216 THE SYSTEM OF FARM- YARD MANURING. 



As shown by the difference of 20 cwt. in the produce 

 of the unmanured plots, the condition of the soil and 

 the store of mineral constituents differed, to some extent, 

 in different parts of the field. Other experiments, which 

 we cannot describe more particularly, showed that the 

 soil was poorer in the centre of the field than on the 

 sides. 



The one great fact most clearly proved by the above 

 table of produce is, that 3 cwt. of superphosphate gave 

 nearly the same crop of turnips as 5 cwt. ; and that a 

 further increase of the manure to 10 cwt. produced no 

 additional increase of the crop. 



No steps were taken, in these experiments, to ascertain 

 which of the constituents of superphosphate of lime had 

 the principal share in increasing the produce of the field. 

 Magnesia and lime, as well as sulphuric and phosphoric 

 acid, are equally indispensable elements of food for the 

 turnip plant; and I have observed that by manuring 

 Avith gypsum and a httle common salt or with phosphate 

 of magnesia, a field will be made to give more abundant 

 crops than by employing superphosphate of lime, although 

 the latter unquestionably proves the most effective ma- 

 nure for most fields. 



To apprehend these facts correctly, we must remember 

 that the law of the minimum does not apply to one con- 

 stituent alone, but to all. Where, in any given case, the 

 crops of any plant are hmited by a minimum of phos- 



