148 EXPERIMENTS WITH PLANTS 



hastens decomposition, and if it is admitted too freely 

 into the compost heap the gases will form more rap- 

 idly than they can be absorbed: if, on the other hand, 

 air is excluded, decomposition almost stops. Hence 

 decomposition may be regulated by regulating the 

 moisture and compactness of the heap. Since manure 

 contains all the mineral constituents needed by the 

 plant, it is by far the best fertilizer for general 

 purposes. 



Another source of nitrogen is decomposing plants, 

 such as leaf- mold, peat, etc. This is known as 

 humus. It contains less soluble nitrogen than manure 

 and decomposes more slowly. The humus in decom- 

 posing forms ammonia and sets free acids which pro- 

 mote the formation of soluble plant-food: these acids 

 often change the color of the soil where it is in imme- 

 diate contact with dead roots, etc. Humus may be ap- 

 plied to the soil by plowing in straw, stubble, weeds or 

 special crops grown for the purpose. 



Rapidly acting nitrogenous manures are urine, 

 guano (manure of sea-birds) , saltpeter and Chili salt- 

 peter, all of which contain large amounts of nitrogen 

 in soluble form. They are especially valuable as 

 "forcing" manures and to tide a crop over a critical 

 period. 



Experiments have shown that not more than one- 

 half to one-third of the nitrogen applied as manure is 

 recovered in the crop: the waste is due partly to the 



