Decomposition of Hunuis 141 



water. Creator losses ot organie matter, up to 60 per cent in 40 years, 

 have been reported under conditions of i:?oor soil management. 



The losses of organic matter as a result of cultivation are at first 

 rapid, later becoming slower, until an equilibrium is attained after 

 about 30 years; a base level of organic matter has then been reached, 

 which is 25-30 per cent lower than that of the virgin soil. Jenny cal- 

 culated that the loss of nitrogen during cultivation of a virgin soil 

 is 25 per cent the first 20 years, 10 per cent during the second 20 

 years, and 7 per cent during the third 20 years. Alway (Table 29) 



Table '29. Influence of Cultivation upon Loss of Ohganic Matter from 

 Prairie Soils (from Alway) 



30-40 year.s in cultivation. 



has shown that the growth of a legume or addition of stable manure 

 reduces considerably the loss of organic matter. The greatest dan- 

 ger to the humus content of the soil was found in clean cultivation 

 and in wind or water erosion. 



Detrimental changes in soil structure and tilth accompany losses 

 of organic matter, which imparts stability to the soil granules. In 

 analyzing the results of 50 years of field experiments at the Woburn 

 Experimental Station, Russell and Voelcker came to the conclusion 

 that soil deterioration under continuous cropping can be prevented 

 only by a combination of farmyard manure with crop rotation. The 

 plots receiving about 8 tons of manure a year showed no deteriora- 

 tion in crop yield and no loss of nitrogen from soil. The fact that 

 artificial fertilizers did not completely stop deterioration suggested 

 that the latter was due to the exhaustion of some soil constituent 



