Soil and Plant Nutrition

Chapter 37: Soil and Plant Nutrition

Terra Preta

• Terra preta: Dark, fertile man-made soil in the Amazon, also called "biochar soils."

Concept 37.1: Soil as a Living Ecosystem

• Plants get water and minerals mostly from the top layers of soil.

• Important soil qualities:

  • Texture (how it feels)

  • What it's made of

Soil Texture

• Soil is made of particles of different sizes:

  • Sand (biggest)

  • Silt (medium)

  • Clay (smallest)

• Soil has layers called horizons.

• Topsoil: Mix of minerals, living things, and humus (broken-down organic stuff).

Soil Horizons

• A horizon: Topsoil.

• B horizon: Subsoil.

• C horizon: Parent rock.

Loams

• Loams: Best topsoil, with equal parts sand, silt, and clay.

• Sandy soil: Doesn't hold enough water. Clay soil: Holds too much water.

Inorganic Components

• Cations (like K^+, Ca^{2+}, Mg^{2+}): Stick to soil particles, so they don't wash away.

• Cation exchange: When cations are swapped on soil particles, usually with H^+.

• This releases cations into the soil, and plant roots can grab them.

• Negatively charged ions: Don't stick to soil well, so they can be washed away easily.

• Roots make the soil acidic, which helps release mineral cations.

Process of Cation Exchange

1. Roots make the soil acidic.

2. CO2 + H2O → Carbonic acid (H2CO3) → Bicarbonate (HCO_3^-) + Hydrogen ions (H^+).

3. This releases mineral cations.

4. Roots soak up these cations.

Organic Components

• Humus: Makes soil crumbly and holds water well.

• It helps the soil hold onto nutrients and acts as a nutrient store.

• Topsoil has lots of living things like bacteria, fungi, algae, insects, worms, and roots.

Soil Conservation and Sustainable Agriculture

• Good soil management (like fertilizing) helped start farming and cities.

• Farming takes away minerals from the soil, uses up water, and causes erosion.

• The Dust Bowl in the 1930s was caused by bad soil management.

• Now, 30% of farmland is less productive because of soil problems.

• Sustainable agriculture: Farming that protects the environment and is profitable.

Irrigation

• Using irrigation in dry areas uses a lot of water.

• Most irrigation water comes from underground.

• Using too much groundwater can make the land sink.

Fertilization

• Fertilizers replace minerals that crops take from the soil.

• Commercial fertilizers have lots of nitrogen (N), phosphorus (P), and potassium (K).

• Organic fertilizers (manure, compost) release N, P, and K slowly.

Adjusting Soil pH

• Soil pH affects how well plants can get nutrients.

• Slightly acidic soil makes it easier for plants to get cations because H^+ ions swap with mineral cations on clay particles.

• How available minerals are depends on pH.

Controlling Erosion

• Ways to stop erosion:

  • Planting trees to block wind

  • Terracing hillsides

  • Farming along the contour of the land

  • No-till farming

Phytoremediation

• Phytoremediation: Using plants to clean up polluted soil.

• Plants and bacteria take out pollutants, then the plants are safely thrown away.

Concept 37.2: Essential Elements for Plant Life Cycle

• Soil, water, and air are needed for plants to grow.

• Water makes up most of a plant's weight (80–90%).

• Carbon dioxide from the air makes up most of a plant's dry weight (96%).

Essential Elements

• Plants have over 50 elements, but only 17 are essential.

• Essential elements: What a plant needs to live and reproduce.

• Hydroponics (growing plants in water) is used to find out which elements are essential.

Macronutrients

• Macronutrients: 9 essential elements that plants need a lot of:

  • Carbon (C)

  • Oxygen (O)

  • Hydrogen (H)

  • Nitrogen (N)

  • Phosphorus (P)

  • Sulfur (S)

  • Potassium (K)

  • Calcium (Ca)

  • Magnesium (Mg)

Micronutrients

• Micronutrients: 8 essential elements that plants need in small amounts:

  • Chlorine (Cl)

  • Iron (Fe)

  • Manganese (Mn)

  • Boron (B)

  • Zinc (Zn)

  • Copper (Cu)

  • Nickel (Ni)

  • Molybdenum (Mo)

Symptoms of Mineral Deficiency

• Symptoms depend on what the nutrient does and how easily it moves in the plant.

