NON-DIFFUSION OF NITRATES IN SOIL 229 



in the same way the amount of nitrates present at each depth 

 in the subsoil water below Plots 6, 7, and 8, is perfectly 

 distinct and characteristic of the manuring applied to the 

 surface. 



Additional evidence of the lack of lateral diffusion of the 

 nitrates in the soil water is to be seen in the permanent grass 

 plots ; although no path separates the plots receiving nitrate of 

 soda from the neighbouring plots, the characteristic vegetation 

 induced by the nitrate of soda manuring shows no tendency to 

 stray across the division line. For example, Plot 14, receiving 

 a complete manure containing 550 Ib. per acre of nitrate of 

 soda, is immediately contiguous to Plot 1, receiving nitrogen 

 only as ammonium-salts ; the vegetation on the two plots is in 

 marked contrast, yet the dividing line is singularly sharp, and, 

 despite the many years Plot 14 has received this last dressing 

 of nitrate of soda, there is not the least sign of its diffusion into 

 the subsoil below the adjoining plot. 



Y. THE NITRATES IN DRAINAGE WATERS. 



The processes of nitrification in soils can also be studied by 

 the examination of the drainage water beneath cultivated land. 

 It has long been known that all the soluble compounds of 

 nitrogen are retained by the soil with the exception of the 

 nitrates, hence an examination of the amount of nitrates 

 present in the water reaching the drains will throw light on the 

 rate at which nitrates are produced in the soil, and on their 

 ultimate fate. 



At Rothamsted the water which percolates through the 

 drain-gauges is stored, and the nitrates are regularly determined 

 in proportionate samples representing the percolation for the 

 month. These results have been combined for twenty-six years, 

 1878-1903, and the averages are set out in the accompanying 

 curves (Fig. 45), which show the rainfall, the percolation in 

 inches through 20 inches of bare soil, the concentration of the 

 percolating water in parts of nitric nitrogen per million, and the 

 total amount of nitric nitrogen reduced to Ib. per acre. 



