TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 



705 



that for curves a and b, but the rate for the first 12 hours is five times 

 that of the last 12 hours. 



The range of influence, defined as the radial distance from the 

 center of the root at which 90 per cent of the total potential drop 

 occurs, has been calculated by A. Klute (1960) as a function of the 

 root radius and the soil diffusivity for a flow period of one day. The 

 data are summarized in Table I. 



The analysis summarized in Figure 8 shows that the AP necessary 

 to maintain a given flow to the root surface is primarily dependent on 

 the transmission constant k and the flow rate F. If a maximum DPD of 



.jdB.IW'.fc.lS*^' — 



&f 



y 



if) 



1.0 10 



SOIL SUCTION - BARS 



100 



Figure 8. Pressure at plant root for different rates of water uptake as a 

 function of the average moisture potential in the surrounding soil. (From 

 Gardner. ) 



