78 PRODUCING ORGANIC MATTER 
able depth with its roots, and if this ideal plant is a 
nitrogen-gathering plant so much the better. 
When organic matter has been produced, that portion 
of it other than the roots of the plants can best be in- 
corporated into the soil by the use of a plow like the 
disc plow or the deep tilling machine. 
It is important that organic matter be put into the soil 
in the right manner to prevent the ill effects of “ soil 
roofing” or soil cavities between the bottom of the 
furrow slice and soil turned under. 
But if the plows above mentioned are used and the 
soil is well rolled and worked down with harrow or 
disc there will no effects from “ roofing.” 
