THE ROTHAMSTED EXPERIMENTS 379 



gen on Broadbalk produced 9.0 bushels of wheat per acre. A 40 

 bushel crop of barley would remove in the grain and straw about 

 56 pounds of nitrogen, in accordance with the average of many 

 analyses; so that, where 4o-bushel crops are produced with only 

 43 pounds of nitrogen supplied, the soil is now being exhausted of 

 its nitrogen content about as rapidly as on the unfertilized land. 

 According to the analyses reported by Dyer, the nitrogen content 

 of the soil to a depth of 27 inches decreased by 528 pounds per acre 

 on plot A4 and by 841 pounds on plot N4 during the 14 years from 

 1868 to 1882, while the nitrogen content of plot 04 actually in- 

 creased by 8 1 pounds per acre. 



This problem is complicated by the fact that there is often con- 

 siderable growth of leguminous weeds (especially of yellow trefoil) 

 on plot 04. The decrease in yield from 24.2 to 15.5 bushels cer- 

 tainly does not harmonize with any actual increase in the nitrogen 

 content of plot 04, but it seems very certain that the nitrogen 

 content of plots A4 and N4 was drawn upon during the 14 years 

 at the rate of 40 to 50 pounds a year, of which probably one half 

 is lost in drainage, as an average. 



In the lower part of Table 66 are recorded some computed effects 

 for different elements under different conditions. Of course, many 

 other similar computations could be made from the data. In 

 computations of this sort, the first effect should be determined for 

 the most limiting element, the next effect for the second limiting 

 element, etc. While it is of interest to compute the effect of apply- 

 ing the most limiting element where all others have been applied, 

 the result has no practical significance, because every application 

 should pay for itself. 



It is evident that nitrogen is the most limiting element for barley 

 on Hoos field, because the ammonium salts produce a greater in- 

 crease alone than either acid phosphate or alkali salts. Phosphorus 

 is as clearly the second limiting element. 



While the alkali salts alone had some power to increase the yields 

 during the earlier years (probably due to their power to liberate 

 phosphorus or encourage nitrification), their stimulating action 

 during those years is indicated by reduced yields during the later 

 25-year period when plot 03 produced less than Oi. Exactly the 

 same conditions appear where alkali salts have been added to acid 



