SI 6 iTO-\V CROPS T'EED. 



er nutritive elements, viz., clilorine and sulphuric ncld, In 

 the Table, ii,311 , leads to the question. May not the aqueous 

 sohxtion of the soil be altogether lacking in some es- 

 sential kinds of mineral plant-food in certain instances ? 

 May it not happen in case of a rather poor soil that it will 

 support a moderate crop, and yet refuse to give up to 

 water all the ingredients of that crop that are derived 

 from the soil ? 



The weight of evidence supports the conclusion that 

 ■water is caj^able of dissolving from the soil all the sub- 

 stances that it contains which serve as the food of plants. 

 The absence of one or several substances in the analytical 

 statement would seem to be no proof of their actual ab- 

 sence in the solution, but indicates simply that the sub- 

 stance was overlooked or Avas too small for estimation by 

 the common methods of analysis in the quantity of solu- 

 tion which the experimenter had in hand. It would ap- 

 pear probable that by employing enough of the soil and 

 enough water in extracting it, solutions would be easily 

 obtained admitting of the detection and estimation of ev- 

 ery ingredient. Knop, however, asserts ( Chem. Central- 

 hlatt, 1864, 168) that he has repeatedly tested aqueous 

 solutions of fruitful soils for phosphoric acid, emj^loying 

 the soils in quantities ranging from 2 to 22 lbs., and water 

 in similar amounts, without in any case finding any traces 

 of it. On the other hand Schulze mentions having inva- 

 riably detected it in numerous trials ; and Yon Babo, in 

 the examination of seven soils, found phosphoric acid in 

 every instance but one, which, singularly enough, was 

 ■that of a recently manured clay soil. In no case did he 

 fail to detect lime, potash, soda, sulphuric acid, chlorine, 

 and nitric acid ; magnesia he did not look for. {Hoff- 

 mann's Jahresberirht der Ag. Chem., I. 17.) 



So Ileiden, in answer to Knop's statement, found and 

 estimated phosphoric acid in four instances in proportions 



