PLANT FOOD IN SOILS. 199 



available plant food in a soil if the extraction of the soil 

 were made with a weak instead of a strong acid. More 

 than twenty years ago dilute acetic acid was used by E. 

 Peters ; and dilute acetic acid, and dilute nitric acid, by 

 Hermann von Liebig. 



If the object is to determine the available potash, it pro- 

 bably does not matter what acid is employed. Schlcesing 

 has taught us that if a soil is diffused in water, and small 

 quantities of hydrochloric or nitric acid are gradually added 

 till the bases in the soil are satisfied, and the solution 

 remains permanently and distinctly acid, that the whole of 

 the ammonia, and the whole of the potash held in the same 

 way as the ammonia, will come into solution. The potash 

 thus dissolved, Schlcesing teaches, is the potash which is 

 available to plants, and this opinion is probably correct. 



If, however, the object is to determine the amount of 

 available phosphoric acid which the soil contains, the nature 

 of the acid becomes important. The phosphoric acid of 

 non-calcareous soils exists principally as a basic ferric, 

 or possibly aluminic, phosphate ; such compounds are not 

 dissolved by acetic acid, which yet is quite capable of dis- 

 solving precipitated calcium phosphate. Deherain and 

 Vogel have proposed to use acetic acid as a means for 

 determining whether a soil stood in need of phosphatic 

 manure, and upon calcareous soils the amount of phosphoric 

 acid soluble in acetic acid is probably an indication of prac- 

 tical importance. It is well known, however, that non- 

 calcareous soils often contain a large quantity of available 

 phosphoric acid, although the existence of this is not shown 

 by extraction with acetic acid. 



The comparative action of various acids upon the phos- 

 phates contained in soil has^ been scarcely studied. Much 

 attention has, however, been given to the determination of 

 the available insoluble phosphates in manures, the reagent 

 usually employed for this purpose being ammonium citrate, 

 or, more rarely, citric acid, or a mixture of the two. Ammo- 

 nium citrate is an excellent solvent for ferric or aluminic 

 phosphate, and a less efficient solvent for calcium phosphate, 

 unless this occurs in a precipitated condition. A weak 



