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 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: 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 Soil Solution Concentration Distance from Root

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 = 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 Cell Membrane Root Soil Solution Carrier Carrier ATP P P ADP

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 NH4+ Fertilizer + N03- Fertilizer % P In plants + + + 4 5 6 7 8 Rhizosphere pH

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