Quality Of Soils Flashcards
What is a soil function?
A service that the soil gives to the environment
Examples of soil functions
Food, fibre, fuel
Carbon store
Water purification
Climate regulation
Nutrient cycling
Habitat
Flood regulation
Pharmaceutical
Infrastructure
Construction material
Cultural heritage
Soil health
Soil quality
Self regulation, stability, resilience, lack of stress
Properties of soil that are fit to perform particular functions
Soil formation factors
Parent material
Topography
Biology
Climate
Time
Soil Horizons
O Horizon = organic top layer
A Horizon = topsoil
B Horizon = subsoil
C Horizon = substratum
R Horizon = bedrock
What does surface area influence?
Increased water retention - more pores
Increased nutrient retention
Nutrient release
Soil particle flocculation
Microorganism activity
When are soils no longer mineral?
10% organic mineral soil
50% pure peat
Primary particle
Sand
Silt
Clay
What is an aggregate?
Soil unit made of primary particles with binding agents
What is a Ped?
Structural unit formed of aggregates
What is a Clod?
Structural unit formed by artificial process eg traffic, cultivations
Friability
Ability of soil to break apart
What is Plastic limit?
Moisture content at which soil starts to behave plastic (smear)
What is Liquid limit?
Moisture content to cause a plastic soil to behave as a liquid
What is Soil aggregation?
Macro aggregates compromised of micro aggregates formed by aggregation
Factors influencing aggregation
Physical-chemical: clay flocculation and cation exchange, volume changes in clays (wet-dry), oxides acting like cement (tropics)
Biological: soil organisms (burrowing, hyphae, glues), organic matter, tillage
Aggregates can be destabilised by:
Forces of impact - rain
Slaking - dry to wet
Micro cracking by swelling - gradual wetting
Dispersion - sodium
Consequence of poor soil aggregation?
Breakdown of structure
Pore clogging
Erosion
Reduced infiltration
No protection of organic matter
Reduced habitat
Less aeration
Bulk Density
Mass of a unit volume of dry soil
- includes solids and pore space
- need known volume of soil (core)
Size and function of pores?
Macropores - transport of water and air, drain under gravity, found in sand, larger aggregates and peds
Mesopores - air/water store
Micropores - filled with water, clay, inaccessible to roots
Effects on bulk density of:
- traffic
- soil texture = more sand
- organic matter
- tillage
- increase = compactability
- increase = less pore space
- decrease = more pore space, OM lighter then minerals
- increase long term = loss of OM, weakened soil structure
Impact of compaction on bulk density
Increased bulk density
Root growth inhibited by soil resistance to penetration, poor aeration, reduced movement of water and nutrients, potential anaerobism
What is Tilth?
Relies on friability - good cohesion of individual aggregates
Depends on texture, aggregate stability, moisture, density, organic matter
Soil capillarity
Water can move up through adhesion and cohesion
More capillary flow in fine structures = smaller pore space
Soil water potential
Gravitational
Matric (pressure) = capillary - unsaturated zone
Osmotic - low to high potential
Field saturation to wilting point
When field at saturation water lost through gravitational
Field capacity = water held by matric
Wilting point = no more water available to plants
Plant available water
Water retained in soil between field capacity and wilting point
Silt has the most
What factors influence soil water for plant uptake?
