[383] SCIENTIFIC MANUAL. 97 



mineral substances by the expanding effects of frozen 

 water, while more of such substances are annually ex- 

 tracted from the soil. 



3. The vegetable matter in the soil is more rapidly de,- 

 composed under the influence of warmth and moisture, 

 though more needed in warm than in cold climates. 



4. Fertilizing gases are liberated more freely in warm 

 climates. This continues throughout the year, where the 

 earth is never covered with snow. Nitrogen, especially, 

 is rapidly removed from the soils of warm climates. 



Humus is of more importance, though less abundant, 

 in the soils of warm than in those of cold climates. It 

 serves the double purpose of absorbing and retaining mois- 

 ture, and thereby cooling the soil, and consequently, is 

 of great benefit independently of the plant-food, which it 

 yields to the soil by its decomposition. 



Our climate necessitates different treatment of soils, both 

 in the preparation and cultivation, from that practiced fur- 

 ther north. 



Winter fallow is less beneficial where the freezes are 

 light, root cutting is more injurious, and level culture, ex- 

 cept on bottom land, a necessity. 



CHAPTER VII. 



FERTILIZERS. 



Fertilizers are classed according to their source as animal, 

 mineral and vegetable. These are also called mechanical, 

 if their principal influence is exerted in affecting the physi- 

 cal condition of the soil ; and chemical, if they produce 

 changes in the soil, or furnish plant-food directly to vege- 

 tation. There is a distinction made also between natural 

 and artificial fertilizers. 



A fertilizer proper is a substance, either simple or com- 

 7 



