SOIL CONDITIONS AFFECTING PLANT GEO WTH 69 



equivalent amounts of the soluble compounds of potassium have 

 practically all the same nutritive value. 



The effect of potassium compounds is more localised than 

 that of phosphates, so that potash starvation can be more 

 readily detected. The colour of the leaf becomes abnormal ; 

 the potash-starved grass plots at Rothamsted have a poor, 

 dull colour, as also have the mangold plots ; the leaves also 



Fig. I2B. — Effect on yield of straw of withholding various nutrients from 

 barley. (Hoos field, Rothamsted.) 



tend to die early at the tips. The stem is weaker so that 

 the plant does not stand up well ; this is apparently a tur- 

 gidity effect, although anatomical differences were observed 

 by Miss O. N. Purvis.^ The most striking effect, however, is 

 the loss of efficiency in making starch, pointed out long ago 

 by Nobbe (2i5<2) ; either photosynthesis or translocation — it is 

 not yet clear which — is so dependent on potassium salts that 



* yourn. Agric. Sci., 1919, 9, 360, 



