Soil 3416

Plant Nutrients in the Environment

 

Basic Soil - Plant Relationships

Plants and Plant Roots: Nutrient Transport and Nutrient Uptake

 

  1. Learning objectives: Lecture 3
  2.  

    1. Understand the three ways nutrients in the soil reach plant roots
    2. Know the major mechanism(s) of supply for each essential element
    3. Know how the characteristics of roots and root systems influence nutrient uptake
    4. Be able to describe how plant roots absorb nutrients

     

  3. Nutrients must reach the surface of a root for plant uptake of essential elements to occur
    1. Accomplished by movement of nutrients and growth of roots

     

    Part A: Movement of Ions from Soils to Roots

     

  4. Three major methods of nutrient supply
    1. Root interception
    2.  

    3. Mass flow
    4.  

    5. Diffusion

     

  5. Nutrient supply vs. nutrient transport
    1. Root interception is solely a supply mechanism
    2. Mass flow and diffusion
      1. Mechanisms of supply and transport
    3. Understanding how nutrients move
      1. Important for understanding environmental impacts as well as nutrient uptake

     

  6. Root interception
    1. Roots grow through soil
    2. Contact soil particle surfaces
    3. Root surfaces contact adsorbed nutrient ions

     

  7. Root cation exchange capacity
    1. Mainly due to carboxyl groups (as in OM)
      1. ¾ COOH Û ¾ COO - + H+
    2. Monocots
      1. 10 - 30 meq/100 g
      2. Take up monovalent cations more readily
    3. Dicots
      1. 40 - 100 meq/100 g
      2. Take up divalent cations more readily

     

  8. Contact exchange
    1. Root hair ¾ H+ K+ ¾ Clay / OM surface
    2. exchange Þ Þ Þ Þ
    3. Root hair ¾ K+ H+ ¾ Clay / OM surface

     

  9. Nutrients intercepted by roots
    1. Quantity depends on:
      1. Soil concentration of nutrients
      2. Volume of soil displaced by root system
    2. Roots occupy <1% up to ~2% of soil volume
      1. Even in surface soil (topsoil) where root density is greatest

     

     

    Percentage of the Total Soil Volume Occupied by Plant Roots of Different Crops

    (in the surface 8-inches of soil)

    Crop

    Root Volume (%)

    Kentucky Bluegrass

    2.8

    Winter Rye

    0.9

    Oat

    0.6

    Soybean

    0.4 - 0.9

    Corn

    0.4

    Adapted from S. Barber, Soil Nutrient Bioavailability, 1984

     

     

  10. Root interception
    1. Relatively small, but still an important contribution to nutrients reaching root surfaces
    2. Most significant for (see Table 2.4 in text):
      1. Nutrients present in high concentrations (e.g. Ca, Mg)
      2. Nutrients required in small amounts (e.g. Zn, Mn, and other micronutrients)

     

  11. Factors affecting root interception
    1. Anything that restricts root growth
      1. Dry soil
      2. Compaction
      3. Low soil pH
      4. Poor aeration
      5. Root disease, insects, nematodes
      6. High or low soil temperature
    2. Root growth is necessary for all three mechanisms of nutrient supply, but absolutely essential for root interception to occur

     

  12. Mass flow
    1. Dissolved nutrients carried in flow of water to plant roots
    2. Flow driven by:
      1. Transpiration (the major factor)
      2. Evaporation
      3. Percolation

     

  13. Nutrients supplied by mass flow
    1. Quantity proportional to:
      1. Rate of flow (volume of water transpired)
      2. Solution concentration of nutrient

     

  14. Nutrients supplied by mass flow
    1. Supplies most of the required amounts of "mobile" nutrients
      1. NO3-, SO42-, Cl-, and H3BO3
    2. Often supplies more than the required amounts of Ca, Mg
    3. Can meet Cu, Mn, and Mo requirements
    4. Can supply a significant portion of required Fe and Zn
    5. (see Table 2.4 in text)

     

  15. Factors affecting mass flow
    1. Soil water content
      1. Dry soil ® no nutrient movement
    2. Temperature
      1. Low temperature reduces transpiration and evaporation
    3. Size of root system
      1. Affects water uptake (and therefore movement)
        1. Both amount of water and the volume of soil it comes from
      2. Root density much less critical for nutrient supply by mass flow than for root interception and diffusion

