CHILDREN'S GARDENS 



held, and the facility and completeness with 

 which a plant can use it. The physical condi- 

 tions of soil are more important than the chem- 

 ical conditions, as plant-food is not available 

 unless in solution. 



Humus in the soil is derived from decompo- 

 sition of aquatic plants in swamps; from the 

 conversion of peat-bogs into humus soil; and 

 from the accumulation of organic matter along 

 lake and seashore and river margins and its de- 

 composition. 



The most important chemical elements in the 

 soil necessary to plants are oxygen^ hydrogen, 

 nitrogen, silicon, carlron, sulphur, phosphorus, 

 aluminum, potassium, calcium, magnesium and 

 iron. 



Oxygen is found in combination with all ele- 

 ments named in more than half of all known 

 rocks. 



Carbon occurs in soil as part of all humus 

 or organic matter and unites with calcium to 

 form carbonates of lime; with oxygen to form 

 carbon dioxid, which plays an important part 

 in the solution of plant- food and is a plant-food 

 in the atmosphere. 



Sulfur occurs in the soil abundantly as cal- 

 cium sulfate, gypsum or land-plaster, an im- 

 portant fertilizer. Sulfur is an important part 

 of organic compounds. 



162 



