166 HOW CROPS GROW. 



The unmanured potatoes -were relatively the richest in 

 lime, phosphoric acid, and sulphuric acid, although several 

 parcels were copiously treated with manures containing 

 considerable quantities of these substances. These facts 

 are of great interest in reference to the theory of the action 

 of manures. 



7. To what Extent is each Ash-ingredient Essential, 

 and how far may it be Accidental? Before the art of 

 chemical analysis had arrived at much perfection, it was 

 believed by many men of science, that the ashes of the 

 plant were either unessential to growth, or else were the 

 products of growth — were generated by the plant. 



Since the substances found in ashes are universally dis- 

 tributed over the earth's surface, and are invariably pres- 

 ent in all soils, it is not possible by analysis of the ash of 

 plants growing under natural conditions, to decide whether 

 any or several of their ingredients are indispensable to veg- 

 etative life. For this purpose it is necessary to institute 

 experimental inquiries, and these have been j)rosecuted 

 with great pains-taking, though not with results that are in 

 all respects satisfactory. 



Experiments in Artificial Soils. — ^The Prince Salm- 

 Horstmar, of Germany, has been a most laborious student 

 of this question. His plan of experiment was the follow- 

 ing : the seeds of a plant were sown in a soil-like medium, 

 (sugar-charcoal, pulverized quartz, j)urified sand,) which 

 was as thoroughly as possible freed from the substance 

 whose special influence on growth was the subject of study. . 

 All other substances presumably necessary, and all the 

 usual external conditions of growth, (light, warmth, 

 moisture, etc.,) were supplied. 



The results of 195 trials thus made with oats, wheat, 

 barley, and colza, subjected to the influence of a great 

 variety of artificial mixtures, have been described, the 

 most important of which will shortly be given. 



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