch of loose soil is of great benefit in 

 preventing evaporation by sun and wind of water from the soil. 

 The shallow coating rapidly becomes dry, but it protects the 

 under undisturbed surface from evaporation, and thus preserves 

 the store of moisture in the soil. 



It may be useful at this point to notice the effect on soil 

 moisture of treading the soil down. This effect is twofold. In the 

 first place, the firmer and more even surface has less tendency 

 than a loose surface to become dried by sun and wind ; in the 

 second place, the consolidation of the soil by treading promotes 

 the passage of water upwards, the drier soil being brought into 

 closer contact with the moister subsoil. Thus the result is a 

 better supply of moisture just below the surface, which supply 

 is necessary for the germination of shallow drilled seeds. The 

 effect of treading is only temporary, and a firmly pressed soil, in 

 the long run, loses more water by evaporation than a looser soil, 

 but this does not matter so long as the object of an immediate 

 supply of moisture to the seed and the tender seedling is secured. 



