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:

Nutrient Mobility

• Estimating Nutrient Movement by Mass FlowConcentration 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: 2 - 4 million L/ha/yr

Nutrient Mobility• Estimating Nutrient Movement by Mass Flow

– Avg. amount of nutrient carried by mass flow• Ca = 60 - 120 kg/ha• Mg = 50 – 100 kg/ha• K = 8 – 16 kg/ha• P = 0.08 - 0.16 kg/ha

– Typical crop nutrient removal (kg/ha)• Crop Ca Mg K P• Corn 5 10 40 25• Soybeans 10 10 50 15• Alfalfa 112 20 200 25

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

Distance from Root

Soi

l Sol

utio

n C

once

ntra

tion

Nutrient Mobility• Diffusion

– Diffusion coefficient

• 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 = 10-10 cm2/sec

– Diffusion distances

• N 1 cm

• K 0.2 cm

• P 0.02 cm

Mean distance between Corn roots ~ 0.7cm

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

Nutrient Mobility

• Nutrient Mobility and Management– Immobile nutrients (Ex. P)

– Mobile nutrients (Ex. NO3-)

Nutrient Uptake• Uptake is dependent on energy

relationships– Ion activities in solution relative to root– Metabolic energy

Root Soil Solution

Nutrient Uptake• Passive vs Active uptake

– Passive uptake • Ions move into the plant down an energy gradient• Non-specific

Root Soil Solution

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

Root Soil Solution

P

ATP

Carrier

Cell Membrane

P

ADP

Carrier

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

Nutrient Uptake%

P I

n pl

ants

Rhizosphere pH

4 5 6 7 8

+

+

+

+

++

NH4+ Fertilizer

+ N03- Fertilizer

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 0.03

NH4+ 5.59 -

Anions

SO42- 0.97 0.33

Cl- 0.45 0.17

H2PO4- 0.37 0.34

NO3- - 4.72