Soil structure degradation - Food and Agriculture · PDF fileSoil structure degradation Soil compaction. ... results in the formation of dense plough pans. Surface compaction • clogging

Soil structure degradationSoil structure degradation

Soil compaction

Soil compactionSoil compaction• most serious form of land degradation

caused by conventional farming practices

• is reversible and its occurrence preventable or at least controllable

• a world-wide problem that spans all soils and levels of farming input

• the most difficult type of land degradation to locate and rationalise as it is principally a sub-surface phenomenon

“Invisibility”“Invisibility”

• requires physical input before it is uncovered and its extent, nature and cause

• no strong surface evidence of its presence

Soil structureSoil structure

• the architecture of soil – the way the solid particles and spaces are arranged

• good soils have a mixture of microporesand macropores: macropores for water entry and drainage, micropores for water storage.

Soil structure

• a prime indicator of a soil’s “health”

• most obvious indicator of restricted rooting when a crop is present is the distribution of the crop roots

The hidden enemyThe hidden enemy

• soil compaction, due to the collapse or diminution of pore spaces, is the most common cause of physical restriction for root growth and development

• individual roots often develop characteristic growth patterns in a dense layer containing few pores

Compacted soilCompacted soil

• Does not provide adequate space for the storage or movement of soil air and water.

• Large, continuous soil pores are lost or are reduced in size, leading to slow water movement and reduced aeration.

• Soil animals and root growth are restricted.

Causes of soil compaction

• forces from agricultural tyres and implements,

• working in moist to wet soil conditions

• the continual use of tillage implements, especially disc ploughs, disc harrows, mould-board ploughs and rotovators, over long periods of time frequently results in the formation of dense plough pans

Surface compactionSurface compaction

• clogging of pores with particles detached from soil aggregates under the impact of raindrops

• the deposition of detached particles on the soil surface as impermeable crusts or seals

• enhanced by excessive tillage, which leads to pulverisation of the soil and the progressive loss of soil organic matter

Effects of compactionEffects of compaction

• loss of rainfall

• restrict seedling germination

• “downward spiral” of land degradation with conventional agricultural practices –traffic and cultivation in moist soil cause compaction, necessitating further cultivation to remove the compaction, and so on

Compacted soils are less productive, Compacted soils are less productive, compared to the same soils with compared to the same soils with

good structure.good structure.This is particularly true in arid and semi-arid regions where optimal soil structure conditions are required to maximise both water entry into the soil and water storage during fallows. In dry seasons, root growth must also be optimal and fine structure is required to enhance root proliferation to fully tap soil water reserves to enhance grain or fibre production through to harvest.

Conventional tillageConventional tillage

• Mouldboard ploughs, sweeps and ducks-foot points, which exert largely horizontal pressures on soils, can cause smearing of soil surfaces.

• This closes pore openings, disrupts the continuity of pores within the soil and reduces infiltration rates.

Water movementWater movement

• Infiltration depends on there being sufficient porosity in the surface soil for rainfall to infiltrate, and in the subsoil and parent material (if shallow) for rainwater to percolate

• If a pore becomes ten times smaller, the amount of water that can flow through it in a given time will be 10,000 times less than before

Improving soils with restricted rooting

Will depend on the cause of root restriction:

• a lack of pores that are large enough to be readily penetrated by roots or which can be sufficiently widened by the growing roots

• chemical restrictions due to the presence of toxic concentrations of aluminium or manganese, high salinity or severe nutrient deficiencies, especially of phosphorus

• lack of oxygen due to a fluctuating water table

Biological methods • to utilize the roots of natural vegetation or

planted cover crops to act as biologicalsubsoilers penetrating the dense root-restricting horizons

• The stability of root channels created by plant roots will be greater than that of channels formed by mechanical methods because of the release of organic substances from the roots that stabilize the channel surfaces

Promising cover crop speciesPromising cover crop species

• Bahia grass (Paspalum notatum), Festuca elatior, Guinea grass (Panicummaximum), alfalfa (Medicago sativa), pigeon pea (Cajanus cajan) and cowpea(Vigna unguiculata).

• Radish (Raphanus sativus), and the nitrogen-fixing shrubs Tephrosia vogelii,Sesbania sesban and Gliricidia sepium

AdvantagesAdvantages

• generally much cheaper to implement

• the benefits are longer-lasting than mechanical methods

• improvement of the physical, chemical and biological fertility of the soil due to the large quantities of organic matter produced and added to the soil

• Factors that enhance biological methods are:

•Zero tillage

•Controlled traffic

•Flotation tyres

Controlled trafficControlled traffic

• The aim is to keep the wheels of all in-field equipment always in the same paths, year after year, and to keep these traffic zones separate from the cropping zones,

Flotation tyresFlotation tyres

• The principal role is to reduce traffic-induced soil compaction

• Conventional agricultural tyres compact soil principally through normal and shear stresses

Mechanical methodsMechanical methods

• are used to break-up compacted or naturally dense root-restricting layers in order to create larger pores through which crop roots can penetrate.

• this is usually accomplished by the implement slightly lifting and breaking the compacted or dense layer

• in CA systems only possible at the implementation of the system

Chemical solutionsChemical solutions• application of P fertilizers to phosphorus-

deficient soils frequently encourages deeper rooting

• application of lime without or with gypsum will reduce toxic concentrations of aluminium and/or manganese to non-toxic levels

• excess quantities of water should be applied sufficient to leach the salts out of the crop’s rooting zone in case of high salt concentrations