122 PRINCIPLES OF TILLAGE. 



bibes heat earlier in the spring, and retains it later in au- 

 tumn, 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 proportionate 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 qual- 

 ity. 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 consists of only four inches. He, 

 however, admits, that this principle cannot be carried to 

 that extent, because the action of the atmosphere must 

 ever afford such an advantage 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 assumes the value 

 of the land to be increased, in the proportion of eight per 

 cent., for every inch of mould beyond the depth of six to 

 ten inches, and to be diminished, in the same proportion, 

 from six to three inches, in soils of a thinner staple. 

 Principes Raisonnesd^Jlg.^ vol. iii. p. 138, §735. These 

 considerations have been hitherto but little regarded in 

 our practice, though they constitute an important feature 

 in the new system of husbandry. 



Good tillage demands, also, the extirpation of weeds. 

 Every plant which grows upon a soil tends to impair its 

 fertility, and weeds more than cultivated crops, because 

 they are generally the most hardy, and the greatest con- 

 sumers of vegetable food. They are particularly preju- 

 dicial to crops in a dry season, as they exhaust the soil 

 of moisture in proportion to their superficies, or the sur- 

 face of their stems and leaves, some species transpiring 

 their weight of moisture every twenty-four hours. The 



