A Short Lesson on Soil 89 



it must also possess certain chemical properties. The me- 

 chanical properties relate to the soil's capacity to admit air and 

 hold moisture and to allow drainage. If soil has too much 

 clay it becomes water-logged, while an excess of sand renders 

 the soil unable to hold moisture. Correspondingly, although 

 a clay soil may be abundantly supplied with moisture it yields 

 up but a small percentage for the use of plants while sandy 

 soil yields a large percentage of its moisture. A desirable soil 

 contains a mixture of sand, clay, and humus clay and humus 

 retaining moisture, while sand renders the soil porous so that 

 water in excess may be drained off and a supply of air 

 admitted. Humus may be applied to a'dvantage by ploughing 

 under leguminous crops. To serve as humus they should be 

 used green ; they will at the same time increase the nitrogen 

 supply (see p. 101). Soil supplied with all the essential inor- 

 ganic elements will be unproductive in the absence of suitable 

 mechanical conditions. 



Food material is obtained from the soil in the form of 

 salts. 1 Soil bacteria play an important part in forming salts 

 containing nitrogen. Certain bacteria combine nitrogen and 

 hydrogen to form ammonia ; others take up the work and effect 

 a combination of ammonia and limestone or other substances 

 to form nitrites and nitrates. These are soluble in water and 

 may thus be absorbed by plants. Should soil contain so much 

 moisture as to exclude air, another set of bacteria breaks down 

 these salts setting free the valuable nitrogen and rendering the 

 soil unproductive. 



Sulphur, which enters into the composition of protoplasm, 

 may be supplied in the form of magnesium sulphate. 



Magnesium seems necessary for forming sugars and re- 

 lated substances in the plant although it does not enter into 

 their composition. 



Phosphorus is of great importance in forming nuclei 

 and in assisting in the process of nuclear division. Unfortun- 



1 A salt is formed by the chemical action of an acid with a base. E.g. 

 potassium sulphate is a salt formed when sulphuric acid unites with the 

 metallic base potassium oxide, and the action of nitric acid upon the base 

 calcium forms the salt calcic nitrate. Not all bases are metallic. 