Texture - finer particles more available, some clays lock up water in micropores
Organic matter - improved structure + high water holding capacity
Structure - reduced porosity and infiltration
Osmotic potential - more soluble salts in solution
Soil depth and horizons
Benefits of drainage
Soil workability
Enhanced root growth
Reduced disease
Rapid soil warming
Reduced gas
Removal of salts
Gases in soil
Oxygen - aerobic respiration
CO2 - toxic above 10%
Water vapour, methane, N2O
Aeration and influenced
To supply oxygen and remove harmful gasses
Drainage, soil respiration, depth, pore size
Effects of Soil temperature
Effects germination, root functions, microbial processes, freeze-thaw
Influenced by pore space, water, slope/aspect, colour, rainfall, soil cover
What is Cation exchange
Positively charged cations are attracted to negative charges on clay and OM
Cations replace one another charge for charge
what is Cation exchange capacity
Sum of total exchangeable cations that a soil can hold
Organic being highest then clay and sand lowest
Why would we use Gypsum
Sodium causes deflocculation
Gypsum works by calcium displacing the sodium and allowing better flocculation
Same with magnesium (causes swelling and shrinking)
Sources of H+
Oxidation of N
Root respiration
OM
Acid rain
Plant uptake of cations
Effects of soil pH
Al and Fe toxicity at low ph
Nutrient availability - lack of Mn at high and Ca at low
Bacteria less active at low
Disease - club root at low common scab at high
Liming
Must displace H+ from exchange sites
Neutralise them in solution
Liming materials with lower neutralising value must be applied in higher cuantities depending on pH and texture
Macronutrients
N
P
K
Mg
Ca
S
Micronutrients
Mn
Cu
B
Mo
Zn
Fe
Co
Se
Ni
Nutrient Interactions
Antagonism - high N = low B
Stimulation - high N = high Mg
N deficiency
Makes up plant proteins
Yellowing of older leaves
Sands, low OM, leaching, anaerobic
P deficiency
Needed for ATP
Stunting
High/low pH, high erosion
How to increase nutrient efficiency?
Nutrient management plan
Ph
Structure for roots
How much carbon in organic matter?
0.58kg C in 1kg OM
What is detritus?
Remains of dead plant and animal material
Undergoes decomposition into humus
What is plant residue?
Sugars, starches, simple proteins
Crude proteins
Hemicellulose
Cellulose
Fats and waxes
Lignin
What is Decomposition?
Microorganisms break down organic compounds in presence of oxygen
Essential nutrients released and/or immobilised
New compounds are synthesised by microbes
Some compounds locked up in soil
What is humus made up of?
Fulvic acids
Humic acids
Humins - water holding, soil structure, stability, CEC
Influence of OM on soil
Soil colour
Soil aggregation and stability - reduces erosion increases aeration and C store
Soil workability - reduces plasticity
Infiltration and water holding capacity - more pore space
Cation exchange capacity
Nutrient slow release
Soil biology
How is OM built up or lost
Plant residues, animal inputs, biomaterial, root residues, rhizodposition
Oxidation, erosion, leaching, removal off-site
Environmental factors influencing OM
Temperature - low = faster accumulation, high = slower accumulation
Moisture
pH
Residue location
CN ratio
CN ratio of residues determine rate of decay and rate of available N
Microorganisms need 8:1 but due to respiration loss need 24:1
High CN ratio causes depletion in N (immobilisation)
Low CN release more soluble N but too much can result in anaerobic
Management influences on OM
Organic amendments - compost best then FYM
Cover crops
Reduced tillage
Grass leys
Microbiota
Viruses
Bacteria
Archaea
Important soil bacteria
N fixing = Azotobacter, rhizobacteria
Nitifrifcation = Proteobacteria
Mineralisation = Actinobacteria
Denitrification = Pseudomonas
Disease = pectobacteria (blackleg)
Important soil Archaea
Extreme halophiles - salt
Methanogens - methane
Importance of soil bacteria and archaea
Release and recycle nutrients
Mutualistic symbionts - biofertilisers
Plant pathogens - biopesticides
Types of fungi
Saprotrophs - feed on dead tissue
Mutualists - mycorrhizal fungi
Parasites or pathogens
Importance of fungi
Release and recycle nutrients
Decomposers
Mutualistic symbionts
Soil structure - glomalin
Microfauna
Protozoa - feed on bacteria, mineralise nutrients and release N
Nematodes - mineralise and release nutrients, plant pathogen
Rotifers - feed on detritus, dead bacteria and Protozoa
Nitrogen cycle processes
N fixation: N2 - ammonium
Nitrification: ammonia - nitrite - nitrate
Denitrification: nitrates into N gas
Mineralisation: organic N into available N
Immobilisation: uptake of N by plants o organisms
Microorganisms with N fixing
Free living e.g. Azotobacter
Bacteria associated within rhizosphere
Phototrophic N fixing bacteria
Symbiotic N fixing e.g. Rhizobium in legumes
Microorganism for nitrification
Proteobacteria for ammonia and nitrate oxidisers e.g. Nitrobacter for nitrate
Causes acidification
Microorganism for Denitrification
Pseudomonas bacteria
P forms in the soil
Soluble P - plant available
Organic P - within microbes and plants
Inorganic P - locked up
What is P mineralisation and immobilisation?
