SUMMARY 345 



interest to maintain growing forests and not wantonly to 

 destroy them. 



SUMMARY 



The soils which have been produced in one way or another, 

 as described in Chapter X, are classified as local or sedentary 

 soil, which is formed from the rocks directly beneath it ; and 

 transported soil, which is generally brought from other 

 localities and deposited by water, ice, or wind. Soils are 

 also classified according to the size of their particles, as 

 gravel, sand, silt, and clay. The best agricultural soils are 

 generally of the consistency of silt, and are called loams. 



Nitrogen, phosphorus, and potassium are the soil elements 

 that are used most freely by the growing plant, but these 

 elements must be in chemical compounds with other sub- 

 stances before they are available as plant food. Plants also 

 require air and water, and are dependent on the activities 

 of soil bacteria. These bacteria cause such changes in 

 organic matter of the soil that it may be used by the plant 

 as food. Partially decayed organic matter in the soil is 

 called humus. Humus is not only a source of plant food, 

 but also serves to mellow the soil and to conserve soil water. 



The most common fertilizers are manures. They contain 

 nitrogen, potassium, and phosphorus in about the proportions 

 needed for ordinary crops. Commercial fertilizers contain 

 one or more of the elements mentioned, in varying pro- 

 portions. The United States is now developing its supplies 

 of commercial fertilizers and bids fair to be independent of 

 foreign supplies. The most common fertilizing agents are 

 the nitrogen-fixing bacteria, moles, gophers, and angleworms. 



Some plants grow with their roots submerged in water, 

 while others can grow only where the moisture supply is 



