Nutrient Uptake b716n

ESSENTIAL ELEMENTS Chemical 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 ions Because roots absorb nutrients in ion form

Examples K 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-, HPO42-, SO42-, the roots release OHinto the soil

Ions Transport Across Root Membrane Electrogenic Pumps

 Mechanisms of nutrient absorption OR uptake by plant roots Root interception Mass flow Diffusion

 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 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 species Specific nutrient Soil texture Soil moisture

 Example  H2PO4-, HPO42- 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 Light Oxygen Soil moisture Soil temperature Amount of nutrient ions applied or in soils Good rooting system Rhizosphere 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 Example Plants 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 Example Poor 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

Example Phosphorus in most cases move in soils by diffusion, so P uptake is sharply reduced in dry soils

 Soil temperature Rates of all chemical reactions both in soils and plants depend on temperature Examples 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 temperature Rates of all chemical reactions both in soils and plants depend on temperature Examples…… 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 ions Increase in amount of ions in the soil via or through fertilization improves absorption Example When 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 Examples Deep 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 Examples Nematodes 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 humus content Good structure Warm soil Deep soil Moist soil Good drainage Fertilization Desirable microbes Near neutral pH

High sand content Loss of organic matter Compaction Cold soil Shallow soil Dry or wet soil Excess irrigation or drainage Erosion Root damaging pH too acid or alkaline