AGRICULTURAL CHEMISTRY. 179 



The operations of tillage, viz: spading, ploughing, harrowing, &c, 

 have the mechanical effect to break up and admix the earth. They 

 convert the surface compacted by rain and sun into a loose and friable 

 mould suitable for the deposition of the seed and for the enlargement 

 of the roots of the young plant. Beyond this, these operations, really, 

 though but to a slight extent, mechanically lessen the size and increase 

 the number of the earthy particles. 



It is chiefly the loosening of the earth and the consequent better 

 admission of water and air, which facilitate the disintegrating effect 

 of these atmospheric agents, whereby, as already explained, the rock 

 fragments are decomposed and dissolved with perpetual increase of 

 the stores of assimilable food. 



Tillage likewise assists, in the same manner, in converting any 

 poisonous matters into innocuous or even salubrious forms. Soluble 

 salts of protoxyd of iron, which might accumulate in the deeper soil, 

 are, by exposure to oxygen, changed into insoluble and harmless com- 

 binations. Exposure of the soil by tillage to the atmosphere also has 

 the effect to increase the absorption of ammonia, and to hasten the 

 process of nitrification. 



Finally, the circulation of water and the consequent distribution 

 of plant food, the removal of excessive moisture after rains, and the 

 absorption of water vapor after droughts, as well as the regulation 

 of the temperature of the soil, are promoted to a most advantageous 

 degree. 



In that stage of agricultural which first follows upon pastoral or 

 migratory husbandry, the simplest modes of cultivation are the only 

 ones practiced ; the amount of tillage is small, just sufficient to prepare 

 way for the seed, and it is accomplished by the rudest implements. 



With the progress of the arts, ploughing, harrowing, &c, are em- 

 ployed to a greater extent. The implements used in these operations 

 are improved in construction, and adapted to all varieties and situa- 

 tions of soil, so that they may be worked at a greater depth and more 

 frequently, as well as at a reduced cost. 



A matter of great importance in tillage is to secure a proper depth 

 of soil. It is obvious that, other things being equal, the deeper the 

 soil the more space the roots of crops have in which to extend them- 

 selves, and the more food lies at their disposal. By deep culture new 

 farms are discovered beneath the old, and it is possible to realize the 

 apparent absurdity of "more land to the acre.'' 



Deep culture is one of the most efficacious means of counteracting 

 drought, as we shall notice presently in discussing drainage. 



Deep tillage is not, however, always practiced. The grain fields of 

 Germany, even in the most carefully tilled provinces, as Saxony, are 

 to this day mostly ploughed with rude wooden tools often not unlike 

 those figured in classical dictionaries as in vogue among the ancients, 

 Avhich merely score up the soil to the depth of two, three, or rarely 

 four inches. In our country, which surpasses every other in the real 

 merit of its agricultural implements, and Avhere the means of deep 

 tilth are in the hands of every farmer, tillage is notwithstanding 



