Soil Water relations & Organic matter Flashcards
What does there need to be a balance of in the soil?
Air and Water
Need a balance for plant roots to function.
What are large pores called?
Macropores.
These pores can drain easily within hours after being saturated.
What are small pores called?
Micropores
These continue to contain water after saturation and drainage has occurred.
What are medium pores called?
Mesopores
These pores hold water or can drain freely. They hold water which is available to the plant.
What is the optimum pore size for plant growth?
A combination of pore sizes where macropores contain air, mesopores and micropores contain water.
What do all soil particles have around them?
A film of water.
What kind of pores do clay soils have?
Many micropores (very small pore spaces). Clay particles cling onto water molecules due to adhesion and cohesion and water stays in the small pore spaces due to capillarity rather than draining away. Sometimes clay soils has a lot of water clinging to the minerals, but the roots do not have enough force to overcome the cohesive forces of water molecules to absorb the water. Even though water is present in the soil it is unavailable to the plant roots.
What kind of pores do sandy soils have?
Many macropores. Large pore spaces. Water flows downwards with gravity and does not stick to soil particles. A thin film of water will adhere to sand particles.
What kind of pores do loam soils have?
A mix of micro, macro and mesopores.
This means they are moisture retentive but free draining.
Which pores join together to form narrow tubes?
Micropores
Which pores join together to form very wide tubes?
Macropores
Mesopores form what sized tubes between them?
Medium sized.
The soil water is held in pores with different strengths. Which pore has the strongest hold?
Micropores
The soil water is held in pores with different strengths. Which pore has the weakest hold?
Macropores
What is infiltration?
Water soaking into the soil with gravitational flow.
Describe surface run off?
Water running off the surface of the soil.
What are the 3 definitions for Water?
Saturation Point
Field Capacity
Permanent Wilting Point.
Describe Saturation Point?
When water occupies all the pore spaces in the soil.
All of the pores (macro, micro and meso) are filled with water.
Indicated by pooling on the surface.
Describe Field Capacity?
The maximum amount of water held by soil after it has been saturated and then allowed to drain freely.
The macropores are filled with air, as the water has drained away.
The micropores hold hygroscopic water which is not available to the plant roots.
The mesopores hold water which is available to the plant roots.
A soil at field capacity has the maximum water content for the optimal plant growth.
Good for most crops a plenty of water available but pore spaces for air.
Define ‘Water holding capacity’?
‘Water holding capacity’ is the total amount of water a soil can hold at field capacity.
Describe Permanent Wilting Point? PWP
At Permanent wilting point there is no more available water in the soil.
The macropores and the mesopores are filled with air.
The micropores has water present but it is tightly bound to soil particles so is not available to plant roots.
What does ‘Available water content’ mean?
This is the amount of water held in the soil between field capacity and permanent wilting point.
The amount of water that is available for plant uptake and is held in the mesopores.
For example:
At Full Capacity At Permanent Wilting P
Coarse sand 26mm 1mm
So Available Water Content is 26mm-1mm = 25mm
Give 4 examples of factors which can make a soil saturated?
Heavy rainfall Poor drainage Iron pan Heavy clay soil High water table.
Give 4 examples of factors which can make a soil dry out?
Sandy soil - soil type.
Over cultivation of sandy soil
Dry weather spell - no rain.
Raised beds dry out more quickly.
Give 4 situations where excess water can become an issue?
Lawn - compaction of clay soils.
Remedy - Aerate with hollow tines.
Veg garden - Soil cultivation pan caused by rotavator on clay soil.
Remedy - Cultivate when soil is dryer to varying depths. Add OM
High water table
Remedy - raised beds.
Newly planted herbaceous border - high rain water or over irrigation.
Remedy - better gardening practise. Hand water or mulch to conserve water instead of watering.
Name two types of drainage systems you could use?
Soakaway
French drain
Give 4 different types of irrigation?
Watering can
Hose
Sprinkler
Seep hose
Pros and Cons of watering can?
PROS Not wasting water. Spot watering - aim at roots not leaves. Warm water better for plants. No wastage. Recycle rain water/grey water. Fertiliser easily added. ------------------------------------------------------------------------ CONS Labour intensive Slow Bad for your back. Only suitable for a small garden.
