368 Thr A railabi/ifi/ of Afineral Planf Food 



from the soil aggregate without any excretion of acid into the soil 

 generally. The cell sap and the soil particles to which the cell is cemented 

 are in such contact as to admit the direct dissolution from particular 

 particles by the sap of the ])articulur cells attached thereto. 



Organic acids have a far greater solvent power for many mineral 

 substances than minoral ;i(-ids have. Oxalic acid will dissolve much larger 

 amounts of iron from the soil tlian hydrochloric acid. Calcium phosphate 

 is much more soluble in citric, acetic, lactic and malic acids than in 

 hydrochloric or nitric acids. But it must be noticed that the solution of 

 mineral matter by the sap and nmciiagc of plant cells is not to be re- 

 garded as essentially an acid dissolution. Many organic compounds 

 besides acids will dissolve iron and aluminium compounds and phos- 

 phates. Tricalcic phosphate, for example, is much more soluble in water 

 containing starch, glue, sugars and many other organic bodies. 



This effect of organic matter on the solubility of phosphates and of 

 compounds of iron and aluminium is very well known and in soil 

 phenomena it must be far reaching. It affords a satisfactory explanation 

 of the uptake of iron fi-oiu cluilky soils, for numy organic compounds 

 have a high solvent power for iron over a wide range of both acid and 

 alkaline reaction. The enormous amount of oxalate in lichens growing on 

 limestone is not without significance in this connection. 



The availability of sparingly soluble phosphates becomes more in- 

 telligible if it is supposed that the root hairs become cemented to the 

 particles thus admitting the absorption of colloidal phosphate and the 

 direcl dissolution of the particle by the organic solvents concerned. .\lso 

 it is easy to see the connection between the root habit of wild white clover 

 and its special response to the presence of these sparingly soluble 

 phosphates. 



The available mineral phosphates have, according to the argument 

 of a succeeding paper^, a hydrophilous surface with which the root hair 

 can make its attachment. The unavailable phosphates appear not to 

 have a colloidal surface. Ferric phosphate loses its colloidal properties 

 on ignition and it also, according to Prianischnikow- becomes unavailable 

 for the plant. Aluminium phosphate retains its colloidal properties after 

 ignition and it remains available for the plant. 



There are, therefore, strong indications that colloidal properties in 

 sparingly soluble mineral plant food are of first importance, perhaps 

 enabhng the plant to absorb colloidal matter; certainly enabling the 



• ThLs \'oluine, p. .372. 

 - liicd. CciUr. 1905, 34. 



