56 FERTILISERS CONTAINING NITROGEN [CHAP. 



condition. Collateral evidence is furnished by some 

 of the other Rothamsted plots ; for example, when 

 the tile drains beneath the wheat plots run, the water 

 percolating from below the nitrate of soda plot is 

 always slightly turbid with fine suspended clay material, 

 while the water from the other plots is clear. This 

 removal of the finest material from the nitrated plot 

 has been so persistent during the fifty years or so of 

 experiment on this field, that the mechanical analysis of 

 the soil now shows a smaller proportion of clay, which 

 removal has only been possible because of the defloc- 

 culation brought about by the nitrate of soda manuring. 

 Again, the soil of the plots receiving nitrate of 

 soda is found to be losing carbonate of lime to the 

 water percolating through it at a lower rate than the 

 soil of the unmanured plot ; this is because the pro- 

 duction of a free base by the plant's own growth has, 

 to a certain extent, saved the carbonate of lime in the 

 soil from attack. The following table (XII.) shows the 



Table XII. — Calcium Carbonate in Broadbalk Wheat Soils. 

 First Depth (i to 9 Inches). 



annual average rate of loss of carbonate of lime for the 

 last forty years from some of the chief plots of the 

 Broadbalk field ; it will be seen that the nitrate of 

 soda has reduced the loss of carbonate of lime from the 