Pros & Cons of hose?
PROS Aim at roots - less fungal disease. Exact penetration No wastage Different nozzle settings so force can be adjusted. ----------------------------------------------------------------------- CONS Trip hazard Uses more water than watering can. Not harvested water. If on a meter can be costly.
Pros & Cons of a sprinkler?
PROS
Can be put on a timer and turned on at night.
Less time consuming.
Good for large scale crops as covers large area.
May scorch leaves in very sunny weather.
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CONS
Evaporation in hot weather
Need high water pressure for it to work.
Wastes water.
Foliage is wetted so can encourage fungal disease such as potato blight.
Pros & Cons of a seep hose?
PROS Targets root zone Less evaporation Slow infiltration. Foliage not wetted. Can be put on a timer ------------------------------------------------------------------- CONS Cost of equipment Need high water pressure. Can waste water. If buried take care not to stab with a fork
When is the best time of day to water?
Early in the morning or evening during summer when it’s cooler and less water is lost immediately to evaporation.
How deep should you water?
Deep watering encourages downward water searching roots not shallow surface roots that are vulnerable to drying out.
Give 5 examples of when irrigation can affect the environment in the garden?
- Surface capping can occur which will prevent infiltration of water and seedling emergence.
- The soil can be saturated which is not good for root growth if over irrigated.
- Surface capping can also lead to surface run off, and cause flooding in other areas of the garden and erosion.
- Watering during hot weather is wasteful as water will evaporate more readily.
- Leeching of nutrients can occur causing nutrient deficiency symptoms of plant.
Give examples of ways to improve water content of soil?
Mulch with well rotted organic matter to prevent evaporation from soil surface.
Incorporate well rotted OM to improve the structure of sandy soil so htere are more mesopores which will hold on to water which is available to plant roots.
Sort out impermeable pans (compaction, cultivation, iron) so that water is able to drain away.
Is there much air in sandy soils?
Sandy soils have macropores so drain more freely. So as long as drainage not impeded sandy soils have good aeration. Roots can burrow as can earthworms and aerobic respiration can happen.
Plants are more susceptible to water stress.
Is there much air in a clay soil?
Clay soils have micropores which cling onto water so have less air space. Compacted clay soils make them dense with lower volume of air space.
Less oxygen means roots have to undergo anaerobic respiration, and a build up of toxins can occur and die.
How can you improve soil?
With soil amendments -
organic - organic matter
inorganic - grit, sand or lime.
What is the goal of soil amendments?
To provide a better environment for root growth.
Why would you add lime to soil?
To change the PH of the soil.
To help break up a clay soil. (flocculation)
Encourages earthworm activity.
To deter the spread of club root in brassicas.
What is the best form of lime for flocculation? (breaking up a clay soil)
Gypsum
Definitition of Organic Soil Amendments?
These are amendments which originate from living organisms, and contain carbon.
What are the 3 categories of organic matter in the soil?
- Living organisms
- Dead organisms
- Humus
Describe ‘living organisms’ in the soil and their benefits?
Convert plant and animal remains into humus and minerals.
Roots, earthworms and other burrowing organisms improve soil structure, aeration and drainage.
Many types of bacteria have a role in detoxifying pesticides and herbicides.
Mycorrhizae fungi have a symbiotic relationship with the plant roots, increasing the surface area for water absorption.
What is the timeline for the breakdown of living organisms?
Living - death - decay - further decay - humus
Describe dead organisms in the soil and their benefits?
Large bulky material such as leaves or twigs. Physically opens up the soil and improves aeration and water movement.
Smaller material improves water holding capacity.
Provides food for soil organisms such as worms.
Increases soil microbial activity.
‘Green’ fresh leaf material decompose quickly and yield nutrients (nitrogen) but brown materials (woody fibrous) decompose slowly and their nutrient release is low.
Give examples of green composting waste? (nitrogen rich)
Vegetable/fruit peelings & scraps Coffee grounds/tea bags Green grass clippings Green garden waste Flowers
Give some examples of brown composting waste? (carbon)
Dried leaves Brown grass clippings Pine & spruce needles Paper, cardboard & newspaper Houseplants Sawdust from untreated wood Straw Prunings and cuttings (help create air pockets)
Describe what humus is and give examples?
