Tillage in agro-ecosystem management Flashcards
Soil provides plants with:
Support
Nutrients
Water
Oxygen requirements of roots (except wetland plants)
Protection of underground buds from temp. extremes
But also a potentially hostile environment:
Pathogens Root grazers Toxins Too much or too little water Inadequate aeration
Soil is a 3-phase system
Solid (mineral, organic)
Liquid (free water, structural/unavailable water)
Gas (enriched in CO2, depleted in O2)
Approximately half of the volume of soil is pore space containing air or water
Other 45% is mineral matter and 5% organic matter(now less than 2% in arable soils)
Soil air is distinct from the atmosphere
and varies with depth.
Soil profile patterns of distribution of organic and mineral components
Cultivated soils which have been tilled have mixed horizons depleted in organic matter
Natural/semi-natural soils have horizons defined with organic layers washed into the mineral horizons
Why are soils tilled?
Prepare a fine loose soil layer for seedlings to establish.
Bury weeds, crop residues and pathogens.
Release nutrients.
Reduce surface and subsurface compaction.
However, there is increasing realization of adverse long-term effects of tillage on soil sustainability and soil erosion.
Types of tillage affects how many times the soil is driven over and the extent to which soil is disturbed and left bare:
Conventional-
1. Inversion tillage (mouldboard plough)
2/3. Disking / power harrow (breaks up clots),
3/4. Seed (drill).
Minimum tillage:
Shallow disc cultivation,
Drill
(and chemical weed kill)
Zero tillage (direct drill):
Slot drill
(and chemical weed kill)
Bulk density
Indirect measure of soil porosity
Dry weight per cm3
Soil minerals have a mean density of ~ 2.65 g cm3
Maximum soil bulk density penetrable by roots ~ 1.8 g cm3
= minimum pore space for root penetration ~ 32% of volume
Rapid adoption of no-tillage in Canada has helped reduce soil erosion
But erosion is still high in general
Abandoning conventional inversion tillage and adoption of minimum tillage and no-tillage arable farming has contributed to reducing soil erosion in the USA-
but erosion rates remain unsustainable by a factor of 10 compared to soil formation.
How do we cultivate in England?
Conventional tillage 60%
Reduced tillage 32% of area
Zero tillage 8%
Conventional tillage causes 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.
No tillage & shallow tillage have less detrimental effects than conventional ploughing
Soil Physical Properties
Texture - particle size distribution relative proportions of gravel, sand, silt, clay
Structure - how particles are organized into aggregates
Water-holding capacity – controlled by porosity, structure and texture
Aeration – controlled by water-holding capacity
Texture:
Based on particle size distribution of mineral fragments in soil.
Atterberg or International classification
Gravel > 2 mm Coarse sand 0.2 - 2 mm Fine sand 0.02 - 0.2 mm Silt 0.002 - 0.02 mm Clay <0.002 mm
Soil water availability depends on soil properties: texture
Clay has a lot of unavailable water, unlike silt loam/loam
Soil texture
Very stable - only changes slowly due to weathering
Affects major soil properties:
Clay soils can store large amounts of water and nutrients but drain slowly and are prone to waterlogging
Sandy soils drain very quickly and store little water or nutrients but provide good aeration to roots
Loam soils- in which there is a mixture of sand, silt and clay- are optimal for most plants.
Structure:
How particles are aggregated and organized together into peds - these are discrete units separated by voids and natural planes of weakness.
Why is structure important?
Soil structure 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:
Clay soil - improved aeration and drainage
Sand soil - improved water and nutrient-holding
Soil organic matter plays a crucial role in this
Structure creates spaces roots can grow through. Most roots > 60 m diameter
Pores are found between and within peds.
Function: root oxygen, water & nutrient
supply
How does tillage affect structure?
There are dramatically fewer water-stable aggregates - prone to flooding
Aggregate stability- resistance to saturation with water is reduced by cultivation- particularly for largest aggregates
Wood-To-Field bioassay of wheat
E. Marshall-Harries MBiolSci Thesis
Crops grew much better on woodland soil that had been untouched
Water Stable Aggregate Fractions
~63% of soils > 1mm (Macroaggregates)
66% loss of Macroaggregates
~22% of soils > 1mm
Loss of soil organic matter as a result of cultivation has compromised soil functioning-
including nutrient and water-holding capacity- limiting crop yields and increasing vulnerabilities to waterlogging and drought.
How does tillage affect pores?
Loss of OM - Loss of macropores - reduced water infiltration - high surface run-off - soil erosion - loss of clays and OM
Ecosystem engineers involved in formation of macroaggregates:
Fine roots, especially of grasses
Mycorrhizal fungi and other fungi
Earthworms and other soil animals
Ploughing just once has a major negative impact on mycorrhizal infection in plant roots.
Can use leys of certain durations (3 years approx.)
Grasses can restore aggregates
Macroaggregates and soil organic carbon may be restored by stopping cultivation and use of grass (leys). Macroaggregate recovery towards the properties of undisturbed soil is faster than for organic carbon
Plant roots, mycorrhizal fungi, and earthworms are ecosystem engineers that restore soil crumb structure and improve soil drainage.
Cultivation practices can disrupt soil structure and damage the ecosystem engineer organisms leading to loss of organic matter, disaggregation, compaction, and impaired drainage- factors that increase soil erosion.
Conservation tillage, reduced tillage, strip cropping and contour-ploughing reduces soil loss and damage
Short-term grass-clover leys, and other perennial vegetation can restore damaged soil - but it can take decades for full recovery even if soil has not been eroded.
Minimum disturbance of soil:
Soil is a living organism and therefore has to be treated as such. Soil organisms are destroyed by exposure to solar radiation and rapid drying of the soil.
Soil tillage, especially inversion by using mouldboard ploughs, enhances the decomposition of organic matter in the soil and leads to soil compaction especially the formation of a plough pan, reduced water infiltration and reduced aeration. Instead, no-tillage and a good soil cover, enhances soil “life” leading to the formation of a deep reaching macropores or biopores, enabling water infiltration and aeration. For most soils –organic matter is central for the storage of nutrients and water in the soil.
