TIIK LAW OF MINIMUM. 217 



plioric acid in the field, these crops will increase by 

 augmenting tlie quantity of phosphoric acid up to the 

 point at which the additional phosphoric acid bears a 

 proper proportion to the next mininmm constituent in 

 the soil. 



If the additional phosphoric acid exceeds the corres- 

 ponding quantity, for instance, of potash or ammonia in 

 the soil, the excess will prove of no effect. Before the 

 supply of phosphoric acid, the available quantity of 

 potash or ammonia w^as a httle larger than the amount 

 of phosphoric acid in the soil, and the excess of the 

 alkahes was ineffective until the phosphoric acid was sup- 

 plied ; similarly the excess of phosphoric acid must 

 remain just as inoperative, as previously the excess of 

 potash. 



\\liilst the produce before was proportionate to the 

 minimum of phosphoric acid, it is now in proportion to 

 the minimum of potash or ammonia, or both alkalies. A 

 few experiments made on ]\Ii\ Eussell's field might have 

 settled the question. Had potash or ammonia been the 

 minimum, after manuring with superphosphate, a suitable 

 supply of potash or ammonia, or both, would have 

 increased the produce. In this same series of experi- 

 ments, 6 cwt. of guano, corresponding to 2 cwt. of super- 

 phosphate, gave a crop of 630 cwt. of turnips, or 130 cwt, 

 more than the superphosphate ; but it is left in doubt 

 whether this increase was attributable to the potash or 

 the ammonia in the guano. 



To return to our Saxon experiments. If we look at 

 the different quantities of dung applied severally on the 

 five fields, we are naturally led to inquire the reason of 

 this diversity. 



Tlie most feasible answer, perhaps, is, that the farmer 

 gives as much manure as he has at his disposal, or tliat 



