PLANT ROOT-SOIL INTERACTIONS 



683 



50 



-^40 

 o 



Q. 



o 30H 

 ■5 



20 



.<!' 



5 10 



Fe = 02510- + 046 

 (r = 0986) 







T — ' — r 

 40 60 



20 40 60 80 100 120 140 

 Minimal root surface area cr, cm.^ 



Figure 9. The uptake of iron by plants in relation to the minimal root-suri"ace 



area. 



These variations in the surface densities of contact sites set in mo- 

 tion the profound differentiation in growth rates shown in Figure 8. 



Generalizing this conclusion for the entire experiment, by stating 

 that the rate of iron uptake is proportional to the numher of contact 

 sites on the root surface, we formulate: 



dFe 

 "dT 



= ki pa 



where p is the density of Fe-sand grains per cm-, a the minimal surface 

 area of the root, and ki a proportionality factor. 



Neglecting in Figure 8 the minute intersect on the y-axis, the iron 

 content of the plant is directly proportional to root area: 



' Fe = hza 

 dFe ki 



dt 



Fe, 



k2 



k.> 



^P 



or Fe = (Fe)„ e 

 For a given age of the plant, and since p = 2i, 



Fe = ( Fe ) „ e'^' 



which is the equation found empirically. It is not known how far 

 beyond 20 per cent Fe-sand the equation retains its validity. 



