W1, Intro + Nutrient movement through soils

Question 1

What’s the mobility and method of movement (% of uptake) for:

Boron (B)

Answer 1

• Mobile
• Mass flow = 65%
• Diffusion = 32%
• Root interception = 3%

Question 2

Describe the pattern of nutrient uptake for wheat during the growing season.

Answer 2

Question 3

Explain the differences between the way nutrients can move to the roots (mass flow, diffusion, interception)

Answer 3

MASS FLOW

• nutrients in solution move with that solution
• driven by transpiration
  
  • the greater the transpiration, the greater the movement of water to the root, and therefore the greater the movement of nutrients (because they’re in solution)
• = [concentration in soil solution] * [water transpired]
  
  • e.g.
  • water use = 1.5 ML/ha = 1,500,000 L/ha
  • [soil solution] = 25 mg/L = 0.000025 kg/L
  • plant uptake (required by plant) = 60 kg/ha
  • % uptake by mass flow = 0.000025 * 1,500,000 = 37.5 kg/ha = 62.5% uptake

DIFFUSION

• nutrient movement is based on a concentration gradient (i.e. nutients move from a region of [high] to [low]
  
  • the greater the concentration gradient (difference), the faster the movement
• slower than mass flow
• roots will create a depletion zone in/around the rhizosheath, which gets replenished from the bulk (surrounding) soil

INTERCEPTION

• nutrient doesn’t move, but rather, the root moves to the nutrient (basically just bumps into it).

Question 4

Explain the antagonistic interactions in ion uptake by roots.

Answer 4

Sometimes, the uptake of one ion affects the uptake of another ion.

e. g. large uptake of Na = ↓ uptake of Ca, so a reduction in growth might not be because of high sodium concentration but because of low [Ca].
e. g. applying lots of Ca decreases uptake of Mg
e. g. high Cl uptake = ↓ nitrate uptake

Question 5

What’s the mobility and method of movement (% of uptake) for:

Phosphorus (P)

Answer 5

• Immobile
• Mass flow = 6%
• Diffusion = 91%
• Root interception = 3%

Question 6

True or false?

Sodium is an essential nutrient.

Answer 6

True, but only for some plants.

Question 7

List the 6 essential macronutrients

Answer 7

• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)
• Calcium (Ca)
• Magnesium (Mg)
• Sulphur (S)

Question 8

Explain what a transient nutrient deficiency is and how it might come about (and be resolved)

Answer 8

Transient deficiencies are those that correct themselves when conditions change.

e.g. a period of prolonged cool, dry conditions could induce transient P, B, Zn, K, Fe, and/or Mn deficiencies due to low rates of diffusion and mass flow, which would be rectified once the soils warms- and wets-up.

K uptake is also related to metabolic activity, so during a period of low/reduced metabolic activity (stress, low light, etc.), a transient K deficiency can develop.

Question 9

True or false?

Silicon (Si) is an essential nutrient.

Answer 9

True; it’s essential for some plants, and beneficial for others.

Question 10

What’s the mobility and method of movement (% of uptake) for:

Sulphur (S)

Answer 10

• Mobile
• Mass flow = 95%
• Diffusion = 0%
• Root interception = 5%

Question 11

Explain what the rhizosheath is and its importance.

Answer 11

• The rhizosheath is the column of soil that adheres to the root as you extract it from the soil.
• The soil sticks because of root hairs and exudates, so its no wonder that the size of the rhizosheath is related to the density and size of the root hairs.
  
  • there’s some evidence to suggest that plants with a greater rhizosheath have the ability to take up more P when it’s limiting (probably due to root hair morphology).

Question 12

How can compaction affect nutrient movement and uptake in soil?

Answer 12

• Interception:
  
  • if root growth is restricted due to compaction, nutrient uptake will also be restricted
  • if root growth isn’t restricted, nutrient uptake will be increased due to greater soil-root contact
• Mass flow:
  
  • Compaction will reduce water movement and therefore nutrient movement (and uptake) via mass flow
• Diffusion:
  
  • If compaction increases the distance between the nutrient and the root surface, nutrient movement and uptake will be reduced.

Question 13

What’s the mobility and method of movement (% of uptake) for:

Nitrogen (N)

Answer 13

• Mobile
• Mass flow = 98%
• Diffusion = 0%
• Root interception = 2%

Question 14

Is Selenium (Se) an essential, beneficial, or toxic nutrient?

Answer 14

It’s essential for animals and humans, but not plants.

Question 15

What’s the mobility and method of movement (% of uptake) for:

Magnesium (Mg)

Answer 15

• Mobile
• Mass flow = 87%
• Diffusion = 0%
• Root interception = 13%

Question 16

Explain the 4 R concept of nutrient management.

Answer 16

Right rate

• Applying enough to meet plant needs (i.e. critical levels) without applying in excess
• To determine what the right rate is, use soil and plant tissue testing and economic assessments (ferts are expensive!)

Right time

• Nutrient requirements change during the growing season
• Therefore, nutrients should be supplied at varying concentrations throughout the season to match demand
• Could include strategic applications of nutrients throughout the season and/or soil/fert management strategies that allow for varied nutrient release

Right place

• Putting nutrients where uptake will be maximised (i.e. foliar, rhizosphere).
• Need to recognise that soil is heterogenous, so need to match fert application with spatial variability

Right source

• Characteristics of the soil determine the availability of nutrients, and therefore which source would be best.
  
