CARBON AND NITROGEN CYCLES IN THE SOIL iSi 



at Bromberg (104^), and they fully confirm the results set out 

 above. 



4. There is no reason to suppose that the amount of 

 nitrogen in an uncultivated soil alters appreciably from year to 

 year so long as the land is untouched. But directly ploughing 

 and cultivation operations begin great losses of nitrogen set 

 in, as shown by Shutt's analyses of the Indian Head soil, 

 Saskatchewan (Table L.). In this particular case there is 

 practically no drainage water, and therefore little or no wash- 

 ing away of nitrates, yet only one-third of the lost nitrogen is 

 recovered in the crop, Snyder (268<5) has given similar results 

 for Minnesota soils, and Swanson for Kansas soils. ^ 



Table L. — Losses of Nitrogen Consequent on Breaking up of Prairie 

 Land, Top 8 Inches. Shutt (265). 



Nitrogen present in unbroken prairie 



„ „ after 22 years' cultivation 



Per Cent. 



•371 

 '254 



Lb. per Acre. 



6940 

 4750 



Loss from soil 2190 



Recovered in crop 700 



Deficit, being dead loss 1490 



Annual dead loss 68 



The exhaustion of the soil is due, therefore, not to the 

 removal of the crop, but to the cultivation. 



Similar losses take place when heavy dressings of farmyard 

 manure are repeatedly applied to land. One of the Broadbalk 

 wheat plots receives annually 14 tons of farmyard manure per 

 acre, containing 200 lb. of nitrogen. Only little drainage can 

 be detected though considerable leaching out of nitrates may 

 occur, but the loss of nitrogen is enormous, amounting to nearly 

 70 per cent, of the added quantity. Alongside is a plot 

 receiving no farmyard manure, from which, in spite of drain- 

 age, there is no apparent loss (Table LI.). 



^ Kansas Bu/^j., 199, 1914; 220, 1918. See also yourti. Ind. Eng, Chetn., 

 1915, 7, 529. Summaries of other results are given by A. W. Blair and H. C. 

 McLean, Soil Sci., 1917, 4, 283-293. 



