CROPS, HOW GOVEKNED. 207 



that where the productive power of an exhausted field 

 is restored, the fertilising effect is to be attributed sim- 

 ply to the manure returning to the field those elements 

 of food which the soil originally contained in the least 

 proportion, and of which it has accordingly lost, by the 

 preceding crops, comparatively the largest fraction. 



Every field contains a maximum of one or several, 

 and a minimum of one or several, other nutritive sub- 

 stances. It is by the minimum that the crops are gov- 

 erned, be it lime, potash, nitrogen, phosphoric acid, 

 magnesia, or any other mineral constituent ; it regu- 

 lates and determines the amount or continuance of the 

 crops. 



Where lime or magnesia, for instance, is the mini- 

 mum constituent, the produce of corn and straw, tur- 

 nips, potatoes, or clover, will not be increased by a sup- 

 ply of even a hundred times the actual store of potash, 

 phosphoric acid, silicic acid, &c., in the ground. But 

 a simple dressing with lime will increase the crops on 

 a field of the kind, and a much larger produce of cere- 

 als, 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 sufficiently explains the dissimilar action upon 

 different 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 

 efficacious on fields deficient in them. 



If two fields have the same abundance of straw-con- 

 stituents, but are not equally rich in corn constituents, 