     

  16. Diffusion
    1. Ion movement from an area of high concentration to an area of low concentration
      1. Roots absorb nutrients from soil solution
      2. Concentration at root surface decreases compared to "bulk" soil solution
      3. Ions diffuse down concentration gradient toward root surface

     

  17. Nutrients moved by diffusion
    1. Important for nutrients that interact strongly with the soil
    2. Primary mechanism for supplying P and K
    3. Important for micronutrients, especially Fe and Zn
    4. (see Table 2.4 in text)

     

  18. Factors affecting diffusion
    1. Fick’s Law
    2. dC/dt = De * A * dC/dX
      1. dC/dt = diffusion rate (change in concentration over time)
      2. De = effective diffusion coefficient
      3. A = cross sectional area for diffusion
      4. dC/dX = concentration gradient (change in concentration over distance)

     

  19. Factors affecting diffusion
    1. Diffusion rate directly proportional to concentration gradient, diffusion coefficient, and the area available for diffusion to occur
    2. Effective diffusion coefficient
      1. De = Dw * q * (1/T) * (1/b)
        1. Dw = diffusion coefficient in water
        2. q = volumetric soil water content
        3. T = tortuosity factor
        4. b = soil buffering capacity

     

  20. Effective diffusion coefficient
    1. Diffusion coefficient in water (Dw)
      1. Includes a temperature factor
      2. Colder = slower diffusion
    2. Soil water content
      1. Drier soil = slower diffusion
      2. Less water = less area to diffuse through
    3. Tortuosity
      1. Pathways through soil are not direct
      2. Around soil particles, through thin water films
      3. Affected by texture and water content
        1. More clay = longer diffusion pathway
        2. Thinner water films = longer path
    4. Buffering capacity
      1. Nutrients can be removed by adsorption as they move through soil, reducing diffusion rate

     

  21. How far can nutrients diffuse in a growing season?
    1. Diffusion distances are very short
      1. K ~ 0.2 cm
      2. P ~ 0.02 cm
    2. Size and density of plant root systems is very important for nutrients supplied by diffusion
    3. Has implications for fertilizer placement

     

  22. Nutrient mobility
    1. Mobility affects fertilizer recommendations and environmental management of a nutrient
    2. Mobility depends on strength of interactions between nutrient and soil
    3. Nutrients supplied primarily by mass flow are considered mobile nutrients
      1. e.g. N, S, Cl, B
    4. Nutrients supplied primarily by diffusion are considered immobile nutrients
      1. e.g. P, K

     

     

  23. Immobile nutrients
    1. Yield proportional to concentration of nutrient near root surface
    2. Nutrient sufficiency levels based on relative yield potential,
      1. Do not vary with growing conditions
      2. In favorable growing conditions, greater root growth increases nutrient uptake
        1. Proportionate feeding
    3. Soil tests are indexes of availability
      1. Estimate relative amounts of available nutrients
      2. Percentage of sufficiency
    4. Fertilizer recommendations are not based on yield goals

     

  24. Mobile nutrients
    1. Yield proportional to total quantity of nutrient in root zone
    2. Nutrient sufficiency levels depend upon growing conditions and absolute yield potential
    3. Root density has limited effects on nutrient uptake
    4. Soil tests must estimate the total quantity of nutrient in the root zone
    5. Fertilizer recommendations depend upon yield goals

     

     

    Part B: Root Growth and Nutrient Absorption

     

     

  25. Healthy, vigorous root systems
    1. Extensive, well-distributed root systems
    2. Draw nutrients from a larger volume of soil
      1. Tap a larger nutrient supply
    3. Take up more nutrients
    4. Important root characteristics are:
      1. Root length
        1. Both vertical and horizontal
      2. Root branching
      3. Root hairs

     

  26. Root surface area
    1. Key factor in uptake
      1. Size of the absorbing surface on roots
      2. Small, fine roots have more surface area than the same mass of large roots
    2. Adaptations to increase surface area
      1. Root hairs
      2. Mycorrhizae

     

