PBL 2: Environment II, The soil and nutrients Flashcards
Photosynthesis equation
6CO2+12H2O+light energy=> C6H12O6+ 6O2+6H2O
Carbon and plants
-Carbon compounds produced by photosynthesis play critical roles in plant growth+respiration because of their dual role as 1.) energy source 2.) carbon skeletons to build other compounds
-Hydrogen (via water from transpiration), Carbon, Oxygen make up 95% of the average plant’s fresh weight. Other 5% of living plant matter comes from soil
Carbon partitioning: Source, Path, Sink
-Source is usually the leaf: chloroplasts
-Carbon then moves to the phloem
-Moves through stem to grain, flowers, fruits, tubers, other parts (Sinks).
-Then there is phloem “unloading” and sink uptake.
Macronutrients
- Nitrogen
- Phosphorus
- Potassium
others: Calcium, Magnesium, Sulphur
-when any of these nutrients are deficient in soil, plant development suffers and symptoms show up quickly
Macronutrients: Nitrogen
-Needed in large amounts by plants but, most universally deficient nutrient
-Occurs in every amino acid: major component of proteins
-Required in enzyme synthesis
-Forms part of chlorophyll and is required
-Nitrogen obtained from ion exchange with soil solution as NO3- or from NH4+.
Deficiency:
-All enzymatic reactions affected
-Yellow colour in plants
-Flowering and fruit affected
Macronutrients: Phosphorus
-Important component of nucleic acids, nucleoproteins, phytin, phospholipids, ATP, several other types of phosphorylated compounds including some sugars.
-built into the DNA of chromosomes and the RNA of nucleus+ribosomes
-cell membranes depend on the phospholipids for the regulation of movement of materials in+out of cells/organelles
Occurs in:
-(As phosphates) metabolic enzymes
-primary cell wall (affects permeability)
-initial reaction of photosynthesis also involves phosphorus (initially reacts with CO2)
Absorption:
-as phosphates from soil solution through plant roots
-low availability of phosphorus in soil
Deficiency:
-Bluish cast on leaves
-Remain dark green, prominent purple anthocyanins on leaf underside
-Root & fruit development severely restricted.
Macronutrients: Potassium
-provides appropriate ionic environment for metabolic processes that take place in the liquid contents of the plant cell (cytosol).
-obtained as cation K+ through the roots as exchangeable ions from adsorption sites in the soil matrix/soil solution.
-regulatory functions: involved in stomatal movement, cofactor for many enzyme systems.
-most metabolic processes affected by potassium
-needs to be present to form starches and sugars+later transport through plant
-involved with cell division+growth, permeability, turgidity, hydration.
-better resistance to disease and environmental stress
Deficiency: disruptions in water balance (Drying tips, curled leaf edges), sometimes higher predominance of root rot. Abundant in soil, but excessive removal through harvest/soil leaching can lead to deficiency.
Micronutrients/Trace elements
- Iron
- Copper
- Zinc
- Manganese
- Molybdenum (Mo)
- Boron
- Chlorine
-Each one plays some vital role in plants but usually in extremely small quantities.
-Most are toxic to the plant when they occur in large quantities in the soil.
-All are taken up from the soil solution through ion exchange at the root surface
-Many inorganic fertilisers carry small quantities of these elements as contaminants, now commonly added to soils that have undergone long period of conventional management: organic fertilisers (especially compost) are rich in micronutrients.
Boron (B)
Carbohydrate transport+metabolism, phenol metabolism, activation of growth regulators
Chlorine (Cl)
Cell hydration, activation of enzymes in photosynthesis
Copper (Cu)
Basal metabolism, nitrogen metabolism, secondary metabolism
Iron (Fe)
Chlorophyll synthesis, enzymes for electron transport
Manganese (Mn)
Basal metabolism, stabilisation of chloroplast structure, nitrogen metabolism
Molybdenum (Mo)
Nitrogen fixation, phosphorus metabolism, iron absorption and translocation
Zinc (Zn)
Chlorophyll formation, enzyme activation, basal metabolism, protein breakdown, hormone biosynthesis
Soil
-The weathered superficial layer of the earth that is mixed with living organisms and the products of their metabolic activities and decay
-Includes material derived from rocks, organic+inorganic substances derived from living organisms, and the air and water occupying the spaces between soil particles
-Is a complex, living, changing and dynamic component of the agroecosystem.
Processes of soil formation and development
Agricultural perspective “ideal” soil:
-45% minerals
-5% organic matter
-50% space (half water, half air)
Soil profile: O horizon lies at the soil surface. In natural ecosystems, the O horizon is the most biologically active part of the profile, most important ecologically
Soil characteristics: Texture
Texture is the percentage, by weight, of the total mineral soil that falls into various particle size classes: gravel, sand, silt, clay.
-Most soils are a mixture of texture classes
Soil texture: Sand
-easily visible by naked eye, porous to water and less able to adsorb+hold nutrient cations.
-good drainage, dries easily and loses nutrients to leaching: can prevent rotting of tubers and make for easy harvesting, (useful in a wet environment)
Soil texture: Silt
-finer than sand, still grain in appearance+feel, but more actively holds water and nutrient ions.
Soil texture: Clay
-particles cannot be seen with naked eye: negative charge, NH4+ attached, NO3 lost through drainage.
-Colloidal in that they can form a suspension in water and are active sites for the adhesion of nutrient ions/water molecules.
-controls the most important soil properties: plasticity+ion exchange between soil particles and water in the soil.
-High clay content: can have problems with water drainage, when dry can exhibit cracking.
-Can be useful in a dry environment: maintains soil moisture and nutrients
Soil characteristics: Structure
-Soil aggregates larger with increasing depth in soil.
-Soil particles that are bound together resists wind+water erosion, especially vegetative cover is minimal.
-Good structure helps maintain low bulk density (weight of solids/unit volume of soil)
-Compaction (loss of pore spaces) indication of the loss of crumb structure, and be caused by the weight of farm machinery, loss of organic matter from excessive tillage or a combination.
Soil structure is dependent on:
-Soil organic matter (SOM) content
-Plants growing in the soil
-Presence of soil organisms
-Soil’s chemical status
Soil structure: Low bulk density
-Has a higher percentage of pore space (higher porosity), more aeration, better water percolation (permeability), more water storage capacity.
-Such soil is easier to till and allows plant roots to penetrate more easily.
-Excessive cultivation accelerates breakdown of SOM an increases the potential for compaction–> advantages of good crumb structure lost.
