Lecture 4 - tillage Flashcards
5 things soil provides plants with?
- Support
- Nutrients
- Water
- Oxygen requirements of roots (except wetland plants)
- Protection of underground buds from temp. extremes
5 reasons why soil can be a potentially hostile environment?
- Pathogens
- Root grazers
- Toxins
- Too much or too little water
- Inadequate aeration
why is soil potentially one of the most complex substances to study?
- Has a 3-phase system:
- Solid (mineral, organic)
- Liquid (free water, Structural / unavailable water)
- Gas (air) (enriched in CO2, depleted in O2)
what is half of the volume of soil?
pore space containing air or water
describe soil air
is distinct from atmosphere and varies with depth due to processes, microbes and root and microbial respiration
4 reasons soils are tilled?
- Prepare a fine loose soil layer for seedlings to establish.
- Bury weeds, crop residues and pathogens.
- Release nutrients.
- Reduce surface and subsurface compaction
what are 3 conventional tillage methods?
- Inversion tillage (mouldboard plough)
- Disking / power harrow,
- Seed (drill)
what does conventional tillage involve?
the use of lots of machinery
2 minimum tillage techniques
- Shallow disc cultivation,
2. Drill inserts seed (plus fertilizer) (and chemical weed kill)
how do minimum tillage techniques work
lighter machinery
often leaves a lot of crop residue on the surface which helps to bind the soil together
what is a zero tillage technique?
- Slot drill adding seed and fertilizer (and chemical weed kill - still not good)- almost complete cover by crop residue
- now alot more common
how does zero tillage work?
direct drill, lighter machinery again
what is the advantage of zero tillage?
- Soil organisms are left largely undisturbed - helps to maintain soil quality in ways that normal ploughing doesn’t
- consequences = reduced erosion
what is the big issue with tillage?
compaction
What is an issue with compaction?
- Compaction can be so tight roots and water can’t get through - restricting roots to shallow part of soil and therefore in a drought the roots can’t reach low level water
how do farmers try to reduce compaction?
increased width of machinery and wheels to spread weight
what are the problems caused by conventional tillage?
loss of organic matter, nutrients, and water storage, increases soil erosion, and damages beneficial fungal symbionts reducing nutrient use efficiency, and reduces earthworm populations involved in aggregation and soil drainage
despite the bad effects of tillage why do farmers continue to do it?
short term consequences are often better
4 physical properties of soil
1)particle size distribution
relative proportions of gravel, sand, silt, clay
2) Structure - how particles are organized into aggregates
3) Water-holding capacity – controlled by porosity, structure and texture
4) Aeration – controlled by water-holding capacity
what is soil texture classified by?
Based on particle size distribution of mineral fragments in soil - how much silt, clay and sand
what soil water availability depend on?
the soil property; texture
describe the stability of soil texture
Soil texture is very stable - only changes slowly due to weathering which
affects major soil properties
describe a clay soil
Clay soils can store large amounts of water and nutrients but drain slowly and are prone to waterlogging
describe a sandy soil
Sandy soils drain very quickly and store little water or nutrients but provide good aeration to roots
describe a loam soil
Loam soils- in which there is a mixture of sand, silt and clay- are optimal for most plants.
what defines soil structure
How particles are aggregated and organized together into peds - these are discrete units separated by voids and natural planes of weakness
why is soil structure important
because it controls the passage of water and oxygen into the soil and affects nutrient-holding capacity
- Structure can overcome some of the limitations imposed by texture
- creates spaces roots can grow through -( Pores are found between and within peds)
what plays a crucial role in structure?
organic matter
what farming technique majorly impacts structure?
- tillage - causes poor drainage
describe the importance of aggregate stability
- Aggregate stability- resistance to saturation with water is reduced by cultivation- particularly for largest aggregates
explain the cascade caused by loss of organic matter
loss of organic matter(carbon) = loss of aggregation due to loss of macropores = reduced water infiltration = plants grow worse and high surface run off = soil erosion = loss of clays = cycles starts again
how are peds formed? (7 ways)
1) colloidal fraction
2) cementing agencies
3) expansion and contraction by wetting and drying
4) plant roots
5) micro-organisms
6) fauna e.g earthworms
7) cultivation and drainage - can improve structure but often causes loss of aggregates
3 ecosystem engineers involved in formation of macroaggregates
- Fine roots, especially of grasses
- Mycorrhizal fungi and other fungi
- Earthworms and other soil animals
whats the issue with loss of macroaggregates?
- Can take decades to restore macroaggregates once they have been broken up
After intense periods of cultivation how can macroaggregates and soil carbon be restored?
the use of grass (leys - e.g. grass and clover) - Leys have very positive effects on physical structures of soil
describe the importance of soil organic carbon
- 1% increase in soil organic carbon increases water storage capacity by nearly 10%
