Hoagland — 162 — Plant Nutrition 



limiting for plant growth. There is difficulty in 

 ascribing the potassium absorbed by plants to some 

 particular form of combination in the soil if the 

 whole system is in dynamic equilibrium. 



Since the total amount of potassium in a soil is 

 sufficient for the requirements of plants for many 

 years, if potassium in all the forms present could be 

 yielded to plants at an adequate rate of supply, there 

 would not be found any soil in which crops would 

 suffer from a potassium deficiency, which is obviously 

 contrary to fact. In many soils the potassium is 

 not capable of release to plants at rates commensurate 

 with their requirements. Consequently it is necessary 

 to distinguish different degrees of absorbability by 

 plants of non-replaceable potassium. Potassium ini- 

 tially present in the soil can be chemically too inert 

 to insure adequate absorption by plants, and some of 

 the potassium added in fertilization may also become 

 so fixed that it is not only non-replaceable by chemical 

 tests, but also more or less non-available to plants.* 



At present it does not appear that the physiological 

 efficacy of the potassium in soils can be consistently 

 appraised without the aid of experiments with plants, 

 of one kind or another, however useful some workers 

 may find certain soil examinations, when these are 

 associated with much experience with particular crops 

 and types of soil. On a laboratory scale the biological 

 test known as the Neubauer method, in which a large 

 number of rye seedlings are grown in a small amount 

 of soil mixed with sand, has been helpful in certain 

 types of study. There is also some promise in the 

 analysis for potassium of a suitable portion of the 



*The interesting- observation has been made in our experi- 

 ments, and in those of one or two other laboratories, that con- 

 tinued cropping- of some soils leads to a fixation of part of the 

 potassium initially added to the soil, in a difficultly replaceable 

 and root absorbable form. This effect may occur even in soils 

 that show very little fixation of potassium in resistant form 

 under laboratory conditions. 



