Dec. 2, 1918 



Greensand as a Source of Potassium 



491 



ties of potassium. These values are indicated as before in terms of 

 pounds of potassium per acre-foot. 



On recalling the experiments with potassium-containing salts (Table 

 II), it will be remembered that the greatest weight of tops was usually 

 seen in the weaker solutions, falling off as the concentration was in- 

 creased. This was regarded as being due to the harmful action of the 

 excess salts. 



Here the case is in general the opposite, the weight of tops increasing 

 as the quantity of marl or of greensand increases. This may be inter- 

 preted as being due to the increasing quantity of potassium obtained by 

 the plants from the greater surface of the potassium-yielding particles. 

 It seems probable that in all cases except in that of the Courtland green- 

 sand the potassium demand is fully satisfied in the cultures containing 

 the maximum quantity of both greensand and marl, in all three cases 

 the plants approaching or exceeding in growth the corresponding control 

 in Shive's complete solution. 



It will be further noted that even where smaller doses of marl are used 

 (i to 5 tons per acre-foot) the yield often exceeds the result obtained 

 with potassium salts in their most favorable concentration. Thus the 

 poorest yield with Red Bank greensand (i .59 gm. of tops with an applica- 

 tion of I ton per acre-foot, containing 126 pounds of potassium) exceeds 

 the yield obtained with potassium sulphate (i .53 gm. with 433.2 pounds of 

 the potassium per acre-foot) , and almost equals the best yield with po- 

 tassium chlorid (1.65 gm. with 866.4 pounds per acre-foot). 



In all cases (Courtland greensand excluded) the best results with marls 

 and greensands exceed the best results with potassium salts (potassium 

 nitrate excepted) . 



A comparison of the total growths made in each series of cultures helps 

 to give an idea of the value of each material in the quantities here used. 

 Such a summary is given in Table IV. 



Table IV. — Total weights 0/ wheat and red clover in potassium salts and greensand marl 



