ix] ACTION OF SOLUBLE SAL.TS UPON SOIL 263 



soda and magnesia without potash, fall further and 

 further behind, though they never reach the low level 

 of Plot II, with no alkaline salts at all. At first the 

 soda and magnesia can do the work of the potash 

 because they can render soluble enough potash in the 

 soil to satisfy the needs of the crop ; but as the treat- 



Table LXXXI.— Effect of Alkaline Salts upon thb 

 Wheat Ckop (Rothamsted). 



ment is continued the readily attackable potash in the 

 soil becomes depleted and the yield falls off despite 

 the great initial store of potash in the Rothamsted soil. 

 That the potash in the soil had been rendered soluble by 

 the soda and magnesia, is made still more clear by a con- 

 sideration of the composition of the ash of the plants 

 from these plots: and Table LXXXI I. shows the 

 average composition of the straw ash for the lo years 



