98 FIELD AND LABORATORY STUDIES OF CROPS 



(1) Weeds rob the soil of food materials that are re- 

 quired by other plants. While the total amount of food 

 elements in the soil is sufficient in amount, the quantity 



Fig. 41. — Horse Sorrel, the Presence of Which Indicates an Acid Soil. 

 (Second Ohio Weed Manual.) 



which is available at any one time is not sufficient to 

 supply both the crop and covering of weeds. 



(2) Weeds rob the soil of moisture. In dry seasons, 

 when the moisture supply is limited, it is easy to see that 



