180 HOW CKOPS GROW. 



lime, phosphoric acid, and sulphuric acid, although sev- 

 eral parcels were copiously treated with manures contain- 

 ing 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 chemical 

 analysis had arrived at much perfection, it was believed 

 that the ashes of the plant were either unessential to 

 growth, or else were the products of growth — were gener- 

 ated 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 uuder natural conditions, to decide 

 whether any or several of their ingredients are indispen- 

 sable to vegetative life.' For this purpose it is necessary 

 to institute experimental inquiries, and these have been 

 prosecuted with great painstaking, and with highly val- 

 uable results. 



Experiments in Artificial Soils. — The Prince Salm- 

 Horstmar, of Germany, was one of the first and most 

 laborious students of this question. His plan of experi- 

 ment was the following : The seeds of a plant were sown 

 in a soil-like medium (sugar-charcoal, pulverized quartz, 

 purified sand) which was as thoroughly as possible freed 

 from the substance whose special influence on growth 

 was the subject of study. All other substances presum- 

 ably 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 tlie influence of a great 

 variety of artificial mixtures, have been described, the 

 most important of which will shortly be given. 



Experiments in Solutions. — Water-Culture. — 

 Sachs, W. Knop, Stohmann, Nobbe, Siegert, and others 



