PBL 2: Environment II, The soil and nutrients

Question 1

Photosynthesis equation

Answer 1

6CO2+12H2O+light energy=> C6H12O6+ 6O2+6H2O

Question 2

Carbon and plants

Answer 2

-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

Question 3

Carbon partitioning: Source, Path, Sink

Answer 3

-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.

Question 4

Macronutrients

Answer 4

1. Nitrogen
2. Phosphorus
3. Potassium

others: Calcium, Magnesium, Sulphur

-when any of these nutrients are deficient in soil, plant development suffers and symptoms show up quickly

Question 5

Macronutrients: Nitrogen

Answer 5

-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

Question 6

Macronutrients: Phosphorus

Answer 6

-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.

Question 7

Macronutrients: Potassium

Answer 7

-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.

Question 8

Micronutrients/Trace elements

Answer 8

1. Iron
2. Copper
3. Zinc
4. Manganese
5. Molybdenum (Mo)
6. Boron
7. 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.

Question 9

Boron (B)

Answer 9

Carbohydrate transport+metabolism, phenol metabolism, activation of growth regulators

Question 10

Chlorine (Cl)

Answer 10

Cell hydration, activation of enzymes in photosynthesis

Question 11

Copper (Cu)

Answer 11

Basal metabolism, nitrogen metabolism, secondary metabolism

Question 12

Iron (Fe)

Answer 12

Chlorophyll synthesis, enzymes for electron transport

Question 13

Manganese (Mn)

Answer 13

Basal metabolism, stabilisation of chloroplast structure, nitrogen metabolism

Question 14

Molybdenum (Mo)

Answer 14

Nitrogen fixation, phosphorus metabolism, iron absorption and translocation

Question 15

Zinc (Zn)

Answer 15

Chlorophyll formation, enzyme activation, basal metabolism, protein breakdown, hormone biosynthesis

Question 16

Soil

Answer 16

-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.

Question 17

Processes of soil formation and development

Answer 17

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

Question 18

Soil characteristics: Texture

Answer 18

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

Question 19

Soil texture: Sand

Answer 19

-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)

Question 20

Soil texture: Silt

Answer 20

-finer than sand, still grain in appearance+feel, but more actively holds water and nutrient ions.

Question 21

Soil texture: Clay

Answer 21

-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

Question 22

Soil characteristics: Structure

Answer 22

-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.

Question 23

Soil structure is dependent on:

Answer 23

-Soil organic matter (SOM) content
-Plants growing in the soil
-Presence of soil organisms
-Soil’s chemical status

Question 24

Soil structure: Low bulk density

Answer 24

-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.

Question 25

Soil characteristics: Colour

Answer 25

Soil colour can influence the interaction of the soil with other factors of the environment (light-coloured reflects light).

-Darker soils are generally an indication of high organic matter content (especially in temperate regions).

-Red+yellow soils indicate high levels of iron oxides, formed under conditions of good aeration and drainage, but these colours can also be directly from the parent material.

-Gray/ yellow-brown soils can be indicators of poor drainage: colours form when iron is reduced to a ferrous form rather than oxidised to the ferric form in the presence of abundant oxygen

-Whitish light-coloured soils often indicate the presence of quartz, carbonates, gypsum (alkaline).

Question 26

Soil characteristics: Cation Exchange Capacity (CEC)

Answer 26

-K, Ca minerals are positively charged ions; Nitrate and Phosphate are negatively charged ions.

-if these dissolved ions are not taken up immediately through plant roots/fungi, they risk being leached out of the soil solution.

-The higher the CEC the better the soil’s ability to hold+exchange cations, prevent leaching of nutrients, provides plants with adequate nutrition

-Varies from soil to soil, depending on the clay/humus complex structure, type of Micelle present, amount of organic matter in soil.

-Measured in milliequivalent of cations (no. of charges) per 100g of dry soil.

-Organic matter in the form of humus is many times more effective than clay in increasing CEC (has more SA:V, more adsorption sites), colloidal in nature.

Question 27

Soil CEC: Micelles

Answer 27

Clay+humus particles, separately or in aggregates that form platelike structures= micelles.

-have negatively charged surfaces that hold smaller, more mobile, positively charged ions in soil.

-Number of sites on micelles available for binding positively charged ions (cations) determines what is the soil CEC

-Cations cling to the negatively charged outer surfaces of the micelles+humates with differing degrees of attraction

Question 28

Soil characteristics: Soil Acidity and pH

Answer 28

-Most plants (especially crops) grow best between pH range of 5-8.

-Acid soil: plaint’s impaired ability to absorb specific nutrients at either a very low or very high pH.

-Legumes are particularly sensitive to low pH due to the impacts acid soils have on the microbial symbiont in nitrogen fixation.
-Bacteria in general are negatively impacted by low pH

Question 29

Soil acidification

Answer 29

Is a result of the loss of bases by:
-leaching of water moving downward through the soil profile,
-uptake of nutrient ions by plants and their removal through harvest/grazing,
-production of organic acids by plant roots and microorganisms

Question 30

Soil characteristics: Salinity and Alkalinity

Answer 30

-Salts released by weathering of parent material+limited rainfall, are not removed by leaching.

-In areas of low rainfall+high evaporation rates: dissolved salts like Na+, Cl- are common (else Ca2+, Mg2+, K+, HCO3-, NO3-).

