350 Journal of Agricultural Research voi.xx, No. 5 



Summarizing this discussion of the results for the individual crops, 

 we have the following average values of the correlation coefficients : 



1911, sugar beets — o. 077 



1912, total alfalfa + ■ 2 4 2 



1913, alfalfa I + - 346 



1913, alfalfa II + • 403 



1913, alfalfa I and II + - 441 



1914, alfalfa I + . 401 



1914, alfalfa II + . 354 



1914, alfalfa I and II + . 407 



1914, alfalfa III + . 366 



1914, alfalfa I to III + .428 



1915, ear corn +0. 167 



1916, ear corn + . 486 



1917, oat straw + . 202 



1917, oat grain + . 289 



1917, total oats + . 293 



1918, silage corn -j- .226 



1919, barley grain + . 141 



1919, barley straw + . 086 



1619, total barley -f- . 126 



General average + . 274 



With the exception of the sugar beets the average correlation for 

 every crop is positive in sign, and in many cases it is of a very material 

 value. 



Returning to the averages for the individual crops, we note from 

 Table IX that the lowest correlation for alfalfa, whether with other 

 cuttings of alfalfa or with the yield of other crops, is that for the single 

 cutting of 191 2. 



It might be suggested that the 191 2 yields of alfalfa are less likely 

 to reflect the real producing capacity of the plots than the yields of the 

 later cuttings of this crop, for the reason that the first cutting of alfalfa 

 when sown without a nurse crop is subject to much variation because 

 of slight differences in surface condition of the soil at seeding time and 

 also because of differences in weediness of different plots. Both these 

 conditions would become relatively less important in their effect on 

 crop yield after the first cutting. 



Because of its nitrogen-fixing capacity and the resistance to decay of 

 the roots and stubble of alfalfa the correlation between the various 

 yields of this legume and the yields of subsequent crops is of especial 

 interest. Fortunately two crops of ear corn were grown immediately 

 after the alfalfa, which was broken up in the fall of. 191 4. 



A comparison of the correlations of these two series of corn yields 

 with the preceding yields of alfalfa is made in Table X. These coefficients 

 indicate a positive correlation between all the yields of alfalfa and the 

 yields of ear corn in both 191 5 and 191 6. 



Of the 19 correlations determined between the yields of alfalfa for 191 2 

 to 1 914 and the yields of ear corn in 191 5 only 9 may be looked upon 

 as probably significant in comparison with their probable errors. Of 

 the 19 correlations between the yields of alfalfa in 191 2 to 1914 and the 

 yields of ear corn in 1916 only one coefficient — that for the 1912 yield of 

 alfalfa and the 191 6 yield of corn — can not be considered as represent- 

 ing a real agronomic relationship between yield of alfalfa and yield of corn. 



The constants for 191 6 are without exception larger and with two 

 exceptions significantly larger in comparison with their probable errors 

 than those for 191 5. 



