166 HOW CROPS GROW. 
The wnmanured 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 prosecuted 
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 asoil-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 presumably necessary, and all the 
usual external conditions of growth, (light, warmth, 
moisture, ete.,) 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. 
