4 i2 AGRICULTURE 



has accumulated to a considerable depth, such soils for example as are found in Man- 

 itoba, the Russian steppes, and the Pampas of South America. Despite the fixing ac- 

 tion of the bacteria associated with the leguminous plants it has always been difficult to 

 regard the enormous accumulation of nitrogen in virgin soils as the result only of past 

 vegetation, since the higher plants are only capable of taking already combined nitro- 

 gen from the soil and restoring it again as they die down. When, however, the vegeta- 

 tion falls back to the soil carbohydrates are added which have been manufactured by 

 the green leaf from the carbon dioxide of the atmosphere by the help of light, and 

 it is by the subsequent oxidation of this non-nitrogenous material by Azotobacter that 

 free nitrogen is drawn into combination and the stock in the soil is added to. 



The examination at Rothamsted, England, of the soil of a wheat field under arable 

 cultivation for more than half a century shows a steady decline in nitrogen content 



approximately equivalent to the removals in crop, any recuperative actions 

 s. at wor k being only sufficient to balance casual losses by drainage, weeds, 



etc. A portion of the wheat field was, however, allowed to go to waste 

 some thirty years ago, and became covered with a natural vegetation of weeds and 

 grasses, which are never cut but allowed to die down each year in situ. The soil of this 

 portion was found to have gained nitrogen at the rate of nearly 100 Ibs. per acre per 

 annum, the greater proportion of which must have been fixed by Azotobacter working 

 on the carbonaceous matter of the dead vegetation. With this clue we can interpret 

 the accumulation of nitrogen in virgin soils carrying a grassy vegetation; similarly forest 

 soils which annually receive carbonaceous matter from the fall of the leaf become en- 

 riched in nitrogen, though in both cases the process will only be effective if the soil con- 

 tains carbonate of lime. Again when land is laid down to grass for a period there will 

 be a regeneration of the nitrogen stock by the action of Azolubactcr, utilising the carbona- 

 ceous matter in the roots and stubble of the grass. 



The last decade has also seen the resources of agriculture enriched by two new 

 nitrogenous fertilisers in which the nitrogen is derived from the atmosphere by artificial 



means. Sir William Crookes had drawn attention to the possibilities of the 

 fertilisers fixation of nitrogen by means of the electric spark, but the process only 



attained commercial realisation through the methods devised by Birkeland 

 & Eyde, now worked on an enormous scale at Notodden in Norway. In this process an 

 alternating current of about 5,000 volts, derived from water power, is set to form an arc 

 between U-shaped copper electrodes, water cooled. The arc is spread out into a broad 

 flat flame by means of a powerful magnetic field, and through it is blown a current of air 

 at gentle pressure. The issuing air at a temperature of 600-700 C. contains about i per 

 cent of nitric oxide, which after cooling combines with more oxygen from the air and. is 

 dissolved out by water and finally by milk of lime. The final products are nitric acid 

 and nitrate of lime, which latter with about 13 per cent of combined nitrogen is now be- 

 ing sold as a fertiliser. The lime base gives it certain advantages over nitrate of soda on 

 heavy soils deficient in lime, and as the nitrogen is equally effective unit for unit, nitrate 

 of lime forms a valuable addition to the stock of nitrogenous fertilisers. 



The other fertiliser depends upon a different principle, the combination of nitrogen 

 with bodies of a metallic nature to form compounds which will give rise to ammonia on 

 decomposition with water. A metal is not, however, the starting point but calcium car- 

 bide, now manufactured on an enormous scale for the generation of acetylene. At a 

 temperature of about 700 C. it combines with free nitrogen to form calcium cyanamide, 

 which by the action of water becomes converted into calcium carbonate and free am- 

 monia. The process is now extensively carried out in conjunction with various calcium 

 carbide works situated where water power is available, as at Odde in Sweden, in Savoy, 

 and at Piano d'Ortenear Rome. The roughly ground carbide is heated in tubes through 

 which is passed pure nitrogen obtained by the fractional distillation of liquefied air. 

 The product contains 18-20 per cent of combined nitrogen with about 20 per cent of 

 free lime; it is sold under the name of nitrolim or kalk-stickstoff and forms an effective 

 nitrogenous fertiliser, especially valuable on soils poor in lime. 



