GOOD TILLAGE. 29 



up and stirring of the subsoil, promotes fertility by 

 increasing the power of the land to absorb water by 

 cohesive attraction. " The power of soils to absorb 

 water from air," says Davy, " is much connected 

 with fertility. This power depends in a great meas- 

 ure upon the state of division of its parts ; the more 

 divided they are, the greater their absorbent power. 

 When this power is great, the plant is supplied with 

 moisture in dry seasons ; and the effect of evapora- 

 tion in the day is counteracted by the absorption of 

 aqueous vapour from the atmosphere, by the interioi 

 parts of the soil during the day, and by both the ex- 

 terior and interior during the night." The soil im- 

 bibes caloric earlier in the spring, and retains it la- 

 ter in autumn, in proportion as it is dry and deep ; a 

 matter Of high consideration in cold climates, where 

 the length of the summer scarcely suffices to mature 

 the crops. The quality and dryness being the same, 

 a soil is fertile and durable nearly in -proportion to 

 the depth of the tillage which it receives : six inches 

 giving nearly double the pasture for plants that a 

 three-inch stratum does ; and a twelve-inch tilth 

 greatly exceeding in productiveness one of only six 

 inches. Von Thaer calculates this difference in pro- 

 portionate degrees in lands which contain a vegeta- 

 tive stratum of soil of four, six, eight, and twelve 

 inches in depth, provided, of course, that it be all 

 of equal quality. If, therefore, each seed were to 

 produce a plant, it would follow that ground which 

 contains eight inches of depth of fertile mould might 

 be sown with double the quantity of that which con- 

 sists of only four inches. He however admits, that 

 this principle cannot be carried to that extent, be- 

 cause the action of the atmosphere must d^er afford 

 that superiority to the surface, that a cubic foot of 

 mould, if divided into two square feet, will always 

 produce a greater number of plants than if the seed 

 were sown upon one foot superficial; but he as- 

 sumes the value of the land to be increased in the 



