How Huniiis Is Foriiiod 



131 



20 



16 



Fig. 56. 



Aerobic decomposition 



Anaerobic decomposition 



2 3 4 5 6 7 8 



Period of decomposition, weeks 



Decomposition of sheep manure under aerobic and anaerobic conditions 

 (from Joshi). 



nient. Because of the difficulty in removing the mass of roots left 

 by the crop, only the surface portions are usually determined. The 

 root mass may equal if not exceed the surface stubble, as shown in 

 Table 25, where the results obtained by Woods in 1888 are reported. 



Table 25. AMOtrxT of Stubble and Roots Left by Various Crops (from Woods) 



Pounds (air-dry) per acre. 



The plant residues vary considerably in chemical composition, the 

 stubble and roots of cereals containing about 0.5 per cent nitrogen, 

 0.1 per cent phosphorus, and 0.5 per cent potassium, whereas the 

 corresponding concentrations of these elements in legume residues 

 are 2-3, 0.5, and 2-2.5 per cent. The chemical composition of stable 

 manures, green manures, and other sources of humus is given in 

 Chap. 14. 



