204 ROTATION OF CROPS. 



716. On a field of clover, gypsum in powder, ashes, 

 and bones dissolved in sulphuric acid with one hundred 

 times its quantity of water, always produce gratifying 

 effects. On grass land similar effects are produced hy the 

 use of liquid manure which has run from the manure 

 heap. 



A poor gentleman in Maryland, suspecting that land 

 which had been worn out by long continued cultivation 

 of tobacco, might be restored by plaster, so as to produce 

 wheat, tried the experiment, which was completely suc 

 cessful. He bought many acres of exhausted tobacco 

 land, and, by fertilizing it with plaster, made himself a 

 rich man. 



This gentleman, not a man of science, was led to make 

 the experiment by reading Sir Humphrey Davy s Chemi 

 cal Lectures. 



717. A Fallow. A field is said to be fallow for a year, 

 when no valuable crop is raised upon it. Such a year is 

 called a year of fallow, and the field itself is sometimes 

 called a fallow. 



It is sometimes well to let a field lie fallow. A field 

 much infested with weeds may be allowed to lie till the 

 weeds arc well grown or beginning to blossom, when they 

 may be turned under with a plough. This is like giving 

 a coat of manure. When another crop of weeds has 

 sprung up, they may be ploughed in, and this may be 

 repeated as often as there arc any weeds to turn under. 

 These green crops may bo advantageously increased by 

 harrowing in, after each ploughing except the last of the 

 season, seed of some rapidly growing plant, like buckwheat. 

 After a year of such fallow, the field will be likely to be 

 comparatively free from weeds, as most of the seeds of 

 weeds will have sprouted and been destroyed. 



