Hoagland —160 — Plant Nutrition 



by its radioactivity there was no doubt that the plants 

 had absorbed the particular ions which had been 

 added and fixed in a chemically or physically more 

 or less resistant form. 



Many other experiments furnish evidence of the 

 absorption by plants from some soils of potassium 

 that is difficult to remove by laboratory procedures 

 with reagents that might be thought to have effects 

 of biological significance. An example from the ex- 

 perience of this laboratory consisted in a comparison 

 of the amount of potassium leached from a soil by 

 ten days continuous leaching with carbonic acid sat- 

 urated water, with the amount of potassium absorbed 

 in a similar period by actively growing rye seedlings. 

 The plants withdrew from this soil more than twice 

 the amount of potassium leached from it in the labora- 

 tory by the carbonic acid. 



A suggestion advanced by one investigator is that 

 soils that supply much potassium to plants from non- 

 replaceable form are generally rich in certain forms 

 of micaceous minerals. The assumption can be made 

 that the total amount of the effective minerals present 

 in the soil and their state of subdivision are important 

 in determining the number of points of root contacts. 



The direct contact theory of absorption of ions 

 may be of value for attempts to understand these 

 phenomena. While studies on the contact mechanism 

 have dealt experimentally with readily replaceable 

 cations, the whole system of soil solution potassium, 

 replaceable potassium and non-replaceable potassium, 

 tends toward an equilibrium state, which, however, 

 is not attained because of biological interference. Non- 

 replaceable potassium ions may migrate into replace- 

 able positions, as ions are removed by plant action 

 either from the soil solution or directly from colloids 

 by contact effects. Gradual hydrolytic breakdown of 

 resistant potassium minerals may also supply potas- 

 sium ions to the soil solution and these would be 

 available for reactions with the colloidal system. 



