Plant Nutrition and Soil Biology

Plant Nutrition and Soil

• About 95% of a plant's dry weight is carbon, hydrogen, and oxygen
• These elements are the building blocks for organic compounds such as carbohydrates
• Primary nutrients include carbon dioxide and water
• Carbon dioxide supplies carbon, and water supplies hydrogen and oxygen
• A mineral is an inorganic substance with two or more elements
• A nutrient is considered essential if:
  • It has an identifiable role
  • Another nutrient cannot substitute for it
  • A deficiency causes plant death
• Essential nutrients are divided into macronutrients and micronutrients based on concentration in plant tissues
• Macronutrients: C, H, O, P, K, S, Ca, Mg
• Micronutrients: Fe, B, Mn, Cu, Zn, Cl, Mo
• Hydroponics allows plant growth when supplied with necessary nutrients
• Hydroponics proved that plants grow well when supplied with sunlight, water, and minerals
• Scientists tested for nutritional deficiencies by omitting nutrients from liquid mediums and observing effects
• Farmers use fertilizers to avoid deficiencies like stunted growth
• The NPK ratio describes the percentage by weight of nitrogen, phosphorus, and potassium in fertilizer

Soil Composition

• Soil has mineral particles, decaying organic material, living organisms, air, and water
• Soil supports plant growth
• Soil formation begins with weathering of rock through mechanical or chemical means
• Organisms such as lichens and mosses play a key role in soil formation
• Humus is decaying organic material
• Sand particles range from 0.05 to 2.0 millimeters in diameter, allowing water to drain readily
• Silt particles range from 0.002 to 0.05 millimeters in diameter
• Clay particles are less than 0.002 millimeters in diameter and consist of water-filled spaces
• Loam soil combines sand's aeration with silt and clay's mineral retention
• Soil needs a balance of clay particles and humus
• Humus increases soil benefits by maintaining 10-20% concentration, decreasing runoff, preventing leaching, and retaining minerals
• Organic matter is broken down by bacteria and fungi and returned to plants
• Living organisms help form soil from rock and penetrate soil to bedrock to open it to water, air, and animals
• Soil-dwelling animals mix and aerate soil, while microorganisms decompose humus and make nitrogen available
• Negatively charged clay and humus bind positively charged minerals like calcium and potassium
• Cation exchange is a process where hydrogen and positive ions exchange places with positive minerals allowing roots to take up nutrients
• Cation exchange capacity (CEC) is the availability of negative charge sites binding positive cations
• Sandy soils have lower CEC than clay and humus mixes
• A soil profile is a vertical section from ground surface to unaltered rock below
  • Soil horizons are parallel layers:
    • A (topsoil): litter and humus
    • B (subsoil): inorganic nutrients
    • C (parent material): weathered rock
• Soil profiles vary depending on parent material and climate in ecosystems

Soil Health

• Soil erosion occurs when water or wind carries soil away, degrading farmland globally
• Fertile soil equal to the size of Texas is lost yearly because of poor land management
• Deforestation and desertification contribute to erosion
• Mitigation includes crop rotation, reduced tillage, and companion crops
• Trees help prevent coastal erosion
• Rhizobium bacteria live in root nodules, fixing atmospheric nitrogen for plants

Water and Mineral Uptake

• Water and minerals enter plant roots through similar pathways
• Water moves between porous cell walls until forced into endodermal cells by the Casparian strip
• Water enters through root hairs and cells across the cortex and endodermis via cytoplasmic strands in plasmodesmata
• Plants require a nutrient to have an identifiable role, no substitution by another nutrient, and deficiency causing death
• Water enters root cells when osmotic pressure is lower than in the soil
• Minerals are actively taken up and transported in the xylem with water
• Minerals are acquired through passive or active transport, requiring energy expenditure
• The Casparian strip prevents backflow and allows higher mineral concentration in root xylem relative to soil
• Roots concentrate minerals up to 10,000 times greater than surrounding soil
• A plasma-membrane proton pump transports hydrogen ions out of the cell
• An electrochemical gradient drives positive ions into the cell, while negative ions are transported by carrier proteins
• Chemiosmosis is the model of mineral ion transport and requires no energy

Plant Adaptations

• Adaptations of roots for mineral uptake include symbiotic relationships, parasitic plants, and carnivorous plants
• Rhizobium bacteria live in root nodules, fixing atmospheric nitrogen, while host plants provide carbohydrates
• Mycorrhizal associations between fungi and plant roots increase surface area for water/mineral uptake and break down organic matter
• Roots get sugars and amino acids from the fungus

Transport Mechanisms in Plants

• Vascular tissues transport water and nutrients
• Xylem transports water and minerals via two conducting cell types:
  • Tracheids
  • Vessel elements
• Water flows passively from high to low water potential
• Phloem transports organic materials through sieve-tube members with companion cells and sieve plates
• Plasmodesmata are extended through sieve plates
• Water potential is the energy of water and moves passively from high to low potential
• Water potential is determined by water pressure and solute concentration
• Pressure potential is the effect of pressure on water potential; water moves from high to low pressure
• Osmotic potential takes into account the presence of solutes; water moves from low to high solute concentration
• Turgor pressure increases because of osmosis
• Xylem vessels form an open pipeline, with vessel elements separated by perforated plates and water moving through pits in tracheids
• Water entering roots creates positive pressure, pushing xylem sap upward which may cause guttation: water forced out of vein endings along leaf edges
• Cohesion-tension model describes passive xylem transport:
  • Cohesion is the tendency of water molecules to cling together
  • Adhesion allows polar water molecules to interact with vessel walls
• Water Loss - Water exits the leaf through the stomata

Cohesion-Tension Mechanism

• Transpiration causes water loss through stomata in leaves which is replaced by the water molecules in leaf veins
• Transpiration exerts a pulling force with cohesion drawing water through the xylem to leaf cells which is prevented due to a waxy cuticle when stomata are closed
• Tension in xylem pulls the water column upward in the stem
• Water enters xylem passively by osmosis in the roots
• The water column extends from root to leaves
• Guard cells border each leaf epidermis stoma
• Stoma regulate opening and closing with light and abscisic acid (ABA)
• Increased turgor pressure in guard cells opens stomata by allowing the active transport of potassium into guard then water follows
• Transpiration creates large volumes if water vapor through a plant