Root Surface Required for Nutrient Absorption

To calculate the inflow, Cta must be found from the concentration in the soil bulk taking into account rates of transport through the soil. Kirk and Solivas (1997) have done this for N uptake by rice growing in flooded soil and used the resulting model to assess the relative importance of root uptake properties and transport through the soil. Their results are summarized in the following. 

Kirk and Solivas measured the time course of N uptake by soil-grown rice plants and the simultaneous changes in soil solution NH4+ and root length density, and then compared the results with the calculated minimum root length densities required to explain the uptake. The calculation was based on the following picture of events. 

The corresponding equations are as follows. For roots uniformly or randomly distributed in volume V of soil at density Ly (length per unit volume), the rate of uptake is 

This is somewhat surprising in dryland crops the total root length is generally far larger than necessary to account for N uptake. An important difference is that, as a result of NH4+ adsorption on the soil solid, unlike for NOs , the concentration of NH4+ in the soil solution is less than for high affinity NH4+ transporters in the root and so V Vmax and a larger root length is required. 

However the calculations indicate an unlikely lack of margin for error in the root length. A possible explanation is that N species other than NH4+ are also being absorbed—such as NO3 or amino acids—as discussed in Section 6.3. 

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