THE BIOLOGICAL CONDITIONS IN THE SOIL 109 



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. 76). 



More definite information can be obtained about the nitrogen com- 

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

 any desired depth. 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. It is this circumstance that accounts for 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 remarkable benefits produced by even 

 small additions of nitrate of soda 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 known as " alkali soils " from 

 the circumstance that sodium and potassium carbonates are often present. 

 In wetter climates the soluble substances tend to be washed out more 

 completely, but notable quantities often persist in heavy clay soils, 

 especially where the drainage is bad, and may produce injurious effects 

 on vegetation. 



