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