120 NATURE TEACHING. 



Most vegetable substances change with moder- 

 ate facility, hence stable-manure, decaying grass, 

 weeds, and bush are valuable sources of nitro- 

 gen. Certain animal substances are also useful, 

 such as blood and refuse from slaughter-houses, 

 the refuse from fish-curing establishments, as 

 well as fish themselves, when caught in greater 

 abundance than required for food. 



4. One fact demands notice. Many sub- 

 stances when mixed with soil, are so firmly 

 held by it that they are not readily washed out 

 by rain and carried away in the drainage water. 

 This is the case with phosphates, potash and 

 ammonium salts. With nitrates, however, it is 

 different; over these soil possesses little hold- 

 ing power, and they are easily washed out and 

 lost. As all nitrogenous matters eventually pass 

 into the form of nitrates, it follows that the 

 supply of nitrogen in the soil is peculiarly lia- 

 ble to become diminished. This is found to 

 occur in practice, for nitrogen is usually the 

 first item of plant food which becomes deficient 

 in the soil, and most of the efforts of the 

 cultivator, in the way of keeping up the stock 

 of plant food, are directed towards supplying 

 nitrogen. 



LEGUMINOUS PLANTS AND NITROGEN. 



.:!, It has just been said that plants are 



