24 



effect of percentage of intertilled crops and number of animal 

 units per hundred acres of crops on the crop index of a number of 

 farms, as worked out by Mr. D. A. Brodie, of this office, in his 

 study of the relation of farm practice to soil fertility. 



Table III shows this relation for a group of farms in Chester 

 County, Pa. A correlation exists here between percentage of 

 intertilled crops and number of arimal units per hundred acres 

 of crops, for the reason that the more heavily stocked a farm i* in 

 this locality, the larger its percentage of corn area. The live- 

 stock population is thus a causal factor in the percentage area of 

 intertilled crops. 



TABLE III.- 



-Influence of area of intertilled crops and animal units 

 on crop yield in Chester County, Pa. 



The table shows that as the percentage of intertilled crops in- 

 creases the crop index increases up to a ceitain point, but beyond 

 this point there is apparently a falling off. It also shows that the num - 

 ber of animal units per hundred acres of crops increases regularly 

 with increase in percentage of tilled crop area. The farms were 

 here grouped on percentage of intertilled crop area, a resultant 

 factor so far as number of animal units per given area is co.tc-crnecl; 

 but no difficulty is thus introduced for the reason that no other 

 causal relation exists to complicate matters, so that the results are 

 practically the same as if the number of animal units per hundred 

 acres of crops had been used as the basis of classification. 



If the number of animals per hundred acres of crops did not con- 

 tinue proportional to percentage of tilled crop area this table would 

 offer no conclusive evidence as to the action of either of these two 



