THE BIOLOGICAL CONDITIONS IN THE SOIL 235 



adduced as evidence of the existence in soils of a considerable 

 number of phosphorus and potassium compounds of varying 

 degrees of solubility, but no such assumption is necessary. It 

 more probably represents the division of these compounds be- 

 tween two solvents, the weak acid and the colloidal complex in 

 which they are present in the soil (see p. 172). 



TABLE LXI. AMOUNTS OF K 2 O AND P 2 O 5 EXTRACTED BY ACIDS FROM ROTH- 

 AMSTED SOILS, PER CENT. OF DRY SOIL. HALL AND FLYMEN 



More definite information can be obtained about the 

 nitrogen compounds. 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 nitro- 

 genous food. This is one of the factors that produce the 

 marked retardation of plant growth in spring when the soil is 

 wet and cold, especially after a wet winter when the washing- 

 out process is complete, and it further accounts for the re- 

 markable benefits produced by even small additions of nitrate 

 of soda or sulphate of ammonia to the soil at this period. 



In dry regions the accumulation of plant food and other 

 soluble decomposition products in the soil may be too great 

 to admit of plant growth, and bare patches or regions arise 



1 For other results see F. Miinter, Zur chemischen Bodenanalyse (Landw. 

 Versuchs-Stat., 1919,91, 181-189). 



