THE CONSTITUTION OF THE SOIL 



75 



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 03 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 (*OOO2 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. 



TABLE XXXII. MTRATES IN ARABLE SOILS (Top 9") AT DIFFERENT SEASONS 

 OF THE YEAR (RUSSELL 240^). 



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

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



6* 



