Lecture 7 — 157 — Potassium Nutrition 



likely to be deficient in potassium supplying power. 

 The amount of replaceable potassium need not be high 

 in an absolute sense — even a few hundred parts per 

 million, calculated on the basis of dry soil, may be con- 

 sidered as high from this point of view. 



The availability of replaceable potassium to plants 

 is a function of the degree of saturation of the colloid 

 with potassium (the proportion of potassium ions to 

 the total number of adsorbed ions), as well as of the 

 total amount of replaceable potassium. Availability 

 further depends on the kinds of other ions adsorbed 

 by the colloid along with potassium, as Jenny and 

 Ayers (1939) have shown. If hydrogen ions are the 

 displacing ions, then they may be used either in dis- 

 placing potassium or other adsorbed ions and the 

 relative strength with which ions of different kinds 

 are held by the colloids is of importance. Adsorbed 

 calcium, for example, is held more tightly by the clay 

 particles than sodium is. Consequently hydrogen ions 

 displace potassium more readily from colloids contain- 

 ing calcium as a complementary ion than from those 

 containing sodium. The kind of colloid on which the 

 ions are adsorbed is likewise of consequence. 



Not less significant than these physical-chemical 

 factors is the biological one; namely, the rapid and 

 selective removal of potassium by the action of plant 

 roots. (The enormous absorbing area developed by 

 roots should once more be recalled) . This is a point 

 that justifies the repeated emphasis I have given it. 

 The action of plants can cause the replaceable potas- 

 sium to be reduced to a very low value in an absolute 

 sense, and also in proportion of potassium to total 

 adsorbed bases. At least this is true of many systems 

 in which calcium predominates. 



In these systems a relatively high content of re- 

 placeable potassium in the soil is reflected in a rela- 

 tively high concentration of potassium in the soil 

 solution. With a given soil, the addition of potassium 

 salts in successive increments results in successively 



