56 SCIENCE PROGRESS 



fineness of which it is composed ; they are produced by the 

 action upon the clay of very small quantities of alkalis or 

 alkaline carbonates, such as would be left in the soil by the 

 withdrawal of an excess of the acid constituents of the neutral 

 salts used as manure. Similar effects have been obtained by 

 Kriiger (Landw. Jahrb. 1905, 34, 761) by pot experiments in 

 the laboratory. 



The attack of the acid root sap upon the mineral particles 

 of the soil formed the theoretical basis of the method of 

 determining the " available " plant food in soils, which was 

 proposed by Dr. B. Dyer in 1894 {Trans. Chem. Soc. 1894, 65> 

 115), and has subsequently been very widely employed. 

 Dyer determined the acidity of the sap of a large number of 

 roots, and found that it might be approximately represented 

 by a 1 per cent, solution of citric acid. He therefore pro- 

 ceeded to attack the soil with this solvent, assuming that it 

 would bring into solution just those compounds of phosphoric 

 acid and potash which the plant could obtain by a similar 

 mode of attack. Hence the materials determined in this way 

 could be regarded as "available" for the plant, and would 

 afford a more accurate means of judging whether a soil required 

 manuring with phosphoric acid or potash than an estimation 

 of the gross amount of these substances, however combined, 

 in the soil. Dyer's method has undoubtedly proved of practical 

 utility, and though for various reasons other acids and other 

 concentrations have been proposed, they have not been 

 generally accepted nor yielded more useful results. With 

 the abandonment, however, of the view of the direct solvent 

 action of the acid root sap, the theoretical basis of Dyer's method 

 falls to the ground, and it must be judged empirically by 

 its success in interpreting the soil conditions established by 

 crop experiments. More recently (Trans. Chem. Soc. 1906, 89, 

 205) it has been shown that the citric acid solution will con- 

 tinue to extract constituents like phosphoric acid from the soil 

 indefinitely if the solvent be removed and repeatedly renewed. 

 This solvent does, however, differentiate between the various 

 phosphates of calcium, aluminium, iron, etc., present in the 

 soil, removing some of them entirely in the earlier extractions 

 and establishing with others solutions of approximately constant 

 composition. 



From several points of view, then, it will be seen that it 



