142 Report on Trials of Plows. 



solutions is unalogous to that by which charcoal separates coloring 

 matters and odoriferous matters from their combinations. This is 

 known to be partly mechanical and partly chemical. The chemi- 

 cal force, like that which causes the solution of substances in 

 water, is very weak; it attracts substances to itself, but does not 

 produce any change whatever upon the character of the sub- 

 stance. The coloring or the odorous matters are held in contact 

 with the pores of the charcoal just as coloring matters adhere to 

 the fibres of cotton or wool quite unchanged in their nature. 

 Neither powdered pit coal nor the hard, glassy charcoal from 

 sugar or blood have much power to attract coloring matters from 

 their solutions, while porous blood or bone charcoal possesses this 

 property in a very high degree, and among wood charcoals, those 

 which have the greatest amount of capillary porosity. 



It is just so with soils, those which have the greatest amount 

 of capillary porosity will condense the greatest amount of manu- 

 rial substances on their internal surfaces; will retain them longest 

 against the adverse solvent action of water, and will give them 

 out most readily to the rootlets of the growing plant. A mass 

 of adhesive clay will absorb but a very slight amount of availaljle 

 manure, but if this same mass is rendered friable by mechanical 

 processes its power of absorption is amazingly increased. In 

 view of what has been stated, it is very clear that plowing land 

 increases its fertility in one Avay by increasing its porosity by 

 pulverization. 



Again, many manurial substances exist in the soil which, being 

 insoluble, exercise no action on the growth of plants, and con- 

 tribute nothing to their nutrition; l)ut by the slow, though regular 

 action of the frosts and the rain, the air and the sunshine, these 

 insoluble and refractory compounds are reduced to a soluble state, 

 whicli are appropriated and held in deposit by the soil to the 

 credit of the next cultivated crop. This routine explains the 

 Avell known fact that soils, which have been cropped to the very 

 verge of barrenness, will recover their fertility if allowed to 

 remain long; enoug-h under the action of these climatic influences 

 to saturate the soil with the necessary plant food which they have 

 unlocked from their chemical combinations and given to the soil 

 in a proper physical condition. 



These changes are brought about much more rapidly when 

 certain mechanical changes of condition are wrought upon the 

 soil. 



