Feb. II. 1918 Effect of Season and Crop Growth on Soil Extract 359 



pression, it is clear that this will not account for the differences noted. 

 With phosphates the probable maximum is 4 to 6 per cent, while with 

 magnesium it would vary from 6 to 10 per cent. All these corrections 

 fail to change significantly the figures recorded. 



At two periods during the season of 191 6 daily studies of the water 

 extract were made with soil 8, and at one period with soil i. The result 

 of these tend to show that considerable fluctuations may take place from 

 day to day. The results of these two studies are plotted in figures 22 

 to 24. It will be seen that the fluctuations in general occur simulta- 

 neously in both containers and so will not alter the relationship of the 

 graphs for the planted and unplanted soils. They show, however, that 

 small differences in the range of the graphs can not be considered sig- 



Fig. 22. — Graphs of the daily studies of the water extract of soils lA and iB, season of 1916. 



nificant. The general range of the graphs is believed to represent an 

 average figure, but small variations between soils should always be dis- 

 regarded. This would also indicate that an expression of the differences 

 in water-soluble material can only be given by a series of related obser- 

 vations. 



DISCUSSION AND DEDUCTIONS 



In the foregoing studies it is believed that the limitations as well as 

 the possibilities of the experimental methods have been pointed out. 

 It is earnestly desired that no deductions should be drawn which are not 

 conservatively justified by the data presented. 



It should be noted that the results have been obtained on 13 soils 

 which comprise only two distinct soil types. These types are, however, 



