1. Nutrient Mobility Nutrient Movement to Roots
Root Volume in 0-15 cm surface horizon of a soil: 0.5 to 1%
Therefore, movement of available nutrients to the plant roots is critical.
Mechanisms of nutrient movement to roots:

2. Nutrient Mobility Estimating Nutrient Movement by Mass Flow
Concentration of nutrients in soil solution X Volume of water transpired by the plant
Avg. Soln. Nutr. Conc.
Ca = 30 mg/L
Mg = 25 mg/L
K = 4 mg/L
P = 0.04 mg/L
Avg. Crop Transpiration: million L/ha/yr

3. Nutrient Mobility Estimating Nutrient Movement by Mass Flow
Avg. amount of nutrient carried by mass flow
Ca = kg/ha
Mg = 50 – 100 kg/ha
K = 8 – 16 kg/ha
P = kg/ha
Typical crop nutrient removal (kg/ha)
Crop Ca Mg K P
Corn
Soybeans
Alfalfa

4. Nutrient Mobility Diffusion
Nutrients will diffuse from a high to a lower concentration
Fick's Law F = -DA dC/dx
F = flux
A = area
D = Diffusion coefficient
dC/dx = concentration gradient
Soil Solution Concentration
Distance from Root

5. Nutrient Mobility Diffusion Diffusion coefficient Diffusion distances
NO3-, Cl- D = 10-6 cm2/sec
NH4+, K+, Ca2+, Mg2+ D = 10-7 cm2/sec
Zn2+, Mn2+ D = 10-8 cm2/sec
H2PO4- D = cm2/sec
Diffusion distances
N 1 cm
K 0.2 cm
P 0.02 cm
Mean distance between Corn roots ~ 0.7cm

6. Nutrient Mobility Ion Corn (kg/ha) Mass Flow Diffusion Root
Interception Ca 45 90 - 40 Mg 35 75 8 K
110 12 95 3 P 30
.12
28.9 1

7. Nutrient Mobility Nutrient Mobility and Management
Immobile nutrients (Ex. P)
Mobile nutrients (Ex. NO3-)

8. Nutrient Uptake Uptake is dependent on energy relationships
Ion activities in solution relative to root
Metabolic energy
Root
Soil Solution

9. Nutrient Uptake Passive vs Active uptake Passive uptake
Ions move into the plant down an energy gradient
Non-specific
Root
Soil Solution

10. Nutrient Uptake Passive vs Active uptake Active uptake
Biochemical energy used to transport ions into the cell against the energy gradient
May be nutrient specific
Cell Membrane
Root
Soil Solution
Carrier
Carrier
ATP P P
ADP

11. Nutrient Uptake Passive vs Active uptake Charge Balance
If cations are taken up, then other cations must be released into the soil solution – Usually H+
If anions are taken up, then other anions must be released into the soil solution – Usually OH-
Major changes in Rhizosphere pH

12. Nutrient Uptake NH4+ Fertilizer + N03- Fertilizer % P In plants + + +
Rhizosphere pH

13. Cation and Anion Uptake Interactions
Uptake (meq/pot)
Ion
NH4+
NO3-
Cations
Ca 2+
0.20
0.46
Mg 2+
0.30
0.52
K +
1.76
1.87
Na +
0.03
5.59 -
Anions
SO42-
0.97
0.33
Cl-
0.45
0.17
H2PO4-
0.37
0.34
4.72