58 MUTUAL EFFECTS OF DIFFERENT SUBSTANCES. 



they are due may be unknown, and thus he may himself be 

 misled in reporting the results of experimental trials. From 

 the operation of disturbing causes, such as I have now illustrated, 

 his results may not only be in discordance with those of others, 

 but may be absolutely inexplicable. 



The influence exercised upon the true and natural action of 

 one substance by the presence of other substances in the soil, is 

 illustrated by another experiment made by the same Mr Eussell. 

 He manured an entire field with twelve loads of farm-yard 

 manure per acre, and applied to separate portions of his crop of 

 green-topped yellow turnips, bone-dust, sulphate of soda, and 

 a mixture of the two, with the following results : 



Dung alone gave ..... 4 tons 2 cwt. of bulbs. 



Do. with 640 Ib. of bone-dust, . . 6 ...12 



Do. with 200 Ib. of sulphate of soda, . 4 ... 2 



Do. with 640 Ib. of bones and 200 of sulphate, 7 ... 7 



Here it is seen, that while the addition of sulphate of soda 

 alone to the dung produced no sensible effect on the produce of 

 bulbs, its presence along with the bone-dust and dung together 

 gave a larger crop than the bones and dung alone had given. 

 Both the crops and differences in this case, however, are so 

 small, that I do not quote them as worthy of much reliance, or as 

 actually proving anything, but only as illustrations of the kind of 

 effect which comparatively small quantities of substances in the 

 soil may produce. They may cause apparently inexplicable 

 diversities, which may tend to dishearten many, and to deter them 

 from continuing to prosecute experimental field-researches. 



In selecting land for experimental trials, therefore, too rigo- 

 rous an inquiry cannot be made into its previous agricultural 

 history. On naturally poor land, scientific experiments may be 

 made with safety, and with the prospect of generally useful re- 

 sults, but on land which has been exhausted by mismanagement, 

 the kind and extent of which may be only partially known, 

 locally useful and interesting experiments only can be under- 

 taken. 



And, viewed as an economical question again, the application 

 of saline and other substances, single or mixed, as top-dressings 



