RECENT WORK IN AGRICULTURAL SCIENCE. 



AGRICULTURAL CHEMISTRY. 



The determination of available plant food in soil by the use of weak acid 

 solvents, part 2, A. D. Hall and A. Amos (Jour. Chem, Soc. [London'], 89 (1906), 

 No. 520, pp. £05-222, figs. 3; ah*, in Analyst, 31 (1906), No. 361, }>. 129).— The rate 

 of solution of phosphates and the concentration of the extract obtained by continuous 

 extraction of soil samples with water saturated with carbon dioxid and with 1 per 

 cent citric-acid solution were studied in the investigations here reported and graphic- 

 ally represented. The general conclusions reached are summarized as follows: 



"(1) The solvent does not at once remove all the phosphoric acid capable of going 

 into solution in the particular solvent employed; instead an equilibrium is estab- 

 lished between the phosphoric acid in the solvent and in the soil. 



"(2) The concentration of the solution in equilibrium with the soil falls with each 

 successive attack of the soil by the same solvent. This indicates the presence in the 

 soil of several compounds of varying solubility, the mass of the more soluble b^ing 

 small an<l of the same order as the amounts going into solution in the earlier extracts. 

 When these more soluble compounds have been removed, an approximate constant 

 equilibrium is attained between the phosphoric acid remaining in the soil and that 

 going into solution at each extraction, indicating that after the more soluble com- 

 pounds have been removed there remains a phosphate in each soil of such low solu- 

 bility that the amount going into solution at each extraction is independent of the 

 mass present in the soil. * 



"(3) With soils which have been for many years manured with a particular phos- 

 phate, the amounts of phosphoric acid going into solution in successive extractions 

 with 1 per cent citric-acid solution follow a logarithmic law of decrement, indicating 

 the presence of one particular phosphate which dissolves in proportion to the mass 

 of it present in the soil. This law does not, however, hold for ordinary soils which 

 have been variously manured. 



"(4) In the case of the Rothamsted soils, the sum of the phosphoric acid dissolved 

 out by the first five extractions with citric acid approximates very closely to the 

 known surplus of phosphoric acid accumulated by the additions of manure to the soil. 



"(5) Assuming that the solvent actions of the soil water and of the weak acid sol- 

 vents employed in the laboratory are comparable, the evidence lends no support to 

 the theory that all soils give rise to a natural soil solution of approximately constant 

 composition, which is not disturbed by the use of fertilizers containing phosphoric 

 acid. 



"(6) For the practical purposes of soil analysis, the evidence afforded by a single 

 extraction of the soil for 20 hours with continual shaking is very similar to that 

 obtained from a series of successive extractions by the same solvent, and leads to the 

 same conclusions as to the manurial requirements of the soil." 



On the determination of phosphoric acid as magnesium-ammonium phos- 

 phate and as ammonium phosphomolybdate, G. Jorgensen (A'. Danske VidertsL 

 Selsk. Skr., 7. ser., 2 (1905), No. 4, pp. 141-238, figs. 3; abs. in Chem. Ztg., 29 (1905), 

 No. 90, Reperl, X<>. 23, p. 342).— -The author critically reviews the literature of this 

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