POTATO DISEASE. 289 



tion, in diminishing or aggravating the malady. It is very 

 generally believed that both high manuring and manuring with 

 unfermented or unassimilated materials, by causing a morbid 

 luxuriance of growth, produces an earlier appearance of the 

 disease, and increases its ravages. This opinion is based on the 

 general experience of cultivators, and has been proved in some 

 instances by careful experiment. The subject is a very impor- 

 tant one, and opens a wide field of inquiry. 



The true method of instituting a course of experiments 

 under this head must consist in selecting, as far as practicable, 

 a similar soil for each crop, and a new soil would be preferred 

 for the first season. In this case we can with more certainty 

 determine how much of the malady, or of the exemption from 

 it, is due respectively to the fertilizer, than if the soil, though 

 apparently uniform, had been in parts previously tilled for differ- 

 ent crops. On the succeeding year the same fields should be 

 retained and divided in the same manner for the different 

 branches of the experiment. 



Under this head would also be included all tests of the differ- 

 ent kinds of native and artificial soil, — determining the effects 

 of an old soil repeatedly tilled, compared with a new or virgin 

 soil ; of a moist compared with a dry soil ; and the different 

 effects respectively of peat land, of salt marsh, of alluvium of 

 various kinds ; of grass land lately turned up, of woodland 

 lately cleared ; of calcareous, siliceous, and all other natural 

 descriptions of soil. 



IV. Other experiments might be made to discover the effects 

 of different modes of mechanical tillage, independent of manur- 

 ing. Observe the different effects of frequent hoeing, of 

 moderate hoeing, and of the entire neglect of this operation. 

 This may be considered a matter of considerable importance, 

 because tillage is to be regarded as one of those luxurious 

 appliances, which, in connection with high composting, may be 

 supposed to have injured the health of the plant. It is, there- 

 fore, a rational subject of inquiry, to see whether an entire 

 neglect of tillage, as well as of manuring, would serve in any 

 degree to restore the original vitality of the plant, by allowing 

 it, as it were, to run back into a state of nature. Many years' 

 experiment, however, with the same seed, would be necessary 



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