• Mobile nutrients: Deficiency shows up in older parts of the plant first.

• Less mobile nutrients: Deficiency shows up in younger parts first.

• Common deficiencies: Nitrogen, potassium, and phosphorus.

Resistance to Aluminum Toxicity

• Aluminum in acidic soil hurts roots and lowers crop yields.

• Plants can be modified to release acids that bind up aluminum.

Smart Plants

• "Smart" plants show when they need nutrients before it's too late.

• A blue color means the plant needs phosphate fertilizer.

Concept 37.3: Plant Nutrition and Relationships with Other Organisms

• Plants and soil microbes help each other.

• Dead plants give energy to soil microbes.

• Roots release substances that help microbes nearby.

Bacteria and Plant Nutrition

• Rhizosphere: Soil right around plant roots.

• Rhizobacteria: Helpful bacteria in the rhizosphere.

• Endophytes: Bacteria that live inside plant cells without harming them.

Bacteria in the Nitrogen Cycle

• Nitrogen: Often what limits plant growth.

• Plants get nitrogen as NO3^- or NH4^+.

• Most nitrogen in the soil comes from bacteria.

• The nitrogen cycle changes nitrogen into different forms.

Nitrogen Cycle Processes

1. Ammonium (NH_4^+) is Made:

   • Ammonifying bacteria: Break down organic stuff and release ammonia (NH_3).

   • Nitrogen-fixing bacteria: Turn N2 into NH3.

   • NH3 becomes NH4^+.

2. Nitrate (NO_3^−) is Made:

   • Nitrifying bacteria: Turn NH3 into nitrite (NO2^−), then into nitrate (NO_3^−).

Nitrogen-Fixing Bacteria

• Plants can't use nitrogen gas (N_2) from the air because of the strong bond between nitrogen atoms.

• Nitrogen fixation: Turning N2 into NH3.

• Legumes (like beans) get nitrogen from Rhizobium bacteria that live in their roots.

Rhizobium and Legumes

• Plants get fixed nitrogen from Rhizobium, and Rhizobium gets sugar and a place to live.

• Each type of legume works with a specific type of Rhizobium.

• Root nodules (where nitrogen fixation happens) form because of chemical signals between Rhizobium and root cells.

Process of Root Nodule Formation

1. Chemical signals attract bacteria, and an infection thread forms.

2. Cells divide in the root cortex.

3. Cells divide in pericycle.

4. Bacteroids form.

5. The nodule grows and develops veins.

Nitrogen Fixation and Agriculture

• Crop rotation uses nitrogen fixation.

• Planting a nonlegume (like corn) one year, then a legume, puts nitrogen back in the soil.

Fungi and Plant Nutrition

• Mycorrhizae: When fungi and roots team up.

• The fungus gets sugar from the plant.

• The plant gets more surface area to absorb water and minerals.

• Mycorrhizal fungi help roots grow and branch out.

Mycorrhizae and Plant Evolution

• Mycorrhizal fungi may have helped early plants get nutrients from the soil.

Types of Mycorrhizae

1. Ectomycorrhizae

2. Arbuscular mycorrhizae

Ectomycorrhizae

• The fungus forms a layer around the root.

• Hyphae (fungal threads) grow into the spaces between root cells.

Arbuscular Mycorrhizae

• Fungal hyphae grow into the root.

• Hyphae form branched structures (arbuscules) inside cells, where nutrients are exchanged.

Agricultural and Ecological Importance of Mycorrhizae

• Farmers often add fungal spores to seeds to help mycorrhizae form.

• Some invasive plants mess up how native plants and their mycorrhizal fungi work together.

• Garlic mustard, for example, slows the growth of other plants by stopping mycorrhizal fungi from growing.

Epiphytes, Parasitic Plants, and Carnivorous Plants

• Some plants get nutrients in unusual ways.

Unusual Adaptations

1. Epiphytes

2. Parasitic plants

3. Carnivorous plants

Epiphytes

• Epiphytes grow on other plants but don't hurt them.

Parasitic Plants

• Parasitic plants steal nutrients from other plants.

Carnivorous Plants

• Carnivorous plants trap and eat animals for nutrients.