KINDS OF SOIL 71 



light subsoil with the surface soil. This may re- 

 duce the crop for a year or two, or until the raw 

 plant food in the subsoil has been acted upon by 

 air, water, and soil acids, but eventually the surface 

 soil is enriched by the fresh material. 



It is advantageous for a sandy soil to rest upon an 

 impervious clay subsoil, and for a clay soil to be 

 underlaid with a sand or gravel subsoil; both sub- 

 soils help to correct the defects of the soil above 

 them. A deep gravel or sandy subsoil, however, 

 is usually a disadvantage, as it allows plant food to 

 leach down beyond the roots of plants. 



ANALYSING THE SOIL AT HOME 



The determination of the relative proportions 

 of sand, silt, clay, and humus in a soil is called a 

 "mechanical analysis," as compared with a "chem- 

 ical analysis," in which the kinds and the amounts 

 of the different plant foods are determined. It is 

 not always possible to have the soil analysed 

 by a chemist, but it is always practicable for a 

 farmer to determine himself, roughly, the relative 

 amounts of the four ingredients that his soil con- 

 tains. A mechanical analysis should point out 

 the deficiencies of the soil much better than simply 

 viewing it on the surface. 



A close examination of a handful of the soil will 

 reveal much concerning its composition, especially 

 if a miscroscope or even a pocket lens is used. 

 Note the colour, whether dark or light ; look closely 

 for the tiny black particles of humus that are likely 

 to be the cause of the dark colour, and are a sign 

 of good texture and large water-holding capacity. 

 Rub the soil gently between the thumb and fore- 

 finger to determine the size of the particles. Are 



