THE BIOLOGICAL CONDITIONS IN THE SOIL in 



uses a saturated solution of CO 2 as being the nearest approach to 

 natural conditions. A 2 per cent, citric acid solution has been sug- 

 gested. 1 Bogdanow 2 in his investigations of Russian soils used 2 per 

 cent, acetic acid. The German " Verband landwirtschaftlicher Ver- 

 suchsstationen " recommends both 25 per cent, and 10 per cent, 

 hydrochloric acid. Lastly Ramann proceeds in a different manner 

 altogether and adopts a method depending on the interchange of 

 bases. The results obtained by different acids are shown in Table 

 XLVI. 



TABLE XLVI. AMOUNTS OF K 2 O AND P 2 O 5 EXTRACTED BY ACIDS FROM ROTHAMSTED 

 SOILS, PER CENT. OF DRY SOIL. HALL AND FLYMEN (117). 



Similar results have been obtained by Engels (95). 



Even strong hydrochloric acid only dissolves a part of the potash 

 and phosphoric acid, the remainder not coming out till after treatment 

 with hydrofluoric acid. 



The different action of the various dilute acids has been adduced as 

 evidence of the existence in soils of a considerable number of phos- 

 phorus and potassium compounds of varying degrees of solubility, but 

 no such assumption is necessary. It more probably represents the 

 division of these compounds between two solvents, the weak acid and 

 the colloidal complex in which they are present in the soil (see p. 77.) 



More definite information can be obtained about the nitrogen com- 

 pounds. The amount of ammonia and of nitrate can be ascertained in 

 any desired depth of soil. On cultivated land the amount is not 

 generally more than enough for one year's crop, any balance being 

 liable to be washed out in winter, so that the plant depends in spring 

 on the activities of the decomposition processes for a regular supply of 

 nitrogenous food. This is one of the factors that produce the marked 

 retardation of planj: growth in spring when the soil is wet and cold, 

 especially after a wet winter when the washing-out process is complete, 



1 By Bergu (Landw. Vcrstichs-Stat., 1901, 55, 19) and recommended by Engels (95). 



2 Expt. Station Record, 1900, u, 130 ; 1901, 12, 725. 



