THE REQUIREMENTS OF PLANTS 



41 



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 tend to die early at the tips. The most striking effect, 

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

 by Nobbe (215) ; either photosynthesis or translocation — it is not yet 

 clear which — is so dependent on potassium salts that the whole process 

 comes abruptly to an end without them. Mangolds, sugar beets, 

 potatoes, and other sugar- and starch-forming crops reduce their pro- 

 duction of sugar with decreasing potassium supply even before the leaf 

 area has been diminished. Thus, in the mangold experiments of 

 Table XII. (p. 34), 7255 lb. of leaf give rise to 14,684 lb. of root 

 where potash food is deficient, while very little more leaf, 8508 lb., 

 give rise to nearly three times as much root, 40,128 lb., where more 

 potassium salts are supplied. The harmful effect of potash starvation 

 on carbohydrate production does not seem to be the result of a patho- 

 logical condition of the chloroplastids. Reed found that they remained 

 normal for two months and even increased in numbers in potash-starved 

 algae. 



A second effect is on the formation of grain ; unlike phosphates and 

 nitrates, potassium compounds have a very marked effect on the weight 

 of the individual grains, as may be seen by comparing Table XVII. 

 with the corresponding Tables IX. (p. 32) and XIV. (p. 38); indeed to 

 withhold potash is the surest way of producing stunted grain. These 

 stunted grains are often sterile on the potash-starved grass plots at 

 Rothamsted. 



Lastly, the tone and vigour of the plant are very dependent on the 

 potassium supply ; potash-starved plants are the first to suffer in a bad 

 season, or to succumb to disease. The Broadbalk wheat plots receiving 

 potassium salts give conspicuously better results than the others when- 

 ever the year is unfavourable to plant growth ; taking the yield on 



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