CHEMISTRY IN RELATION TO HORTICTL1TRE. 



Ill 



which thrives in an acid condition of soil. I have myself, at the "Woburn 

 Experimental Farm of the Royal Agricultural Society of England, been 

 able to show that, by the continued use of nitrogenous top-dressings of 

 sulphate of ammonia, the lime in the sandy loam that occurs there can be 

 so far removed as to cause the land to become sour and no longer capable 

 of producing a corn crop, whereas as soon as a dressing of lime is applied 

 the fertility is once more restored. 



While, however, lime may have this importance as regards farm crops, 

 I am not forgetful that it may not have it to the same extent as concerns 

 horticulture. The ill effects of w hard " water, i.e. water containing much 

 lime, on heaths, azaleas, kc., has been mentioned, and the same applies 

 to soils rich in lime, inasmuch as these plants will not grow upon them. 

 So also with trees, for while some, beech for example, thrive on chalky 

 soils, others, like elm and lime, will not do so well where there is much 

 lime in the soil. Conifers, too, appear to be independent of lime. 

 Absolute deficiency in lime and its presence in too great excess are, how- 

 ever, two very different things, and, while agreeing as to the unsuitability 

 of the latter in certain cases, I cannot believe in almost complete absence 

 of lime being productive of other than sourness and ultimate sterility. 



Magnesia is generally associated with lime in a soil and, though an 

 essential constituent, its supply would appear to be easily met, and its 

 special application in manurial form is never called for. Sulphur is a 

 constituent of the Brassicacea?, and, as already noted, enters into the 

 composition of albuminoids. 



Silica is abundant both in soils and in plant structure, and yet there 

 is no evidence to show that it is an essential ingredient. It enters largely 

 into the formation of bark in trees. Other constituents, such as soda, 

 chlorine, manganese, and occasionally lithium and other rare bodies, 

 also occur in soils, chlorine mainly in the form of sodium chloride in 

 plants of maritime origin, while soda, so long as pDtash is present, takes 

 a subordinate place. The influence of manganese, lithium, and similar 

 bodies requires to be still further studied, but there is evidence that, 

 though not in themselves essential, they may yet exercise some benefit. 

 Lithia, for example, is found largely in the ash of the tobacco plant. It 

 would seem probable, from work recently done, that the influence of these 

 rarer constituents will be found when they are used in quite minute 

 quantities, and not as ordinary constituents of manurial applications. 



Nitrogen, besides being supplied from the atmosphere to certain 

 leguminous plants in the special way already described, is also conveyed 

 to the soil by rain in the several forms of ammonia, nitric acid, and 

 nitrous acid, and these, by the processes of nitrification and oxidation, are 

 converted into nitrates, the form in which they become available for use 

 by the plants, their absorption taking place through the roots. The 

 change into nitrates is effected by the presence within the soil of one or 

 more organisms known as " nitrifying " organisms, and the activity of 

 which depends upon the presence in the soil of a base, such as lime, of 

 oxygen, and of a suitable temperature. 



Certain plants, such as Drosera, have the power of digesting nitro- 

 genous organic matter supplied to them direct, and these are known as 

 "carnivorous" plants, while others, such as the great class of orchids, 



