8 



STATION BULLETIN 355 



There was no difference between heavy-and light-dropping trees 

 in density of population of small roots in any given soil zone. In 

 both cases the greatest density of population of small roots occurred 

 in the B and C layers, the least in the A layer. The latter condition 

 ma\- possibly be due to the fact that few small roots were encountered 

 in the top 3 inches of soil because of frequent injury by the cultivator 

 discs. 



Fig. 2 shows that there were the same average number of small 

 roots in the top 9 inches of soil under both heavy-and light-dropping 

 trees. Between the 9-and 21-inch level there were more small roots 

 under the light-dropping trees. Below 21 inches the greatest number 

 of roots were found under the heavy dropping trees without relation 

 to zone. Thus, conditions seemed less favorable for the development 

 of small roots at the deeper levels under light-dropping than under 

 heavy-dropping trees. Whether the greater number of small roots 

 below 21 inches enabled the heavier-dropping trees to obtain more 

 water and nitrogen in not known. That there were exceptions to the 

 general rule is indicated by the fact that trees #88 and #90 located 

 in topsoil only one foot deep were heavy-droppers while tree #70, a 

 light-dropper, A\-as growing in soil only 14 inches deep. Dickson'*, 

 MacDaniels^^. and Southwicki-'* concluded that increased nitrogen 

 supply results in heavier drop. It is possible that more deeply root- 

 ing trees were able to obtain a greater total amount of nitrogen, es- 

 pecially in dry weather, thereby being a factor contributing to heavier 

 drop. 



NUMBER 

 OF ROOTS 



500 



400 



300 



200 



100 



T 



Light Droppimg Trees 



Heavy 



36" 4'2' 48' 



to 4-2.' to-4-8" ttjS-^' 



Fig. 2. Number of roots 2 mm. or less in diameter located at different 



depths below the soil surface. 



