TECHNICAL DISCUSSION 165 



1 to 4 per cent Ca ; whereas the amount in the soil averages about 

 0.2, 0.8, 1.5, and 0.5 per cent respectively, only a fraction of 

 which is available to the plant. Both the mechanical mixing and 

 the action of digestive secretions favor the decomposition of the 

 organic matter and of soil minerals. The resultant product con- 

 tains a lower concentration of plant- food than the plant residues 

 but a higher concentration than the soil. The process may be 

 likened to the consumption of grass, hay, and grains by cattle 

 and the subsequent return of the manure to the soil, with this 

 difference, however, The cattle (or the milk from cows) are 

 sold from the farm, resulting in net loss to the soil of a certain 

 amount of plant- food. Also, some losses occur in the manure 

 before it is incorporated with the soil. The earthworm, on the 

 other hand, dies in the soil and its decomposed body returns plant- 

 food to the soil without loss. It has been found that the increased 

 nitrification that takes place when earthworms are introduced 

 into the soil is due, in part at least, to the decomposition of their 

 own bodies (6, 8). Russell (8) reported the nitrogen content of 

 worms to be 1.5 to 2.0 per cent or about 10 mgm. of N per worm. 

 That yields may be increased by the presence of earthworms 

 has been demonstrated in pot culture studies (5, 8). On a field 

 scale, however, no accurate quantitative comparisons have been 

 made, to the knowledge of the writers. Inasmuch as any practice 

 that favors earthworm activity is also favorable to plant growth, 

 it is extremely difficult in the field to determine to what degree 

 the worms are responsible for any increase in yields or improve- 

 ment in quality of the crop. Obviously one should avoid any 

 practice that would materially reduce earthworm activity. 

 Whether or not it is practicable deliberately to increase the worm 

 population is another question and one which still lacks an an- 

 swer. 



SUMMARY 



Samples of earthworm casts and of unworked soil from sev- 

 eral depths were collected from a cultivated field and from four 



