2^4 THE SOILS AND CROPS OF THE FARM. 



ton of pig- weed contains as much phosphoric acid as 

 fifteen bushels of wheat, as much nitrogen as twenty 

 bushels, and as much potash as seventy-five bushels. 



2. Weeds shade the ground. Plants require a 

 certain degree of warmth of the soil to grow satisfac- 

 torily. By shading the ground, weeds may prevent it 

 from obtaining the necessary warmth. 



3. Weeds occupy space. Plants require a certain 

 amount of room, both for roots and tops. If occupied 

 by weeds it cannot be occupied by useful plants. The 

 Canada thistle is probably more harmful in occupying 

 space than in any other way. 



4. Weeds take water from the soil. All growing 

 plants transpire large quantities of water. The quan- 

 tity will vary with the humidity of the air. Lawes 

 found in England that 150 to 270 pounds of water 

 were transpired for each pound of increase of dry sub- 

 stance in different cultivated crops, Hellriegel found 

 in Germany that about 300 pounds of water was tran- 

 spired for each pound of increase of dry matter. 

 Plants cannot reach their full development without an 

 abundance of water throughout their entire growth. 

 Weeds rob the soil of its water, and thereby restrict 

 their growth. An area covered by vegetation evapor- 

 ates much more water than bare soil, or even a similar 

 area of water. 



Sturtevant says that the vineyardists on the uplands 

 of New Jersey find weeds injurious; those on the low- 

 lands do not. On the wetter land the transpiration 

 by the weeds cause that dryness of soil that is bene- 

 ficial to the grape. 



5. Weeds are troublesome and injurious to stock. 

 The cockle burr is more troublesome to stock than it is 



