262 MATERIALS OF INDIRECT VALUE [chap. 



effect an exchange with the bases in the zeoHte, to a 

 greater or less degree according to the relative mass 

 of salt and zeolite, becoming itself insoluble and bring- 

 ing the other bases into solution. Thus in practice the 

 application of any soluble salts of calcium, magnesium, 

 and sodium to the soil results in potash going into 

 solution and thus becoming available for the crop, 

 always supposing that the soil is provided with a 

 normal amount of clay containing zeolites derived 

 from potash felspar. This principle serves to explain 

 the fertilising value of all such bodies as gas lime, 

 gypsum, salt, and sulphate of magnesia, and also the 

 irregularity of their action, because they are only 

 effective when the soil contains potash and yet the 

 crop requires more than the soil can normally furnish. 



This general action of soluble salts in increasing 

 the supply of available potash for the plant may be well 

 illustrated from the Rothamsted experiments. Taking 

 the wheat crop, there are five plots treated alike as 

 regards their supply of nitrogen and phosphoric acid, 

 but whereas one receives nothing further, one each of 

 the others also receives sulphate of sodium, potassium, 

 or magnesium respectively, and the fifth plot all three 

 of these salts, with the results set out in Table LXXXI. 

 for five successive ten-year periods. 



It will be seen that in the first decade the lack of 

 any alkaline salt on Plot 1 1 caused a serious reduction 

 of crop, but on the other plots there was much the same 

 yield, the mixed sulphates giving somewhat the highest 

 and the sulphate of potash itself the lowest yield. From 

 this alone it might be concluded that it is a matter of 

 indifference to the plant which of the alkaline salts it 

 receives, but as time goes on it will be seen that Plot 

 13, receiving potash, remains but little behind Plot 7 

 receiving all the salts, but that Plots 12 and 14, receiving 



