EFFECT OF NITEOGEN ALONE 157 



the same proportion of weeds and the same general aspect as 

 the continuously unmanured plot. 



II. Use of Nitrogenous Manures alone. 



Three of the plots 17, 5, and 1 show the effect of the 

 long-continued use of nitrogenous without any mineral manures. 

 Plot 5 has been receiving 86 Ibs. of nitrogen as ammonium- 

 salts, Plot 17 half the quantity of nitrogen in the shape of 

 nitrate of soda, and Plot 1 the same half quantity of nitrogen 

 as ammonium-salts, though on this plot dung was applied in 

 each of the first eight years of the experiment. It is very 

 evident when a nitrogenous manure is used alone for grass, 

 nitrate of soda is far more effective than the ammonium-salts ; 

 e.g., on Plot 17 it has given an average crop of 34 cwt. 

 against 26 cwt. produced by double the quantity of nitrogen 

 in ammonium-salts on Plot 5. 



For this superiority of the nitrate of soda two reasons may 

 be traced; being completely 'soluble it sinks deeply into the 

 soil, and encourages grasses of a deeply-rooting habit, which 

 not only obtain more food from the soil, but also are better 

 able to withstand the droughts of spring and early summer. 

 On Plot 17 (nitrate) deep-rooting grasses like Meadow 

 Foxtail and Downy Oat Grass are prominent, but latterly 

 Sheep's Fescue (14 per cent.) and Bent Grass (14 per cent.) are 

 well represented. The plots receiving only ammonium-salts 

 are almost wholly occupied by Sheep's Fescue and Common 

 Bent, whose feeding roots are close to the surface, where the 

 ammonium-salts are caught and retained by the humus in 

 the soil. 



The continued use of large applications of ammonium- salts 

 has also had an injurious effect upon the reaction of the soil, 

 since it behaves as an acid, and continually removes carbonate 

 of lime. The creeping surface vegetation tends to accumulate, 

 and decays into a substance resembling peat ; at the same time 

 the vegetation shrinks into tufts, between which are bare 

 patches of black soil, showing an acid reaction to litmus 



