Lecture 7 — 163 — Potassium Nutrition 



plant at appropriate stages of growth, as the crop is 

 growing in the soil it is desired to appraise. 



Using procedures like these implies adoption of 

 the view that the plant itself is the best indicator of 

 what it can absorb. But whatever sort of examina- 

 tion is made on a small scale, there is always to be 

 faced in its application to practical agriculture an 

 array of factors influencing the growth of crops and 

 many of these factors can be only crudely predictable 

 or are too little understood to be assessed. Thus the 

 improvement of crop production by fertilizer appli- 

 cations of potassium or other elements rests on com- 

 bining and interpreting knowledge and experience of 

 many kinds. 



A Physiological Aspect of Potassium Supplying 

 Power of Soils : — In appraising the requirements of a 

 crop for potassium there is one physiological factor that 

 demands special attention, and here I should like to 

 return to the prune die-back disease. This disease 

 appeared, except in the most extreme type of potas- 

 sium deficient soil, as a sequence of heavy bearing of 

 fruit. In the localities concerned the trees tend to 

 set far more fruit than similar varieties of trees do 

 in most other localities. This tendency to heavy bear- 

 ing is seemingly the result of an interreaction of cli- 

 matic factors, not yet clearly understood. 



The development of a heavy prune crop means a 

 great draft on potassium, which migrates from the 

 foliage into the fruit. The too great depletion of 

 potassium from the tree, or metabolic disturbances 

 resulting from this depletion, can be assigned as a 

 cause of injury. In other words, the capacity of the 

 tree to absorb potassium is not great enough to meet 

 the physiological requirements of the entire tree, in- 

 cluding the developing fruit. LiLLELAND and BROWN 

 (1938) have shown that removing the fruit at an 

 early stage prevents the appearance of dieback symp- 

 toms, assuming the soil condition is not too bad. It 



