128 NATURE TEACHING 



manuring* Thus thorough tillage of the soil, and the 

 careful maintenance of the conditions necessary for the 

 activity of the useful bacteria, are in themselves most 

 important manurial operations. At the present day, 

 however, the word manure is only applied to the actual 

 substance added to the soil. 



Manures may be classed according to the substances 

 they contain. Thus we have nitrogenous manures, 

 potassic manures, and phosphatic manures, which add, 

 respectively, nitrogen, potash, and phosphates to the 

 soil. Such substances as farmyard manure and guano, 

 which add all the requisite substances for an ordinary 

 crop, are known as general manures. Farmyard manure, 

 guano, etc., are also organic manures, being the direct 

 product of living beings, as opposed to such substances 

 as nitrate of soda, basic slag, etc., which are spoken of as 

 artificial or cliemical manures. 



General Manures. 



Farmyard manure and stable-manure contain all 

 the constituents of plant food in well-adjusted propor- 

 tions. The actual amount of plant food contained in 

 these manures is often comparatively small. Their 

 great value is due to the fact that they add a large 

 amount of organic matter to the soil. Light soils are 

 thus enabled to retain more water, and the crops on 

 them to withstand droughts better. Heavy soils are 

 rendered more porous and easier to work. Such 

 manures, therefore, are of great value to the cultivator, 

 and are useful for almost all soils and crops. When 

 * See derivation of the word in the Glossary, page 181. 



