EXHAUSTION OF POTASH 49 



abundant in the soil, and when in consequence the nitrogen 

 and phosphoric acid together had practically the effect of a 

 complete manure. Latterly, as the potash has become ex- 

 hausted by the continual cropping, the yield with nitrogen and 

 phosphoric acid has been but little superior to that produced 

 by nitrogen alone. Similarly, in the earlier years of the experi- 

 ment the crop on Plots 12 and 14, where soda and magnesia 

 are added to the superphosphate and ammonium-salts, was 

 but little inferior to that of Plot 13, which receives 

 potash. The results in later years show, however, that neither 

 magnesia nor soda can replace potash, their good effect in the 

 first few years being due to the fact that the addition of any 

 soluble salts to the soil brings into action some of the dormant 

 potash. At first this is sufficient to grow as large a crop as 

 where a potash manure is directly supplied, but in course of 

 time the available potash becomes reduced, and there is a 

 manifest decline on the plots receiving magnesia or soda only, 

 till finally the steady state is attained, the soda giving per- 

 manently a higher yield than the magnesia. Plot 7, which 

 differs only from Plot 13 in receiving magnesia and soda in 

 .addition to the potash, phosphoric acid, and nitrogen applied 

 to 13, gave until recently a somewhat higher crop. This is not 

 due to any specific effect of magnesia and soda, because Plot 

 JL3 does not show any progressive decline as compared with 

 Plot 7, although its soil must be becoming exhausted of these 

 constituents by their constant removal in the crops. Doubtless 

 the effect of the sulphates of magnesia and soda on Plot 7 is due 

 to their action as soluble salts, maintaining in a more soluble 

 condition the other manurial constituents necessary to the crop. 



D. Retention of Manures by the Soil. 



It has already been stated that, as a rule, 100 Ib. of the 

 .ammonium-salts are applied in the autumn when the seed is 

 sown, the rest being reserved for a top-dressing in the 

 spring. On one of the plots, however, Plot 15, the whole 

 400 Ib. of ammonium -salts is applied in the autumn, 



