THE CONSTITUTION OF THE SOIL 73 



most useful for making plant food, but there is no evidence in favour 

 of this view. 



No examinations have been made of the part of the organic matter 

 insoluble in alkalis, but there is not the least reason for supposing that 

 it is any less important than the soluble part. 



Wax-like Constituents. 



Some of the soil organic matter is wax-like in properties, interfering 

 very much with the wetting of the soil and the movement of the water. 

 As it only decomposes slowly it tends to accumulate in rich soils 

 and to become rather troublesome. It can be extracted by organic 

 solvents, e.g. toluene, and obtained as a yellowish-brown mass contain- 

 ing appreciable quantities of nitrogen (a soil yielded "003 per cent, of 

 a substance containing 3 per cent, of nitrogen in one of the writer's 

 analyses). 



The Nitrogen Compounds in the Soil. 



It is convenient to collect together the main data connected with 

 the nitrogen compounds of the soil. The total nitrogen in arable soils 

 is usually about 0-15 per cent, in pasture soils about 0*3 per cent. ; 

 higher amounts are present in chalk soils and still higher in fen, moor- 

 land, and black prairie soils. About half of the nitrogen in arable soils 

 is contained in compounds soluble in alkalis, and a small proportion in 

 unstable compounds readily breaking down to ammonia. The amount 

 of nitrogen present as free or combined ammonia is about -oooi per 

 cent. (i.e. I part per million) of arable soils not rich in organic matter, 

 and some ten times this quantity in pasture or heavily dunged arable 

 soils. There is considerable variation in the amount of nitrogen 

 present as nitrate ; rich garden soils may contain 60 or more parts per 

 million (-006 per cent.), arable soils 2 to 20 parts (-0002 to -002 per 

 cent.), pasture soils rather less and woodland soils still less. 1 No soil 

 constituent fluctuates more in amount than nitrates ; plants and rain 

 rapidly remove them and bacterial action rapidly forms them. The 

 producing agencies are most active in spring, and work throughout 

 summer and autumn, while the removal agencies are active in summer 

 and winter. Thus the amount of nitrate actually present in arable 

 soil is highest in spring, falls in summer, often rises somewhat in 

 autumn, and falls again in winter as shown in Table XXXII. (p. 74). 



1 It is sometimes stated that woodland soils do not contain nitrates and are unsuited 

 for nitrification, but Weis (301) has shown this to be incorrect. 



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