112 AGRICULTURE 



to a great extent by the superficial layers 

 of the soil, and therefore does not reach the 

 roots of a top-dressed crop with sufficient 

 rapidity. Of these two manures nitrate of 

 soda is the better for application to the 

 hay crop, and this chiefly because we have 

 hereto do with a crop that can only be manured 

 by top-dressing, so that the added plant food 

 must find its way down through a greater or 

 less depth of soil before reaching the roots. 

 The superior merits of nitrate of soda, as 

 contrasted with sulphate of ammonia, as a 

 top-dressing for hay, are well illustrated 

 in the long series of experiments conducted 

 on the permanent meadow at Rothamsted. 

 Without exception, nitrate of soda has pro- 

 'duced a much heavier crop of hay than 

 ammonium salts, and botanical analysis has 

 -shown that the hay is also of superior quality. 

 Nitrogen in the form of ammonia is largely 

 retained in the surface soil, and it has there- 

 fore the effect of encouraging the development 

 of shallow-rooted plants, such as Sheep's 

 Fescue, Agrostis, Yorkshire Fog, Sweet- 

 scented Vernal, and Smooth-stalked Meadow 

 Grass ; whereas nitrate-nitrogen, by sinking 

 deep into the soil, encourages the growth of 

 plants whose roots are similarly disposed, 

 notably Meadow Foxtail, Tall Oat Grass, 



