The rhizosphere is the soil zone immediately surrounding plant roots that is influenced by root activity and exudates. Compared to bulk soil, the rhizosphere has higher microbial activity, changes in nutrient concentrations due to root uptake and exudation, and changes in pH and redox potential. The size of the rhizosphere varies by plant species but typically extends 1-3 mm from root surfaces. Nutrient availability in the rhizosphere depends on rates of mass flow, diffusion, and root uptake compared to concentrations in bulk soil.

1. Rhizosphere
& Nutrient
A
vailability.

2. Rhizosphere
• The soil zone in the immediate vicinity of
plant roots which is directly influenced by
roots, is called the rhizosphere.
• The soil loosely adhering to the roots is called
rhizosphere soil.
• The soil closely adhering to the roots is called
rhizoplane soil.
• The rest of the root zone is called bulk soil.
• Rhizosphere extends about 1 to 3 mm from the
root surface into the bulk soil.

4. Rhizosphere vs bulk soil
• In rhizosphere:
– Depletion/accumulation of the nutrients
b/c of root uptake.
3 2
– R. release H+, HCO - & CO → pH
changes.
– R. consume/release O2 → Red. Pot.
Changes.
– Low. Mol. wt. root exudates mobilize
mineral nutrients.
– High microbial activity due to high C.

5. Spatial Extension of
Rhizosphere
Species Rh. zone (mm)
Maize 0.4-1.9
Clover 0.8
Wheat 1.5
Ryegrass 1-3.5
Rape 2.4->4.0
Onion 2.0

6. Size of Rhizosphere
• The size of Rhizosphere may extend
well beyond the first few mm of soil
adhering to plant roots
• The variation in size of Rhizosphere
depends upon
– Water uptake, Mob. Nut. Uptake
– Poorly Mob. Nut. uptake

7. Ion conc. in rhizosphere &
bulk soil
• Can be lower, higher or similar
depending on:
• conc. of specific ion in the bulk soil
solution.
• the delivery of the ion to the root surface;
– mass flow : transport of b. soil soln. along WP
gradient.
– Diffusion : nut. transport along conc. Gradient.
• rate of ion uptake by the root
– For example:
• In ryegrass depletion of K in the
rhizosphere occurred in a few days
resulting in release of interlayer K.

8. Estimated Contribution of Root Interception,
Mass Flow & Diffusion in Mineral Nutrition of
Maize
Mineral
Nutrient
Uptake Interce
p- tion
Mass
flow
Diffusio
n
kg ha-1
N 190 2 150 38
P 40 1 2 37
K 195 4 35 156

9. Estimated Contribution of Root Interception,
Mass Flow & Diffusion in Mineral Nutrition of
Maize
Mineral
Nutrient
Uptake Intercep
- tion
Mass
flow
Diffusio
n
kg ha-1
Ca 40 60 150 0
Mg 45 15 100 0
S 22 1 65 0

10. pH of rhizosphere & bulk
soil
• Rh.pH may differ from B.soil up to 2 units b/c
of:
– Imbalance in cation /anion uptake ratio.
– Difference in net release of H+ & OH- &
– Excretion of organic acids.
– Release of org. C. →↑ micr. prod. of org.
acids.
– Release of CO2 that get dissolved in soil soln.
• pH diff. of up to 2 units can also occur :
– Along the root axis &
– b/w the primary & lateral roots.

11. Factors governing
rhizosphere pH
– The pH buffering capacity of the soil
– Initial soil pH
– Plant spp. b/c of diff. cation/anion
uptake ratios
– The form of nitrogen supplied
• NO3 supply is accompanied by H+ consumption
• NH4 supply is accompanied by H+ excretion
• In general NH4 is taken up more; ↓pH generally
• N2 fixing plants
– Have high cation /anion uptake ratio and high H+
release
– Can take up more phosphate, iron and manganese
• Neutral or alkaline soils;
– NH4 supply improves P & micronutrient uptake
• Acid soils;
– NO3 supply improves P availability as pH ↑

12. The form of nitrogen supplied
N
suppl
y
Rh.
pH
Contents in shoot dry matter
mg g-1 µg g-1
K P Fe Mn Zn
NO3-N 7.3 13.6 1.5 130 60 34
NH4-N 5.4 14.0 2.9 200 70 49

13. Factors governing
rhizosphere pH
• Nutritional status of the plants e.g.
– Zn & Fe def. results in ↑ H+ release and
↓ pH.
– P def. results in Rh. acidification;
dicots.
– Such acidification usually occurs in
apical root zone.

14. Ionic Conc. Changes
• Accumulation
– Of those nutrients which move through mass
flow
– Higher conc. In soil solution
– Higher rate of mass flow than uptake
• Depletion
– Mostly those nutrients which move
through diffusion
– Lower Conc. In soil solution
– Higher rate of uptake than diffusion

15. Redox potential in
Rhizosphere
• Red.Pot. is a measure (V) of affinity of a
substance for electrons
• Aerated soil: +500-700mV.
• ↑ in water contents ↓ Red. Pot.
• Microsites of low RP may occur in aerated soils
particularly in Rhizosphere; imp. for Fe & Mn
nutrition & gaseous loss of N
• Red. Pot. Results in
– Changed solubility of Mn, Fe & P
.
– Accumulation of phytotoxic organic solute

16. A Sequence of Reduction Reactions Occur
after Flooding a Soil

17. There are concomitant changes
in the redox potential and pH
of the soils. The pH of acid soils
gradually changes. The
sequential changes in the Eh
are shown below
Redox Potential