24 INTRODUCTORY [chap. I. 



stances, nitrogen, phosphoric acid, and potash, which 

 alone among the various elements necessary to the 

 nutrition of the plant cannot be supplied by cultivated 

 soils in amounts sufficient for profitable crop production. 

 The soils do contain these substances in comparatively 

 enormous quantities, but the distinguishing feature of a 

 fertiliser which makes it effective when supplied in 

 quantities comparable with those removed by the crop, 

 is its " availability." 



A distinction is often drawn between natural and 

 artificial manures ; properly speaking, the latter should 

 include only such materials as are the results of some 

 manufacturing process, e.g.^ sulphate of ammonia, super- 

 phosphate and basic slag. But practically speaking, any 

 concentrated fertiliser that is brought on to the farm in 

 bags, though its origin be as natural as the sea birds' 

 excrement constituting "guano," or the ground seeds 

 known as " rape dust," gets called an artificial manure, 

 in contradistinction to the farmyard manure which is 

 the normal product of the farm. In all the published 

 reports dealing with the Rothamsted experiments it has 

 been customary to distinguish such substances as are 

 found in the ash of a plant — the phosphates, the sul- 

 phates, and chlorides of the alkalis or alkaline earths — 

 as "mineral manures"; the compounds containing nitro- 

 gen are regarded as distinct, since they are ultimately of 

 organic origin, even when they consist of such obviously 

 mineral substances as nitrate of soda or chloride of 

 ammonia. The term "cinereals" has also been pro- 

 posed in place of mineral manures or ash constituents ; 

 none of the terms are satisfactory, but since attempts 

 at corrected terminology only result in increased con- 

 fusion, the term " mineral manures," however imperfect, 

 will continue to be used throughout this book for 

 fertilising substances containing no nitrogea 



