38 



loss by drainage ; and plots 7 and 9, which yield the greatest 

 increase in the crop, show the least loss by drainage. 



The excess in the soils (over plot 6) is obviously much more in 

 the order of the increased yield in the crops. Plot 10, with the 

 least in the increase of crop and the most in the drainage, shows the 

 least excess in the soil ; whilst plots 7 and 9, with, the greatest 

 increased yield in the crop, and the least loss by drainage, show the 

 greatest excess in the soil. 



It is clear, therefore, that whilst the excess in the soil has no 

 direct relation to the amount supplied in the manure, it has a very 

 obvious relation to the increased yield in the crop ; in other words, to 

 the amount of growth. The last column of the table brings this 

 out more clearly. Excepting in the case of plot 10, with the ammo- 

 nium salts alone, there is a general uniformity in the proportion 

 of the excess in the soil over plot 5 to the increased yield in the 

 crop over plot 5 ; and the variations, such as they are, have an 

 obvious connection with the conditions of growth. Thus, plots 11, 

 12, and 14, all with a deficient supply of potash, show approximately 

 equal proportions retained in the soil for 100 of increase in the crop. 

 Plots 13, 7, and 9, again, all with liberal supplies of potash, show 

 higher, but approximately equal, proportions retained in the surface 

 soil for 100 of increased yield in the crop. 



Upon the whole, it is obvious that the relative excess of nitrogen 

 in the soils of the different plots is little, if at all, due to the direct 

 retention by the soil of the nitrogen of the manure, but is almost 

 exclusively dependent on the dift'erence in amount of the residue of 

 the crops — of the stubble and roots, and perhaps of weeds. 



Recurring to the main point which it is our object to elucidate, 

 there can be no doubt that the determinations of nitrogen in the sur- 

 face soils of the plots of the experimental wheat field, at different 

 dates, establish the fact that the decline in the yield of nitrogen in 

 the crops, when none is supplied in manure, is accompanied by a 

 decline in the stock of nitrogen in the soil. 



It will be well to consider, as far as the data at command will 

 allow, what relation the yield of the nitrogen in the crops bears to 

 the loss of nitrogen by the soil ? 



On this point it may be stated that, taking the average of thirty 

 years, 1852 — 1881, it is estimated that the nnmanured plot yielded 

 18'6 pounds of nitrogen in the crops, and lost 10"3 pounds in the 

 drainage, or in all 289 pounds per acre per annum over that period. 

 In like manner, it is estimated that plot 6, which received nitro- 

 genous as well as mineral manure during the preceding eight yeais. 



