6o THE PHYSIOLOGY OF PLANTS 



stance, as long as the soil remains porous enough to allow the 

 access of oxygen ; but still in a thoroughly saturated soil the 

 formation of carbonic acid does not cease entirely. 



Of course the absolute amount of carbonic acid and nitric 

 acid produced depends mainly upon the nature of the substance 

 which is being decomposed and upon the substances with 

 which it may be mixed. The rapidity of the decomposition of 

 various substances used as manures can be roughly gathered 

 from the following figures, which represent the volumes of 

 carbonic acid contained in i ooo volumes of air after i gramme 

 of the substance has been decomposing for twenty-four hours : 

 — Steamed bone-meal liberates 3 1 ; Peru guano, 24 ; Pigeon- 

 droppings, 26 ; fresh swine-dung, 1 4 ; partially decomposed 

 horse-dung, 1 2 ; fresh cow-dung, 1 3 , and after three months 

 about 9 volumes of carbonic acid ; while peat gives oft' only 2 , 

 sawdust 5, ground horn 6, dead leaves 7—8, pine leaves 9, corn- 

 stalks 17-19, and pea and bean stalks 22 volumes of carbonic 

 acid. Though, generally speaking, the addition of quicklime 

 during the later stages of decomposition becomes advantageous 

 on account of the combination of the lime with humic acid, 

 which decomposes more rapidly than the humic acid alone, 

 quicklime generally retards the initial stages of decomposi- 

 tion (Wollny). In the same way, mineral acids even in small 

 quantities retard decomposition, while weak solutions of alkaline 

 carbonates stimulate decomposition. The latter will also be 

 stimulated by small additions of sodium nitrate, but is retarded 

 by large amounts of this salt. Common salt (sodium chloride) 

 always retards decomposition. 



As a general guide for the rapidity of decomposition, we 

 may place the manures in the following order. The most 

 rapid of decomposition are bone-meal, flesh-guano, flesh-meal, 

 the droppings of birds ; straw and stable manure are slower ; 

 still slower are leather scrapings, horn, dried leaves and saw- 

 dust ; peat is slowest of all to decompose. 



Other things being equal, the heat developed will be pro- 

 portional to the rapidity of decomposition. Horse-dung heats 

 the soil most effectively, cow-dung least. Decomposing corn- 

 stalks give off less heat than the stalks of the pea, which 

 contain more nitrogen. 



