40 EXPERIMENTS UPON WHEAT 



on the dunged plot, and shows no evidence of a decline in 

 fertility. The manure on this plot supplies 86 lb. of nitrogen 

 per acre, whereas the average crop has taken away not more 

 than 50 lb. The phosphoric acid and potash supplied are also 

 in excess of the requirements of the crop. On Plot 6 only 

 43 lb. of nitrogen per acre are supplied, little more than the 

 amount removed in the crop. If we consider the other sources 

 of loss of nitrogen to the soil, such as the removal of weeds, 

 drainage, etc., it becomes clear that the 43 lb. of nitrogen 

 in the manure are not sufficient to repair the annual with- 

 drawals of nitrogen. Consequently we should expect some 

 diminution of fertility on this plot, and analyses of the soil 

 seem to show that it is slowly losing nitrogen. The curve 

 expressing the crop on Plot 6 is very similar to that of the 

 unmanured plot, indicating a considerable fall in fertility 

 during the first ten years, and a comparatively constant 

 position for the last forty years. Thus this plot like the 

 unmanured plot seems to have reached a position of com- 

 parative stability, when the annual withdrawal of nitrogen 

 by crop and drainage, etc., is almost balanced by the 

 additions from all sources, so that the fertility of the land 

 is declining very slowly, if at all. Though no material to form 

 humus has been supplied to Plots 6 and 7, and analysis shows 

 that the soil is gradually being deprived of its original stock, 

 yet the wheat crop so far seems to be unaffected by the loss 

 of this important constituent of the soil. 



Plot 10 has received an annual dressing of nitrogen only, in 

 the shape of 400 lb. of ammonium-salts since the earliest date 

 of the experiments. It will be evident from the curve showing 

 the crop production that, despite this long-continued use of a 

 manure supplying but one element of plant nutrition, the crop 

 has been wonderfully maintained. Whereas the average pro- 

 duction over the whole period is increased by the supply of 

 minerals to the extent of 1'8 bushels, the nitrogen alone has 

 produced an average increase of 7 '6 bushels, the unmanured 

 plot being taken as the standard in either case. The curve. 



