400 Decomposition of Organic Matter in Soil 



bacterial numbers more fully until the other organisms have been 

 studied in more detail. Investigations are now going on in our labora- 

 tory to discover means of counting the protozoa of the soil. 



We must now turn to a closer discussion of the nitrate curves. 



The difference between cropped and fallow land is sharply shown 

 in the nitrate content of the soils. On the fallow land receiving 

 farmyard manure (Fig. 8) the nitrate steadily accumulates from April 

 right up to September, and then drops to the winter level in January. 

 The highest point reached on the dunged plot is no less than 170 lbs. 

 nitric nitrogen per acre in the top 18 inches ; an enormous amount and 

 representing a gain of 100 lbs. over the winter level. The added dung 

 only contains 200 lbs. nitrogen per acre. 



The cropped land at first contains similar amounts, but as soon as 

 plant growth becomes active nitrate ceases to accumulate, no more 

 than 40 lbs. being present, — 90 lbs. less than in the fallow land at the 

 same date, and 120 lbs. less than the maximum there. 



This enormous difference in nitrate content between cropped and 

 fallow land has been known for many years. " The way," says Bacon^, 

 " to hasten the breeding of Saltpeter is to forbid the Sun and the growth 

 of vegetables. And therefore if you make a large Hovel, thatched, 

 over some quantity of ground; nay, if you do but plank the ground 

 over, it will breed Saltpeter." 



The difference is due to the crop but apparently some other action 

 is concerned besides absorption of nitrates : if the amount of nitrogen 

 in the crop (which presumably is equal to the nitric nitrogen absorbed) 

 is added to the amount of nitric nitrogen in the soil the total falls short 

 of the amount found in the fallow land. 



The differences are only small and in the case of the Broadbalk 

 unmanured plot they are within the error of experiment, but the per- 

 sistence with which they appear in the other cases indicates that they 

 are real. The 1912 result is abnormally high because the crop was 

 exceedingly poor, and in such circumstances weeds are always trouble- 

 some. The phenomenon has been discussed in the earlier paper^ where 

 instances are given from the work o'f other investigators : it can only 

 be satisfactorily studied in the laboratory, and we hope to be able 

 to do this. The results are: 



^ Sylva Sylvarum, p. 123. 



- See this Journal, 1914, 6, p. 34. An arithmetical error has crept into the September 

 figures on p. 36, wliich vitiates tiic numerical results given there and also the top para- 

 graph on p. 37 but does not affect this general conclusion. 



