Crops. 



179 



ter, but this increase is obtained at small expense in plant food. 

 According to Warington, the production of '3000 pounds of dry 

 pine timber requires the consumption of only 2% pounds of 

 potash and 1 pound of phosphoric acid per acre yearly. Harder 



Note the difference in the extent of the root system of the two plants, 

 alfalfa and barley. 



woods require rather more of these constituents. The amount of 

 nitrogen in wood is very small, amounting to an average of about 

 10 pounds for an annual growth of beech wood. Trees produce 

 seed only at mature age and then at the expense of material 

 stored in the leaves and wood. 



