CROPS, now GOVERNED. '213 



Avliicli the soil originally contained in the least propor- 

 tion, and of which it has accordingly lost, by the pre- 

 ceding crops, comparatively the largest fraction. 



Every field contains a maximum of one or several, and 

 a minimum of one or several, other nutritive substances. 

 It is by the minimum that the crops are governed, be it 

 lime, potash, nitrogen, phosphoric acid, magnesia, or any 

 other mineral constituent ; it reo;ulates and determines 

 the amount or continuance of the crops. 



Where hme or magnesia, for instance, is the minimum 

 constituent, the produce of corn and straw, turnips, pota- 

 toes, or clover, will not be increased by a supply of even 

 a hundred times the actual store of potash, phosphoric 

 acid, sihcic acid, &c., in the ground. But a simple dress- 

 ing with hme will increase the crops on a field of the 

 kind, and a much larger produce of cereals, turnips, and 

 clover will be obtained by the use of this agent (just as 

 is the case by the application of wood-ashes on a field 

 deficient in potash) than by the most liberal use of farm- 

 yard manure. 



This sufiiciently explains the dissimilar action upon 

 chfferent fields of so composite a manure as farm-yard 

 dung. 



Only those ingredients of farm-yard manure which 

 serve to supply an existing deficiency of one or two of 

 the mineral constituents in the soil act favourably in 

 restoring the original fertility to a field exhausted by 

 cultivation; all the other ingredients of the manure, 

 which the field contains in abundance, are completely 

 without effect. 



A field rich in straw-constituents cannot be made more 

 productive by manuring with straw-constituents in the 

 dung, whereas these constituents will prove most effi- 

 cacious on fields deficient in them. 



