336 Journal of Agricultural Research vol. xx.no. s 



data. He gives a table which indicates that there is such a change. 



He says: 



It is noticeable that the variability as measured by the standard deviation becomes 

 less in each succeeding year. This suggests the question as to whether continued 

 cropping might not tend to induce uniformity. The records of a few of these plots 

 which were continued in corn for three years longer do not support such a conclusion. 



It must be noted that in Smith's experiments seasonal conditions 

 varied greatly from year to year. Thus 1895, which was exceedingly 

 dry and also cool in the early part of the season, was highly unfavor- 

 able. The two following years were unusually favorable for corn. As 

 a result the yields were, respectively, 31.6, 91.6, and 71.4 bushels per 

 acre in the three years. 



Lehmann in his work at the experimental farm near Bangalore at- 

 tempted to use the experience of previous years in the standardization of 

 experimental plots. His data will be considered in some detail below. 



II.— METHODS AND RESULTS 



The permanency of the differentiation of plots in their capacity for 

 crop production may be measured in terms of correlation. If the plots 

 of a field differ among themselves in a more or less permanent way 

 there will, with reasonably uniform climatic conditions, be a correla- 

 tion between the yields of the plots of a series in two or more successive 

 years — in short, an intera nnual correlation (2). 



The problem of the correlations between the yields of identical plots 

 in different years is one of very great interest. If this correlation be 

 high it should be possible to standardize a field of plots by one or more 

 sowings to the same variety. A chief difficulty in the standardization 

 of the field by the carrying out of a preliminary test in which the pro- 

 ductive capacities of the plots are determined once and for all lies in 

 the fact that the factors which determine yield are in part edaphic — 

 that is, pertaining to soil conditions — and in part meteorological. For 

 example, in a very dry year sections of a field which are lower may pro- 

 duce the heaviest crops because adequate moisture is longer retained 

 in these places. In a wet year the case may be just the reverse, for the 

 crops in the lower-lying portions may be too wet for the best plant 

 growth. Thus, it is quite possible that in cases in which there is a 

 profound influence of environmental factors there may be a negative 

 correlation between the yield of the same plots in different years. 



It is conceivable, therefore, that the interannual correlation for yield 

 per plot may range from negative to positive values, zero correlation 

 being found in cases in which edaphic and meteorological factors exactly 

 counterbalance each other in their influence upon the yield of the plots 

 of a heterogeneous field. 



