210 THE SYSTEM OF FARM-YAKD MANURING. 



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 ascer- 

 tain 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 with gypsum and a little common salt or with 

 phosphate of magnesia, a field will be made to give 

 more abundant crops than by employing superphos- 

 phate of lime, although the latter unquestionably proves 

 the most effective manure for most fields. 



To apprehend these facts correctly, we must remem- 

 ber that the law of the minimum does not apply to one 

 constituent alone, but to all. Where, in any given case, 

 the crops of any plant are limited by a minimum of 

 phosphoric acid in the field, these crops will increase 

 by augmenting the quantity of phosphoric acid up to 

 the point at which the additional phosphoric acid bears 

 a proper proportion to the next minimum constituent 

 in the soil. 



If the additional phosphoric acid exceeds the corre- 

 sponding 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 was a little larger than the amount 

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

 alkalies was ineffective until the phosphoric acid was 

 supplied ; similarly the excess of phosphoric acid must 

 remain just as inoperative, as previously the excess of 

 potash. 



"Whilst 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 Mr. Russell's field might 

 have settled the question. Had potash or ammonia 



