224 Physics of the Soil. 



the possible yield of any crop lessened. No soil can mature 

 a maximum crop of corn when weeds are permitted to grow 

 with it. Neither is it possible for an orchard of any kind to 

 come into bearing as quickly or to produce as vigorous 

 trees where the soil between and beneath them is occupied 

 by either weeds or grass. It may be thought that so long as 

 the weeds are destroyed upon the ground they return to it 

 whatever they have taken out and therefore cannot leave 

 the soil poorer. To this it must be said that whatever 

 moisture is removed is a positive loss because it is carried 

 away by the winds ; the nitric acid that is taken up and the 

 potash, phosphoric acid and other ash ingredients are also 

 largely a positive loss so far as that season is concerned for 

 they are removed from the soil moisture and converted into 

 dry matter in the tissues of the weeds where the crop can- 

 not use them. Even if the weeds are killed while the crop 

 is yet on the ground they cannot furnish food for it for 

 they are likely not to decay soon enough to become at once 

 available. 



273. The Best Time to Kill Weeds. The best time to kill 

 weeds is just as the seeds are germinating or while they 

 are yet very small. When this is done but little moisture 

 is lost through them and they render but little plant food 

 insoluble. In the thorough and early preparation of the 

 seed-bed many weeds are destroyed by killing them just as 

 they are coming up. So, too, in the case of a grain field, 

 which is rolled after being seeded and is then harrowed, the 

 rolling hastens the germination of the weed seeds and the 

 harrowing then throws them out into a dry soil which kills 

 them. If such a field is again harrowed just after the gram 

 is up a second crop of weeds may be destroyed and the 

 yield made greater as a consequence. 



In the case of potatoes and corn it is very easy to destroy 

 at least two crops of weeds before the corn or potatoes are 

 large enough to cultivate, by harrowing before and just 

 after the plants are up. This is very important because it 

 not only saves plant food for the crop but it can be done 