  27. Root hairs
    1. Perpendicular extensions of epidermis just behind root tip
    2. Increase root surface area by 2-10 times
      1. Length ~0.1-1.5 mm, depending on species and environment
    3. In low P soils, root hairs are longer
    4. Lifespan a few days to a few weeks
      1. Actively growing "feeder" roots required to continually renew these important absorbing surfaces

     

  28. Mycorrhizae
    1. Function as extensions of plant root system
      1. "fungus roots"
      2. Myco = fungus, rhizae = roots
    2. Symbiotic associations between soil fungi and plant roots
      1. Mycorrhizae obtain photosynthate (food) from plant roots
      2. Plants receive additional water and nutrients

     

  29. Mycorrhizae
    1. May extend over 3-inches into surrounding soil
    2. Increase root surface area up to 10-fold
    3. Particularly important for P uptake
      1. In low P soils
      2. Can also increase Zn and Cu
    4. Tillage disrupts mycorrhizae
      1. Also less extensive when nutrient levels are high

     

  30. Soil properties affecting root growth
    1. Structure
      1. Compaction
    2. Air - water balance
      1. Aeration
      2. Water-holding capacity
    3. pH
    4. Disease, insects, nematodes
    5. Temperature

     

  31. Ion absorption by plants
    1. Most nutrients are absorbed by roots in an inorganic form
    2. After reaching a root surface, nutrient ions are transported to plant leaves in a series of steps
    3. Passive root uptake
    4. Active root uptake
    5. Translocation

     

  32. Root structure
    1. Ions must move through (or around) several layers of root tissue
      1. See Figs. 2.15 and 2.16 in text
    2. Epidermis - outermost layer of cells
    3. Cortex - large, irregularly shaped cells with extracellular space between them
    4. Endodermis - cell layer with suberized band, Casparian strip, barrier to movement into stele
    5. Stele - contains the xylem, which transports water and ions to the shoot

     

  33. Passive movement
    1. Diffusion and ion exchange
    2. From epidermis ® through cortex ® to endodermis
    3. Apoplast (or apparent free space)
      1. Extracellular - within and between cell walls
    4. Root CEC is in cell walls

     

  34. Active movement
    1. Must cross cell membrane
    2. Symplast
      1. Intracellular - interconnected cytoplasmic pathway between cells
    3. Active transport across membrane
    4. Selective uptake of nutrient ions

     

  35. Active ion uptake
    1. Energy required to move nutrients across cell membrane
    2. Concentrations higher within cell than outside cell
    3. Movement against an electrochemical gradient
    4. Energy comes from cellular metabolism

     

  36. Ion carriers
    1. Transport across membrane mediated by carriers
    2. Carriers located within membrane
      1. Bind to ion on outer boundary ® move across membrane ® release ion into cytoplasm
    3. Carriers are selective
      1. Specific carriers for most ions

     

  37. Active Transport
    1. Enables plants to be selective about what elements enter roots
    2. To maintain electrical neutrality in root cells, roots release H+ and OH-
      1. Cation uptake: release of H+
      2. Anion uptake: release of OH-
    3. Cation uptake generally > anion uptake, so rhizosphere pH ¯

     

  38. Rhizosphere (rhizo = root)
    1. Zone of soil immediately adjacent to plant roots (~1-4 mm)
    2. Area of active microbial activity
      1. Organic root exudates provide a food supply
    3. Both rhizosphere pH and microbial activity affect nutrient availability
      1. e.g. solubility and chelation

     

  39. Rhizosphere
    1. Rhizosphere and nutrient availability
      1. Lower pH and organic acids can increase solubility
      2. Roots and rhizosphere microbes can both produce chelates
      3. Roots and microbial activity can also increase solubility by lowering redox potential
    2. Both rhizosphere pH and microbial activity affect nutrient availability

     

  40. Active Transport
    1. Enables plants to accumulate essential nutrients
    2. Plants differ in their ability to accumulate nutrients at low soil concentrations
    3. Genetic differences in uptake, translocation, root growth, root metabolism, rhizosphere environment, and other factors

     

  41.  Plant roots
    1. Unseen and often ignored
      1. Below ground and hard to study
    2. Not just passive absorbers of nutrients
      1. Active transport and selective uptake
      2. Modify soil around the root to increase nutrient availability
    3. Soil fertility is not complete without considering the "organs of uptake"

 

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