THE AFTER-CULTIVATION OF CROPS 165 



LOSS OF WATER THROUGH WEEDS 



We learned in a former chapter that during their 

 growth farm plants require an amount of water 

 equal to from 300 to 500 times their dry weight. 

 Weeds require just as much water and some of 

 them probably more than the cultivated plants. 

 This water is largely absorbed by the roots and sent 

 up to the leaves where it is transpired into the air 

 and is lost from the soil, and therefore is unavail- 

 able to the growing crop until it again falls onto 

 the soil. 



In some parts of the country, particularly the 

 semi-arid West, the rainfall is not sufficient to sup- 

 ply the soil with enough water to grow such crops 

 as it could otherwise produce. In the moister re- 

 gions the rainfall is not evenly distributed through- 

 out the growing season, and there are longer or 

 shorter intervals between rains when the loss of 

 water through evaporation and weeds is apt to be 

 greater than the rainfall. For these reasons it is 

 best to check these losses and save the water in the 

 soil for the use of the crops. 



SAVING THE WATER 



This can be done by : 



Preventing the growth of weeds and by checking 

 losses by evaporation with a soil mulch. 



