THE CHEMISTRY OF THE SOIL. 



Frapps has identified a fourth organism, which it is stated converts nitro- 

 genous organic matter directly into nitrites and nitrates. The process takes 

 place, more or less vigorously, in all soils not absolutely destitute of organic 

 matter, but there are certain conditions which are more particularly favour- 

 able to the process. They are — 



1. The presence of organic matter prone to oxidation, represented by 



humus. 



2. The presence of suitable food for the growth and development of the 



organisms, such as lime, potash, sulphates, and phosphates, and free 

 carbonic acid. 



3. Suitable temperature. The optimum temperature being about 35° to 



36° C. Nitrification is stopped at temperatures over 50° to 55° C, 

 or at 0° C. 



4. The presence of a base to combine with the free nitrous and nitric 



acid formed. Carbonate of lime is the most suitable. The presence 

 of free acid prevents nitrification, so does excessive amount of 

 alkali (carbonate of soda). 



5. Moisture. 



6. Absence of too strong light. 



7. Presence of sufficiency of oxygen (the process is essentially one of 



oxidation). 



8. Good drainage. 



(b). — From the Nitrogen of the Air. 



We have, so far, discussed the question of the plants' supply of nitrates 

 (which are the principal source of the plants' nitrogen), and we have seen 

 that these are derived from the decomposition of animal and vegetable nitro- 

 genous matter within the soil. How is this material, in its turn, derived 

 from the free nitrogen of the air, for that must be its ultimate source? 

 Atmospheric nitrogen is converted into organic material in nature in 

 several ways. 



Quite a number of free-living organisms have been isolated during recent 

 years which have the power of fixing the free nitrogen of the air, and thereby 

 enriching the soil in nitrogenous material and nitrates. 



The Azotobacter chromococcum (for example) of Baeyerink is capable of 

 fixing atmospheric nitrogen in a medium in which nitrogenous matter is 

 absent, the fixation being due to the presence of lime, and is much more 

 active in -soils rich in lime. 



These free-living organisms do best in soils containing organic matter, but 

 poor in nitrogen. Hall thinks that, as in the case of the nitrifying organisms, 

 the fixation depends upon the oxidation of carbohydrates, which supplies the 

 energy. 



Henry has found that decaying leaves of forest trees, such as oak or 

 beech, possess the power of fixing free atmospheric nitrogen in consider- 

 able quantities. The fallen foliage on the surface of the ground in an oak- 

 forest accumulates 13 kilogrammes of nitrogen per hectare annually (ll 1 /^ 

 lb. per acre). In the case of beech leaves, 22 kilogrammes is annually accu- 

 mulated per hectare (19*4 lb. per acre). 



(c). — By the Root-nodules of Leguminous Plants. 



An extremely interesting instance of the absorption of free nitrogen by 

 a certain class of plants was first observed by Hellriegel and Willfarth. 



The roots of leguminous plants when vigorously growing develop small 

 nodules or excrescences, which contain bacteria capable of fixing the free 

 nitrogen in the interstitial atmosphere of the soil, and of handing it over to 



