70 



SOIL CONDITIONS AND PLANT GROWTH 



the whole process comes abruptly to an end without them. 

 Mangolds, sugar beets, potatoes, and other sugar- and starch- 

 forming crops reduce their production of sugar with decreasing 

 potassium supply even before the leaf area has been dimin- 

 ished Thus, in the mangold experiments of Table XVIII. 

 (p. 63), 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 pathological condition of the chloro- 

 plastids. 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 phos- 

 phates and nitrates, potassium compounds have a very marked 

 effect on the weight of the individual grains, as may be seen 

 by comparing Table XXI. with the corresponding Tables XV- 

 (p. 60) and XIX. (p. 66) ; indeed, to withhold potash is the 

 surest way of producing stunted grain. At Rotham'sted the 

 average weights per bushel of wheat for the ten years 1910- 

 1 9 1 9 were : — 



Table XXI. — Effect of Potassium Salts on the Development of Barley. 

 Hellriegel (130^). 



KgO supplied, mgs. 

 Dry matter in crop, grams . 

 Grain, per cent of dry matter 

 Weight of one grain, mgs. . 



2'27I 



23-5 

 5'4i4 

 4-8 



5 



47 



9*024 

 21-5 



9-5 



70-5 

 if636 

 27-2 

 13 



94 



15*302 

 301 

 17 



188 

 20-946 



38-5 

 26 



282 

 29*766 

 427 

 34 



Lastly, the vigour and healthiness 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 