Saline Soil: high concentration of neutral salt (NaCl, NaSO4).
-In cases where sodium is combined with weak anions (HCO3-), alkaline soils develop (pH >8.5: saline soil usually=alkaline soil)

-Soils with high levels of neutral salts are a problem for plants due to osmotic imbalances. Alkaline soils are a problem because of excess OH- ions, difficulty in nutrient uptake.

-Proper irrigation and soil water management become a key part of dealing with these conditions.

Question 31

Limiting nutrients

Answer 31

When a particular nutrient is not present in sufficient quantity, it is called a limiting nutrient and must be added.

-The presence of a nutrient does not necessarily mean it is available to plants: factors like pH, CEC, soil texture, determine the actual availability of nutrients.

-Because of the loss/export of nutrients out of the soil due to harvest, leaching/volatilization, fertilisers must continually be added in large amounts to most agroecosystems. However, costs are increasing and so is pollution into surface water.

-Issues with fertiliser: leaching, eutrophication

Question 32

Soil organic matter (SOM)

Answer 32

Soil (also called pedosphere) is a living part of the agroecosystem where it contains a diverse assortment of living organisms (soil biota). Intensively managed agricultural soil often has very little organic matter.

-Humus= level of organic matter in the soil.

-SOM includes surface litter, dead roots, plant residue in various stages of decomposition, microbial metabolites, humid substances, excretion of animals.

-Key component of good soil structure, increases water+nutrient retention, provides important mechanical protection of the soil surface.

-Most important function: serves as a food source for soil biota.

Question 33

Natural soil management: Crop residue

Answer 33

Returning to the soil the parts of the crop that are not destined for human/animal use.

-Major concern has been how to deal with potential pest/disease organisms that residue may harbour and pass on.

-Proper timing of incorporating this residue, rotating crops, composting residue away from field are possible solutions

Question 34

Natural soil management: Covercrops/ Green manure

Answer 34

Where a plant cover is grown specifically to produce plant matter as ‘green manure’ into the soil.

-Usually grown in rotation with a crop or during a time of the year that crop cannot be grown.

-E.g. legumes: fixate nitrogen and add more organic matter. The quality of biomass can be greatly improved.
Leftover can be incorporated into soil or used as protective mulch until it decomposes.

Question 35

Living mulch

Answer 35

also earthworms play critical roles in enhancing soil quality: composters!

a non crop species is planted between the crop rows during the cropping cycle.

-can provide+conserve nitrogen for grain crops
-reduce soil erosion
-reduce weed pressure
-increase SOM content

Question 36

Animal Manure

Answer 36

Animal manure is added to soil to improve SOM. Applications can simultaneously increase SOM and supply nutrients for crop growth.

Can have many drawbacks:
-Nitrogen loss through ammonification can be high
-Runoff of nitrates/other soluble materials can be an issue

-Often a waiting period for decomposition and stabilisation before planting can take place.

Question 37

Composts

Answer 37

An attractive way to add organic matter because
-particle size distribution of compost favours uniform field application
-ratio of carbon to nitrogen is optimal
-compost is usually free of weed seeds
-soil diseases are often suppressed by compost addition

-goes through the first stages of decomposition+humification so it is stabilised: waste is turned into a new resource.

Question 38

Other soil amendments

Answer 38

-Humates, kelp, fish meal, animal by-products, mined guano

-Sewage sludge (Removed from wastewater treatment): can be applied as dried cake/granules or liquid slurry. Complicated because oftentimes industrial waste is not removed from sewage

Question 39

Soil cultivation pros vs cons

Answer 39

Pros: convention that soil must be cultivated to control weeds, incorporate organic matter, allow root growth

Cons: cultivation can degrade soil structure, reduce organic matter content, disrupt soil biota, simplify soil food web, lose productivity

Question 40

Tillage

Answer 40

often a 3-step process involving 1. a deep plowing that turns the soil, 2. secondary tilling for peroration of a seed bed, and 3. postplanting cultivations (often +herbicides to control weeds)

Soil erosion, loss of good soil structure, nutrient leaching.

Question 41

No-tillage

Answer 41

respects the need for a fallow period to control weedy vegetation and to allow natural soil-building processes to replace removed nutrients.

-soil cultivation is limited to actual seedbed, done at the time of seed planting.

cons: developed a great dependence on herbicides

Question 42

Ridge-till systems: Reducing herbicide use

Answer 42

1. ridge tillage: initial plowing n which the planting bed, only focus on seed planting, weed management, SOM incorporation, and movement from ridge top to valley.

Question 43

Dutch Nitrogen Crisis

Answer 43

(Started in the 1980s, addressed today): nitrogen crisis in the Netherlands primarily related to nitrogen emissions and their environmental impact. Nitrogen compounds, particularly ammonia and nitrogen oxides, contribute to air and water pollution, affecting ecosystems and human health.
The crisis involves concerns about excessive nitrogen deposition in sensitive natural areas, leading to biodiversity loss and ecosystem damage. Agriculture, especially intensive livestock farming, has been a major contributor to nitrogen emissions. The issue gained prominence due to legal challenges and the Netherlands failing to meet European Union air quality standards.
The Dutch government faced pressure to address the nitrogen crisis, and measures were being considered to reduce nitrogen emissions from various sources. These measures included changes in agricultural practices, such as modifying livestock feed and improving manure management. Additionally, there were discussions about spatial planning and infrastructure projects to mitigate the environmental impact of nitrogen emissions.