Hoagland — 156 — Plant Nutrition 



tive stages. Probably even lower values would be 

 adequate if present methods of aerating solutions were 

 employed. 



The adaptation of plants to extremely dilute potas- 

 sium solutions is achieved because of the remarkable 

 ability of roots to absorb potassium ions rapidly when 

 the roots are growing and accumulating potassium by 

 metabolic processes. Despite this important considera- 

 tion, soil solutions from some soils are found in which 

 concentrations of potassium are maintained at such 

 low levels that doubt arises that the concentrations 

 are really adequate, or above critical levels. Neverthe- 

 less, certain of these soils supply enough potassium 

 to plants as evidenced by the growth and yield of the 

 latter. It is difficult, however, to evaluate the efficacy 

 of the soil solution concentrations as ascertained be- 

 cause of uncertainty as to the total extent of absorb- 

 ing root surfaces. Furthermore, we do not have ac- 

 curate knowledge of soil solution concentrations in 

 localized zones of the soil or of rates of entry of potas- 

 sium into the soil solution. 



If the soil solution seems to fail as an intermediary 

 for the transfer of potassium from the solid phase 

 of the soil to the plant root the direct contact theory 

 of absorption outlined before, may be invoked. Com- 

 plete proof that this method for the absorption of 

 potassium must perforce operate to explain the results 

 of the experiments on plants growing in soils, is not 

 yet available, but I have mentioned elsewhere that 

 Jenny, Overstreet and others have offered several 

 lines of evidence that contact absorj^tion of some ions 

 by roots does take place. 



However this question of mechanism may be de- 

 cided, it is clear now that when a root grows in contact 

 with or close proximity to colloidal particles bearing 

 potassium ions in replaceable form, these ions are in 

 general readily available to the plant. Hydrogen ions 

 act as chief replacing agents. Thus a soil with a rela- 

 tively large amount of replaceable potassium is not 