  • can overcome this with new formulations or delivery systems (e.g. liquid vs granular, surface vs in-furrow, etc.)
• Influenced by price and availability

Question 17

What are the consequences of fertiliser form, soil properties, and the mobility of nutrients on diffusive transport?

Answer 17

• The greater the solubility, the greater the concentration gradient created, and therefore the greater the rate of diffusion (and uptake)
• Soil texture, moisture, and temperature affect diffusion and influences uptake
  
  • ↑ clay content = ↓ tortuosity (impedance) = ↑ diffusion rate
    
    • soil texture also influences the CEC and buffering capacity of the soil, which will effect the rate of diffusion (↑ CEC/buffering capacity = ↓ diffusion)
  • ↓ moisture = ↑ tortuosity (pathway distance) = ↓ diffusion rate
  • ↓ soil temp = ↓ diffusion rate
• immobile nutrients are more likely to move via diffusion, and the amount that they’re affected by these other properties (e.g. moisture and temp) is affected by their mobility
  
  • For example, P relies on diffusion as the means of transport more than K (they’re both immobile), and the rate of P diffusion is affected by temperature more than the rate of K diffusion.

Question 18

What’s the mobility and method of movement (% of uptake) for:

Calcium (Ca)

Answer 18

• Mobile
• Mass flow = 72%
• Diffusion = 0%
• Root interception = 28%

Question 19

What’s the mobility and method of movement (% of uptake) for:

Copper (Cu)

Answer 19

• Immobile
• Mass flow = 20%
• Diffusion = 10%
• Root interception = 70%

Question 20

True or false?

The concentration of nutrients in the vacuole is greater than in the cytosol.

Answer 20

False. The vacuole is used for nutrient storage so the plant can buffer growth against changes in nutrient supply.

Question 21

What’s the mobility and method of movement (% of uptake) for:

Potassium (K)

Answer 21

• Immobile
• Mass flow = 20%
• Diffusion = 78%
• Root interception = 2%

Question 22

Why are higher amounts of phosphorus (P) required to maintain production in Australia compared to the rest of the world?

Answer 22

Because many soils in Australia are very low in P or are highly P fixing.

Question 23

True or false?

Plant roots can modify the environment around them.

Answer 23

True; they can create localised changes in pH to enhance the uptake of nutrients in that area.

There’s considerable genetic variation between species in their ability to change the rhizosphere pH.

Question 24

Is Aluminium (Al) an essential, beneficial, or toxic element?

Answer 24

Potentially toxic.

Question 25

What’s the mobility and method of movement (% of uptake) for:

Zinc (Zn)

Answer 25

• Immobile
• Mass flow = 30%
• Diffusion = 40%
• Root interception = 30%

Question 26

True or false?

The nutrient concentrations within the vacuole are maintained within quite a narrow range.

Answer 26

False; the vacuole is used as a temporary storage facility for nutrients, where they can be drawn into the cytoplasm (where the pH is considerably higher (7.3-7.6 vs 4.5-5.9) and the nutrient concentrations are maintained within a narrow range) when needed.

Question 27

Is cobalt essential, beneficial, or toxic, and why?

Answer 27

It’s essential for N₂ fixation but not for plants.

Question 28

What’s the mobility and method of movement (% of uptake) for:

Iron (Fe)

Answer 28

• Immobile
• Mass flow = 10%
• Diffusion = 40%
• Root interception = 50%

Question 29

List the 8 essential micronutrients.

Answer 29

• Iron
• Manganese
• Zinc
• Copper
• Boron
• Molybdenum
• Chlorine
• Nickel

Question 30

Explain how root morphology varies depending on the soil.

Answer 30

• Roots can detect, and will preferentially grow in areas of higher nutrient concentration, lower resistance, and where there is an absence of soil constraints.
  
  • = more roots in topsoil
  • = in the subsoil, roots will prefer to grow through points of weakness (fractures, root channels, large pores, etc.)
    
    • = unequal distribution of nutrients (low concentrations in/around the fractures/channels/pores and high concentrations remain outside/between them).

Question 31

Briefly describe the variation in root morphology (within the same plant) for woody perennials (like grapevines), the rate of turnover for the different root components, and what the various types of root structures are used for.

Answer 31

• The thickest, oldest roots are mainly used for support and have a low turnover rate
• The smaller-diameter woody roots are mainly used for structure/support and water and nutrient transport (esp. water). They’ve also got a low rate of turnover.
• The fine roots (< 2 mm) have a high rate of turnover, account for up to 50% of the carbon allocation within the plant, are often mycorrhizal, and are the source of the majority of nutrient acquisition (from the soil).
  
  • young roots take up nutrients more rapidly than older roots

Question 32

True or false?

Plant nutrition influences tolerance to stresses.

Answer 32

True; adequate nutrition will increase a plant’s ability to tolerate biotic and abiotic stresses, but usually not to the point that would be achieved by a chemical control. NB: that’s not to say that it should be foregone in favour of chemical control, but that sole reliance on it (nutrition) may not be wise either.

Question 33

What’s the mobility and method of movement (% of uptake) for:

Manganese (Mn)

Answer 33

• Immobile
• Mass flow = 5%
• Diffusion = 80%
• Root interception = 15%