102 THE PRINCIPLES OE AGRICULTURE. 



The benefit to be derived from both fertilizer and soil 

 depends partly iipon their combination with each other. 



5. To kill weeds. 



Weeds are injurious to growing crops in various ways : 



(1.) By occupying the soil to the exclusion of culti- 

 vated crops, and forming a shade, depriving them of the 

 beneficial influences of sunlight. 



(2.) By withdrawing moisture from the soil, which 

 is taken up as sap and transferred to the atmosphere 

 through the leaves. The amount of water thus removed 

 from the soil, where weeds are numerous, may cause a 

 crop to suffer from want of moisture, while a crop upon a 

 neighboring plot free from weeds will be amply supplied. 



(3.) By withdrawing food from the soil upon which 

 crops would otherwise feed. 



The quantity of available plant food in the soil at any 

 time is limited. The strongest, most vigorous, and quick- 

 est growing plants are sure to obtain the greatest share. 

 Most agricultural plants are not so vigorous when young 

 as the ordinary weeds. Weeds grow much faster at first, 

 and so tend to starve out other crops. 



Weeds should always be destroyed when young and 

 small. If allowed to grow until they attain considerable 

 size, they damage the crop, and are more difficult to ex- 

 terminate. They are plants, some varieties of which 

 might be of service in their proper places, but they can- 

 not be made useful when growing in the midst of other 

 crops. 



Although returned to the soil, they reduce the avail- 

 able fertility of the farm by taking up the elements of 

 plant food and converting them into vegetable tissue, 

 some of which may require years to be again trans- 

 formed into soluble matter. 



