226 NITRIFICATION 



soil, probably because the closeness of textm^e permits but 

 little capillary movement of water to take place. This we 

 learn from the comparison of the yields on Plot 5, receiving 

 minerals but no nitrogen every year, and on Plot 17 or 18, 

 which receives alternately minerals and ammonium-salts. As 

 has already been pointed out (p. 51), in the years this plot 

 17 or 18 receives minerals but no nitrogen, its crop sinks 

 almost exactly to the level of the crop on Plot 5, although it 

 had received 86 lb. of nitrogen as ammonium- salts the year 

 l^efore. Clearly, then, on the Rothamsted soil ammonium- salts 

 are not retained as such for more than the season of applica- 

 tion, nor are the nitrates resulting from them able to return to 

 the surface to feed the succeeding crop. On other soils of 

 better textm'e for allowing the movements of water by capil- 

 larity there can be no doubt that the nitrates in the subsoil 

 water will retmni to the suiface and be of service to the crop. 



It must not be supposed, however, that dressings of 

 manures like nitrate of soda and sulphate of ammonia, which so 

 readily wash away as nitrates, are entirely without action on 

 the succeeding crops. Because of the very fact that they cause 

 a large growth, there is left behind in the soil a corre- 

 spondingly large development of root and stubble, which will 

 decay for the benefit of succeeding crops. Especially is this 

 the case where some considerable proportion of the crop grown 

 is not harvested, but is returned at once to the land, as is done, 

 for example, with the leaves of mangels or the haulm of 

 potatoes. A striking example is seen at Rothamsted, on the 

 plots which grew potatoes for twenty-six years, from 1876 to 

 1901, and were then sown with barley without fuither manuring. 



Table LXXX. shows the total produce (grain and straw) 

 of the first and second crops (barley) and the third crop (oats), 

 after the manuring had been discontinued. 



It will be seen that the change from potatoes to barley was 

 followed by enormous crops of grain wherever nitrogenous 

 manure has been used for the potatoes ; the two plots which 

 had previously been dunged gave over 70 bushels of grain per 



