164 THE PRINCIPLES OF 



case an application of lime would be of benefit. By the aid 

 of chemical analysis you might detect the want of sulphuric 

 acid in the soil, which would lead you to employ a cheap 

 source of sulphuric acid, such as gypsum ; or you might find 

 a deficiency of magnesia, which would lead you to use sulphate 

 of magnesia ; or you might find a lack in organic matter, or in 

 nitrogen, or in some other material which you could remedy 

 by liberal application of special manures containing the 

 missing substances. 



In the next place, special manures are needed for certain 

 crops. Cereals long for nitrogenous manure, root crops long 

 for phosphatic manure, potato crops cannot be grown to 

 perfection without abundance of potassic substances in the 

 soil. These substances may occur in the soil in sufficient 

 quantities for the requirements of other crops, but not for 

 the crops mentioned, and as one more example, we may take 

 mangel wurzel. The parent form of mangel wurzel is a 

 maritime plant, the lela maritima, which grows wild near 

 the coast in situations where chlorine in the form of chloride 

 of sodium is abundant. It is well known that while even 

 in inland districts some twenty pounds of chloride of sodium 

 per acre is yearly brought down in the rain-fall, nearer 

 the coast where sea-frets are common, a very much larger 

 quantity is yearly poured down over an acre. The mangel 

 wurzel being a cultivated form of beta maritima, appears 

 from long usage to require a large quantity of common salt, 

 and the -application of this substance increases the yield by 

 many tons per acre, especially upon soils of light loamy 

 character. These cases seem to show that special manures 

 are of use in a manner quite distinct from soil requirements. 

 We have, in fact, now before us two explanations of the use 

 of these special substances to which I shall add a third. 



We hear nitrate of soda and certain special manures 

 spoken of as " whips " and " stimulants." I do not much 



