nutrient uptake
TRANSCRIPT
ESSENTIAL ELEMENTSChemical element required for normal plant growth without which plants cannot complete their life cycles.
(Brady and Weil, 1999)
Primary macronutrients
They are mineral elements usually not available in large enough quantities for best growth
The primary macronutrients are N, P, and K
N, P, and K are mostly added to soils by fertilizer
Importance of knowing nutrients as ionsBecause roots absorb nutrients in ion form
ExamplesK K+
S SO42-
Because soil particles adsorb nutrients in ion form
Note
Absorb: take in something like sponge absorbing water
Adsorb: to attract a thin layer of molecules to a surface, where they stick
Nutrient uptake
Nutrient uptake
When plant roots (root hairs) take cations such as NH4
+, Ca2+, Mg2+, Na+, and K+, the root cells release H+ into the soil
When plant roots take anions such as NO3-,
H2PO4-, HPO4
2-, SO42-, the roots release OH-
into the soil
Ions Transport Across Root Membrane
Electrogenic Pumps
Mechanisms of nutrient absorption OR uptake by plant roots
Root interceptionMass flowDiffusion
1. Root interception
Interception of nutrients in soil solution by the extension of root systems
Root interception contributes to only small amount of nutrients (<1%) taken up by plants because roots have very little contact with soil
2. Mass flow
Absorption of nutrient ions by plant roots due to flowing of water containing the ions towards plant roots
Results from transpirational water uptake by plants
3. Diffusion
Movement of ions toward roots from surrounding soil
i.e. ions diffuse through soil water instead of being carried with the water
Diffusion
Movement of ions by random kinetic motion of molecules, sometimes referred to as Brownian Movement. When a concentration gradient exists, net movement occurs by diffusion from locations of high concentration to locations of low concentration.
Distant of diffusive nutrient movement: 0.1 – 15mm
Note
Interception, mass flow, and diffusion occur at the same time
Relative importance of interception, mass flow, and diffusion depends on:
Plant speciesSpecific nutrient
Soil textureSoil moisture
Example
H2PO4-, HPO4
2- and K+ are better taken up by plants via diffusion than mass flow
Generally, diffusion is the most important mechanism
Root interception contributes least to nutrient uptake
Factors influencing nutrient uptake
LightOxygenSoil moistureSoil temperatureAmount of nutrient ions applied or in soilsGood rooting systemRhizosphere organisms
Light Nutrient uptake involves energy
consumption by roots. Roots get this energy from plants through photosynthesis. Photosynthesis is affected by light. Therefore anything that affects photosynthesis affects nutrient uptake
ExamplePlants growing under low light make less
sugar to send to roots, so they take up fewer nutrients
Oxygen Plants use oxygen for respiration so any
condition that limits or affect oxygen supply will limit nutrient uptake
ExamplePoor drainage and soil compaction slow
movement of oxygen into the soil. Therefore, these conditions also limit the ability of plants to absorb nutrients
Soil moisture
When a soil dries, it lowers nutrient uptake because lack of water retards nutrient flow toward root hairs by mass flow and diffusion
ExamplePhosphorus in most cases move in soils by
diffusion, so P uptake is sharply reduced in dry soils
Soil temperatureRates of all chemical reactions both in soils
and plants depend on temperatureExamples
Plants respire less in cold soils, so plants have less energy to take up nutrients
Root growth is slow in cold soil therefore limiting root interception of nutrients
Soil temperatureRates of all chemical reactions both in soils
and plants depend on temperatureExamples……
Low temperature slows decomposition of organic matter to release more nutrients, so nutrient uptake in cold soils is low
Phosphorus and Fe deficiencies are common in spring when soils are cold and wet
Amount of soil nutrient ionsIncrease in amount of ions in the soil via or
through fertilization improves absorption
ExampleWhen K is high in soils, plants take up more
than they need (luxury consumption)
Note: If something happens to slow uptake, the excess K stored in the plant cells is used
Rooting system
Nutrient uptake is fast when plants grow well to produce a lot of roots
Poor drainage, compaction, and low nutrients status reduce root growth, so these affect nutrient uptake
Rooting system
ExamplesDeep rooted plants need less fertilization
than shallow rooted plants
Soils with restricted zones such as high water table, bedrock, plowpan, can use shallow rooted plants
Organisms
ExamplesNematodes or root-rotting fungi damage
ability of roots to take up nutrients
Micorrhizal infections improve availability of nutrients more
Insects feed on plant roots to disrupt nutrient uptake
Raises fertility Lowers fertility
High clay content High sand contentHigh humus content Loss of organic matterGood structure CompactionWarm soil Cold soilDeep soil Shallow soilMoist soil Dry or wet soilGood drainage Excess irrigation or drainageFertilization ErosionDesirable microbes Root damagingNear neutral pH pH too acid or alkaline