Hoagland — 152 — Plant Nutrition 



factor in the prune die-back disease, then a simple 

 cure would be found in the application to the soil of 

 potash fertilizers. Actually several instructive diffi- 

 culties are met in the effort to overcome the conditions 

 of disease in the trees by adding fertilizers to the soil 

 or by other soil treatments. These difficulties involve 

 both the physiology of the plant and the chemistiy 

 of the soil. First I shall discuss several features of 

 soil systems in relation to the potassium status of 

 soils. 



Reactions of Potassium in the Soil : — In a previous 

 lecture, stress was given to the point that when a 

 fertilizer salt is applied to soil this is not merely a 

 process of adding something to an inert medium. 

 Potassium can react with soil colloids by a base ex- 

 change reaction, displacing calcium, magnesium or 

 sodium ions. By this reaction, potassium becomes 

 "fixed" by the colloid. In turn it can be displaced by 

 other cations, including hydrogen ions. The exchange- 

 able potassium later becomes available to plants 

 through this exchange, since hydrogen ions are pro- 

 duced by root respiration or the metabolic activities 

 of microorganisms. The equilibria of the system are 

 constantly disturbed because of the rapid absorption 

 of potassium ions by roots. In order that this potas- 

 sium may become effectively available to plants, the 

 roots must act close to or in contact with colloidal 

 particles on which the potassium is held. 



If most of the absorbing roots are active below 

 the zone of fixation, but little of the added potassium 

 may reach them within the necessary periods of time. 

 The roots of fruit trees, under some agricultural con- 

 ditions, do not develop absorbing systems in the upper 

 zones of soil in which the fixation of potassium first 

 occurs. If surface applications of fertilizer are made, 

 a problem of fertilizer placement arises in those soils 

 that have a high fixing power for potassium, when 

 deep rooted plants must be treated. 