Humus is the final stage of decomposition and is the end stage of organic matter.
Humus coats soil particles with a black colloid. (like a gel)
Makes soil darker, so it warms up quicker in spring.
Sticky so aids crumb formation.
Improves water holding capacity.
It has cation exchange capacity just like clay thus increasing nutrient availability and reducing leeching of nutrients.
Surface charge in humus can bind with clay thus making heavy soils less sticky and more friable.
What are the 3 main sources of OM in the garden?
- Natural decomposition of leaves and plants and animals after death.
- From the garden - digging in crop residues/garden compost/green manure.
- Brought in - composted straw, composted wood fibre, municipal compost, well rotted farmyard manure.
Give 6 benefits of adding OM to the garden?
- Increases nutrient availability.
- Encourages crumb structure
- Improves drainage.
- Improves water holding capacity.
- Darkens soil to warm up quicker.
- Increase CEC (Cation exchange capacity) for nutrient holding.
Why should organic matter be well rotted before adding to the soil?
Fresh farmyard manure can be very acidic and scorch leaves and roots due to high levels of ammonia.
Nitrogen can be robbed as bacteria use up nitrogen from the soil to break down the organic matter containing carbon. Eg. wood chips.
Risk of passing on disease.
Fresh OM can acidify soil as it breaks down and may alter other nutrient availability as it changes the PH.
List some examples of organic matter?
Farmyard manure
Pro improves soil structure & has high potash so good for fruit.
Cons - if not rotted down well can scorch plants. High ammonia. May have weed seeds. Nutrient levels will vary.
Garden compost
Pros - Helps keep soil open and crumb like. Dark so warms soil up. Soil improver adds nutrients.
Cons - May contain treated material, weeds, or diseased material.
Peat
Pros - stable, moisture retentive and porous.
Negatives - not sustainable, destruction of species habitats, carbon sink, absorbs excess water so more flooding.
What is a mulch?
A mulch is a layer of organic or inorganic material applied to the soil surface. It is not worked into the lower soil layers.
4 Examples of mulch
Garden compost
Leaf mould
Gravel
Ornamental bark chippings.
What are the benefits of mulching?
Water preservation. Prevents evaporation from soil surface.
Heat trapping - if applied when soil is warming if dark in colour.
Can use to create pathways.
Weed prevention and control
Protecting roots and from cold temperatures. (insulation)
Controls and prevents soil erosion.
Reduces surface capping and protects the surface of the soils during winter.
Can be decorative, tidy and prevent soil splashing onto plants.
Can deter some pests.
Barrier to prevent edible crops making direct contact with soil - strawberries.
OM mulch can improve soil structure.
May fertilise the soil.
What are the limitations of mulching?
Organic mulch needs to be at least 5-8cm thick to suppress weeds.
Organic mulching can cause Nitrogen robbing as it breaks down - eg. wood chip.
May need replacing such as gravel.
Can be expensive.
Pros and cons of Farmyard manure as an organic mulch?
PROS
Bulky organic matter.
Maintains and improves soil fertility.
Will eventually be incorporated in overall structure.
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CONS
Varies widely
May contain weeds.
High in ammonia if not well rotted, can scorch plants.
Pros and cons of compost as organic mulch?
Pros Dark so helps to warm soil in spring. Fertiliser Few pests Improves soil fertility -------------------------------------------------------------- Cons May have weed seeds Requires space to produce your own compost on site.
Pros and cons of shredded prunings and bark chippings as a mulch?
Pros Last long while - various grades. (sizes) Suppresses weeds if thick layer 5-8cm Retains moisture ----------------------------------------------------------------- Cons May cause nitrogen deficiency Requires energy to shred. May transfer honey fungus.
Give 5 examples of inorganic mulch?
Black polythene Woven fibres (weed suppressant) Gravel Glass Rubber
What is a green manure?
Green manures are fast growing plants that prevent erosion, smother weeds, supply nutrients and improve soil structure.
Give 3 examples of green manure?
Crimson Clover
Mustard seeds
Rye grass
