NITROGEN 121 



plowed up. This is partly due to the humus added by the 

 decaying roots, and is undoubtedly partly due to the 

 fixation of nitrogen. Probably the humus has much to 

 do with the nitrogen fixation. 



In the regions where soils have been so farmed as to 

 become unproductive, the fields are commonly abandoned 

 for one or more years, then they will produce crops again. 

 Where the soils are not quite so far exhausted, one or two 

 tilled crops are grown and are then followed by hay a 

 few years, after which small crops can once more be raised. 

 The same principle should be applied in regular farming. 

 Under most conditions, the land should be in sod one 

 to three years out of every five. The poorer the land, the 

 more time it should be in sod. If legumes can be com- 

 bined with this sod, so much the better. The same results 

 may be accomplished in other ways, as by plowing under 

 green-manure crops. 



119. Losses of Nitrogen from Soils. There are other 

 organisms in the soil which accomplish the opposite re- 

 sults. They act on nitrogen compounds and break them 

 up so that the nitrogen escapes into the air as free nitro- 

 gen. This is called denitrificntion. When manure is left 

 in loose piles, much of the nitrogen is lost by denitrifica- 

 tion. 



Nitrogen may also be lost by being made soluble too 

 rapidly, in which case it may leach out of the soil. The 

 humus in a sandy soil is likely to be burned out so rapidly 

 that the nitrogen may be lost in this way. 



Soils in Minnesota that were kept continuously in 

 grain lost 140 pounds of nitrogen by the destruction of 

 organic matter for each 25 pounds that was removed in 



