X.] SLOW DIFFUSION OF FERTILISERS IN SOIL 289 



amounts of soluble fertiliser, e.g., 550 lb. per acre of 

 nitrate of soda, or 600 lb. per acre of ammonium salts, 

 march with plots receiving either no fertiliser or a 

 characteristically different one, yet in neither case is 

 there any sign in the herbage that the soluble fertiliser 

 has diffused over the boundary. Although the treat- 

 ment has been repeated now for fifty-two years, 

 the dividing line between the two plots remains per- 

 fectly sharp, and the rank herbage produced by the 

 excess of nitrogenous fertiliser on one side does not stray 

 6 inches over the boundary. Again, on the Rotham- 

 sted wheatfield the plots were 24-7 feet in breadth, 

 and were separated by unfertilised strips only about a 

 foot in breadth; in 1893, each plot was sampled down 

 to a depth of 7-5 feet, and the amount of nitrates was 

 determined in each successive sample of 9 inches in 

 depth. The amount of nitrates found was in each case 

 characteristic of the supply of nitrogen to the surface of 

 the plot, and right down to the lowest depth there were 

 no signs of the proportions approximating to a common 

 level, as they would have done had any considerable 

 amount of lateral diffusion been taking place. Con- 

 sidering that the plots are only separated by a foot or 

 so of soil, and each had been receiving its particular 

 amount of nitrogen for forty and in some cases for fifty 

 years, the sharp differentiation of plot from plot in the 

 amount of nitrates at a depth of 7 feet is sufficiently 

 remarkable, and is evidence that the movements of the 

 soluble salts in the soil are almost wholly confined to 

 up and down motions due to percolation and capillary 

 uplift, lateral diffusion taking place only to an insignifi- 

 cant extent. 



From these considerations we may conclude that 

 when a fertiliser is mixed with the soil, each particle 

 will establish round itself a zone of a comparatively 



T 



