Plant Nutrition

Macronutrients (N-P-K)

  • Nitrogen: proteins, nucleic acids, chlorophyll

  • Phosphorous: ATP, DNA, phospholipids

  • Potassium: enzyme activation, stomata movement, water balance

These are the primary components in fertilizers

Micronutrients (Fe)

  • Iron: electron transport, chlorophyll synthesis

Even though needed in small amounts, iron deficiency severely affects photosynthesis

When plants lack essential nutrients, three main treatments are used:

  1. Chemical fertilizers

    • Quickly provide nutrients

    • Can cause environmental pollution if overused

  2. Organic fertilizers & mineralization

    • Nutrients released by decomposition

    • Slow, long-term nutrient supply

  3. Foliar sprays

    • Nutrients sprayed directly onto leaves

    • Used for rapid correction of deficiencies

Soil nutrients come from two main processes:

  1. Weathering

    • Rocks break down into smaller particles

    • Releases nutrient ions

  2. Microbial Decomposition

    • Bacteria and fungi break down dead organic matter

    • Releases nutrients back into the soil

Topsoil contains a diverse ecosystem:

  • Roles of Soil Organisms

    • Burrowing animals → loosen and aerate soil

    • Microbes & fungi → act as decomposers

    • Earthworms → act as soil refiners

  • Earthworm Benefits - Their castings contain:

    • 5× more Nitrogen

    • 7× more Phosphorus

    • 11× more Potassium

    • 3× more Magnesium

    • 2× more Calcium

This greatly increases soil fertility and plant growth.

Soil particles, especially clay, carry (Anions) negative charges from:

  • Aluminates (AlO₂⁻)

  • Silicates

These Anions attract and hold positively charged ions (cations) such as:

  • K⁺

  • Ca²⁺

  • Mg²⁺

  • NH₄⁺

This prevents nutrients from being washed away and allows plant roots to absorb them gradually.

Nitrification (Soil Bacteria Process)

Plants cannot directly use atmospheric nitrogen (N₂) — it must first be converted.

two-step oxidation process:

  1. Nitrosomonas bacteria

    • Convert NH₄⁺ → NO₂⁻

    • This reaction releases energy

  2. Nitrobacter bacteria

    • Convert NO₂⁻ → NO₃⁻ (nitrate)

✅ Nitrate (NO₃⁻) is the main form of nitrogen absorbed by plants.

Nitrogen Fixation

Nitrogen fixation = the conversion of N₂ gas → NH₃ (ammonia).

Performed by:

  • Nitrogen-fixing bacteria

  • Cyanobacteria (Anabaena)

  • Root nodule bacteria in legumes

  • Azolla (aquatic plant with symbiotic cyanobacteria)

Root Nodules in Legumes

  • Found on soybean and other legume roots

  • Contain nitrogen-fixing bacteria

  • Provide plants with a direct nitrogen source

  • Plants supply bacteria with sugars and protection

The enzyme nitrogenase performs nitrogen fixation:

N₂ + 8e⁻ + 8H⁺ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pi

  • This is energy expensive

  • Ammonia (NH₃) is quickly converted to NH₄⁺

  • Plants use NH₄⁺ to make amino acids and protein

Oxygen Control in Nitrogen Fixing Bacteria

  • Requires anaerobic conditions (no oxygen)

  • But bacteria still need oxygen for respiration

✅ Solution:

  • Leghemoglobin

    • Oxygen-binding heme protein

    • Controls oxygen levels in nodules

    • Made partly by the bacterium and partly by the plant

Exam Summary

  • Plants require specific essential elements.

  • Most nutrients come from soil ions.

  • N, P, and K are the most important fertilizer components.

  • Soil organisms play a major role in nutrient cycling.

  • Nitrogen is biologically unavailable unless fixed by bacteria.

  • Nitrogenase is oxygen-sensitive and ATP-intensive.

  • Root nodules and leghemoglobin make nitrogen fixation possible in legumes.