150 PROCEEDINGS OF SECTION B. 
same time, much mineral and nitrogenous material that was 
previously insoluble, o difficultly soluble, owing to it being 
in organic combination with the tissues, becomes easily 
soluble in the soil water. A great number of organisms 
take part in the rotting, and during the process there is 
much diminution of the organic matter, with an evolution 
of quantities of carbon dioxide. There is also a loss of 
nitrogen, which goes off partly as free gaseous nitrogen and 
partly as ammonium carbonate, which is volatilised by the 
heat evolved in the fermentation. The former loss is caused 
by the denitrifying bacteria, which find part of their food in 
xylan or wood gum—a substance that is found in straw. 
When the manure is ripe, and ready for working into the 
ground, the xylan has been destroyed, and the denitrifying 
bacteria, bemg no longer able to find a suitable food, do not 
denitrify the valuable soil nitrate. It is partly on this 
account that well-rotted farmyard manure is so highly 
esteemed. In the decomposition of manure, the albuminoid 
matter is converted into substances of the nature of urea, 
and finally into ammonium salts. The process from albumi- 
noids to ammonia is known as ammonisation, and can be 
done by many bacteria; but one of the most active in this 
work is Bac. mycoides. This is also one of the most active 
of the bacteria capable of decomposing bone. 
Once a salt of ammonia is formed a special class of bac- 
teria take up the running, and change the ammonia into 
nitrite and then nitrate. This is known ag nitrification, 
and although it has been said that nitrification is necessary 
before the plant can absorb combined nitrogen, this is not 
the case. The plant exercises its selective action only after 
the manurial constituents have passed by osmosis from the 
soil into the tissues. | What is required is used, and what 
is not wanted remains dissolved in the fluids, and no more 
of that constituent passes into the plant. Since any soluble 
nitrogenous constituent will pass into the plant, the ultimate 
nitrification is unnecessary, if the constituents are not fixed 
by the silicates and humates of the soil. Ammonia salts are 
fixed, and must be nitrified into the diffusible nitrate. 
It is not so very long ago that nitrate of potash was made 
by fermenting a mixture of soil, wood ashes, and liquid 
manure in the nitre beds. Now Chili saltpetre—the product 
of the action of bacteria upon masses of seaweed in the past 
ages—is the source of most of our nitrate. Nitrate of potash 
is still obtained by leaching the surface soil of the nitre 
fields, which are the sites of old encampments and villages, 
where wood was burned and manure thrown down. 