Organic P into P
P into organic P
Influenced by soil moisture, temperature, pH and microbial population
P adsorption
P attached to soil particles by clay, Al, Fe oxides
pH dependent, highest available at 6.5
Factors influencing P availability
OM - mineralisation + release P from soil
Clay - locks up P
Soil mineralogy - Fe and Al locks up P
Soil pH - 6.5
Abiotic factors
P microorganisms
Bacteria, fungi
Mycorrhizal fungi form Mutualistic symbiosis - P in exchange for carbohydrates
Can be outside (ectomycorrhizas) and inside (arbuscular mycorrhizas) the root
Benefits of mycorrhizal fungi
Improved nutrient uptake
Increased soil stability
Resistance against pathogens and herbivores
Improve water balance
Alleviate abiotic stress
What is the rhizosphere?
The soil volume around roots that is strongly affected by root functioning
Roots release enzymes, H+, CO2, exudates
Roots uptake nutrients, O2 and water
Rhizosphere interactions
Positive: Mutualistic symbiosis, growth facilitators, biocontrol
Negative: Phytotoxins, pathogens, root herbivores
Allelopathy and Autotoxicity
Chemical mediated plant-plant interference where phytotoxins released to reduce survival of neighbours
Plant or decomposing residues release toxins to prevent germination of same species
Farm practices and the rhizosphere
Crop rotation - reduce allelopathy, crop cover maintains temperature and moisture
Tillage - less tillage increased biodiversity
Nutrients
Mulching
Mesofauna size and examples
0.2 micromilimeters - 2mm
Tardigrades - feed on bacteria, plant cells, Protozoa, Rotifera and nematodes. Survive extreme conditions
Springtails - decomposers, feed on bacteria, fungi, nematodes. Different ecotypes.
Mites - break down OM and feed on fungi and bacteria
Important for nutrient cycling by feeding and being prey
Macro fauna size and examples
Over 2mm
Pot worms - feed on bacteria, fungi, OM decomp
Thrips
Centipedes - predators, venomous
Millipedes - detritivores
Pseudocentipedes - decaying vegetation, seeds, roots
Earthworms
Wireworms
Ground beetles
Types of Earthworms
Epigeic - small, litter dwelling
Endogeic - small to medium, horizontal burrows
Anecic - large, deep vertical burrows
Good indicators of soil health - improve structure and decomposers
Earthworm counts - AHDB
Assessing soil quality
Physical: Texture - hand, laser diffraction, VESS - 1-5, infiltration rates, penetrometer, bulk density
Chemical: pH, PK, Mg - lab, W pattern 15cm, N - blocks of 30cm, C
Biological: earthworm count, SOM - loss of ignition (LOI), microbial activity - respiration rates underpant test, taxonomy ID, DNA
Gas analysis
Flux tower
Dynamic chambers
Static chambers
What is conservation ag and regen ag
Minimum soil disturbance, permanent soil cover, diverse crop rotation
Plus: living roots, livestock
Management options for improving soil quality
Tillage - mintill, no till, strip till
Rotation - early winter cereals, grasses, spring cereals after cover crop
Cover crops
Leys and grazing
Mulching
Organic manures
Drainage
Nutrient management plan
Pesticide use
Biostimulants
Windbreaks/shelterbelts
Agroforestry
Buffer strips
Soil policy
Sustainable development goals
4 per mile - increase SOM by 0.4% a year
25 year environmental plan - soils must be managed sustainably
ELMS - SFIs - soil management plan, cover crops, herbal leys, no till farming,
NVZ
Farming rules for water
Catchment sensitive farming
