January, 1910.] 



47 



Scientific Agriculture. 



As a rule, we can do this most readily 

 by repeating the measurement, chang- 

 ing, if possible, the process and the 

 instruments used ; a consideration of 

 the differences in the results obtained 

 will then show us what is the most pro- 

 bable result and within what limits it is 

 likely to be correct. If, for example, 

 successive measurements of a piece of 

 land bring out the area as 184,184*3, 

 183*5, 184*6, and 183*3 square yards, we 

 may accept 184 square yaids, the mean 

 of the results, as the most probable 

 area, and we may further conclude 

 that we are then not likely to be 

 more than a quarter of a square 

 yard wrong on one side or the other. 

 The more measurements we make 

 the nearer the mean will be to the 

 truth, always provided that there is 

 not some definite source of error which 

 is repeated in all the experiments, such 

 as would be caused by want of truth 

 in the measuring tape in the example we 

 have been using. 



Field trials, whether they are to test 

 the effects of different manures, or dif- 

 ferent varieties of the same crop, or 

 variations in the cultivation, are gener- 

 ally recognised as being subject to a 

 large number of sources of error, so 

 that it becomes of considerable import- 

 ance in drawing conclusions from such 

 experiments to know what degree of 

 accuracy in the results we can expect, 

 supposing all the conditions have been 

 favourable. Of course, after a set of 

 field plots have been laid out, great 

 variations in the soil may reveal them- 

 selves, due either to changing subsoil 

 and drainage or to some past irregularity 

 of manuring or cropping. Again, the 

 plots may suffer most irregularly from 

 some insect or fungoid attack. In these 

 cases one must ignore the results entirely 

 and begin afresh. But supposing the 

 field to be sensibly uniform and a good 

 stand to inve been obtained, what sort 

 of differences in the yields from two 

 plots may be taken to indicate an effect 

 ot the treatment they have received, 

 and what must be regarded as covered 

 by the natural variation due to unknown 

 causes ? 



We may take the Rothamsted experi- 

 ments as satisfying all the external con- 

 ditions of accuracy ; the land is reason- 

 ably uniform, more care is given to the 

 plots than would be possible under 

 ordinary farming conditions, while the 

 staff have both experience and organi- 

 sation to ensure accuracy in weighing 

 and measuring the produce. If we then 

 select from the Rothamsted records 

 various pairs of plots receiving the same 

 treatment, we find at once that they 



do not give similar yields year by year, 

 but vary with considerable irregularity. 

 As an example, we may take the two 

 unmanured plots on the grass field and 

 set down both their actual and relative 

 yields for the last fifty years. If the 

 soil of the two plots is identical, they 

 should show the same result after a 

 certain number of years ; but if there is 

 some permanent difference between the 

 two revealed by the averages, it will be 

 possible to see how far this difference 

 would be made evident by a single 

 year's trials. 



Looking at the relative yields set out 

 in Table I., we see that Plot 12 in 37 

 years out of the 50 gave a bigger crop 

 than Plot 3, but on thirteen occasions it 

 gave less : taking the mean of the whole 

 period, its relative yield is 110 against 

 100 for Plot 3. Granting, however, that 

 it is really about 10 per cent, the better 

 plot, there have been many years when 

 it gave a 30 per cent, better yield, and 

 in one year it was 96 per cent, better; 

 on the other hand, it was on two oc- 

 casions 10 per cent, below Plot 3. Mathe- 

 maticians have devised a process where- 

 by we can calculate from such a collec- 

 tion of results the value we may safely 

 attach to the result, and using this 

 method, we shall find that the "pro- 

 bable error " of the mean result is about 

 2 per cent, ; i.e., from the fifty years' 

 results we may conclude that there is 

 an inherent superiority in Table I. 



Actual and Relative Yield on Two 

 Unmanured Grass Plots, 

 Rothamsted. 



93 

 102 

 111 

 111 

 105 



90 

 100 



90 



95 

 118 

 131 

 125 



95 

 100 

 111 

 118 

 117 

 133 

 135 

 168 

 119 

 104 



97 

 119 



TTo- 



Yield of Hay. 



11 



Yield of Hay. 









K 



yield < 

 Plot 











Plot 3. 



Plot 12. 





Plot 3. 



Plot 12. 



1856 



2,615 



2,351 



93 



1882 



2,524 



2,340 



L857 



2,856 



2,592 



91 



1883 



2,266 



2,322 



L858 



2,472 



3,360 



136 



1884 



1,804 

 2,101 



1,91)6 



859 



2,540 



2,576 



101 



1885 



2,339 



L860 



2,760 



2,884 



104 



1886 



2,547 



2,672 



L861 



2,844 



3,304 



116 



1887 



1,471 



1,330 



862 



3,052 



3,424 



112 



1888 



2,296 



2,298 



1863 



2,284 



2,844 



125 



1889 



2,638 



2,383 



864 



2,688 

 1,296 



2,808 



104 



1890 



1,648 



1,565 



i860 



1,932 



149 



1891 



2,060 



2,422 



1866 



2,660 



3,012 

 3,048 



113 



1892 



1,627 



2,130 

 487 



867 



3,332 



91 



1893 



391 



868 



i,9eo 



2,676 



137 



1894 



2,685 



2,638 



869 



4,266 



4,352 



102 



1895 



1,402 



1,399 



870 



644 



1,260 



196 



1896 



1,144 



1,272 



871 



2,844 



2,860 



104 



1897 



1,742 

 1,922 



2,018 



l872 



1,644 



2,252 



137 



1898 



2,256 



873 



1,372 



1,804 



131 



1899 



1,342 



1,788 

 1,859 



L874 



1,412 



1,642 



116 



1900 



1,379 



875 



3,620 



4,232 



117 



1901 



455 



765 



L876 



1,384 



1,599 



116 



1902 



1,004 



1,200 



877 



2,360 



2,165 



92 



1903 



1,509 



1,571 



878 



1,848 



1,832 



99 



1904 



2,949 



2,872 

 2,297 



879 



3,028 



3,167 

 1,081 



104 



1905 



1,936 



880 



848 



127 









881 



1,480 



1,393 



94 



Av. 



2,057 



2,254 



