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- Ted Floyd
Urban Soils
Soil Loss and Degradation
The productivity of farms decreases when soils become unhealthy and plant growth is reduced.
Water erosion Structure breakdown Beating raindrops Organic matter decay Wind erosion Land degradation Nutrient erosion Surface seals Silt deposition Soil compaction Landslides Salting Nutrient harvesting Desertification Acidification Soil contamination Scalds Mining Nutrient leaching Urban spread Acid sulphate soils Roads and railways Riverbank erosion Water storages Buried soils PEAK SOIL
Ted Floyd
June 2011Keywords: Soil erosion, Soil degradation, Raindrop splash, Soil carbon sinks
Agricultural land is now destroyed faster than new land becomes available for farming.
It is estimated good land in the world is lost 10x faster than new land becomes available for farming.
Peak Soil is when the loss of soil is faster than new soil is developed for agriculture.
Peak Oil is when oil is consumed at a rate faster than the discovery of new oil fields.
Many people warn the consumption of oil in the world will soon reach Peak Oil and oil shortages will occur causing many problems in our modern economy.
Many soil scientists warn we are rapidly losing good agricultural land and Peak Soil is a bigger problem than Peak Oil.
In the future it will become difficult to feed everyone in the world. The worlds population is increasing and good farmland is decreasing. Increasing population in some countries is forcing people to try and increase food production at the expense of soil degradation.
New technology has increased food production in the past and it is hoped sustainable technology will improve productivity in the future.
Sustainable technology exists and farmers should be encouraged to conserve soils.
PLANTS PROTECT SURFACE SOILS
Growing plants intercept raindrops, breaking them up into smaller droplets and reducing their velocity.
The soil surface is protected by plants from the beating action of raindrops reducing structure breakdown. Leaf litter also protects soil.
Infiltration of rainfall into soils is greater when plants protect the soil surface.
Erosion is less when plants reduce runoff and improve surface structure. Plant roots, soil organic matter and microbes especially fungi bind soil particles into stable aggregates below the soil surface.
Loss of soil organic matter is the most common form of soil degradation and can lead to soil structure breakdown and erosion. Soil degradation can occur when farming systems are not suitable to the present ecosystem and attempts are made to obtain high production at the expense of maintaining good soil health.
Many ancient civilizations have failed because of soil loss and land degradation. This is in contrast to the terraced paddy fields in Asia where food is still grown sustainably after 2000 years of farming.
Water erosion
Soil loss and degradation by water erosion occurs on many farms. Erosion decreases plant growth and farm productivity and also the deposition of silt downhill can be damaging covering roads and choking rivers and harbours.
Raindrop splash
The power of raindrops is underestimated, breaking down surface soil structure and causing sheet erosion. Falling raindrops contain a high level of energy. In thunderstorms raindrops are larger, fall faster and have higher energy. Plants help to protect soils from raindrop splash and bare soils are easily eroded.
Sheet erosion
The loss of a uniform layer of soil by raindrop splash and water runoff. It is difficult to notice when sheet erosion has occurred. The build up of soil against a fence is a good sign of the damage caused by sheet erosion.
Rill erosion
Small erosion channels able to be crossed by farm machinery. 5-10 cm deep.
Gully erosion
Erosion gullies are too large to be crossed by farm machinery. Gullies can be large enough to damage roads and buildings.
Sheet, rill and gully erosion often occur together. Sheet and rill erosion are often ignored when ploughs are able to till the soil and crops planted. When large gullies form the land becomes unusable and this has an obvious reduction in farm productivity.
Organic matter and renewable energy
Soil organic matter is solar energy stored in soils, providing energy for the growth of microorganisms and soil animals.
Energy originates from the sun during photosynthesis when plant materials are manufactured from carbon dioxide and water.
Plant material falls onto the ground forming a leaf litter and is incorporated into the soil. Soil organic matter is a renewable resource.
RAINDROP SPLASH ON BARE SOILS
Beating raindrops hit bare soil surfaces forming small craters.
Raindrop splash occurs when droplets carrying small soil particles are flung into the air.
Raindrops break down clods and destroy soil structure forming a surface seal.
Beating raindrops slow down infiltration into soils and runoff increases.
Erosion is more severe when surface clods are broken down into small particles and become easily carried by water. Higher runoff will increase erosion.
On a sloping surface more raindrop splash occurs downhill than uphill and this results in downhill erosion occurring directly from beating raindrops.
Raindrop splash increases sheet erosion.
Runoff is able to carry more soil particles when raindrops stir up surface water.
Organic matter contains bioenergy and is essential for the growth of microorganisms and soil animals. Minerals and vitamins are also present in organic matter.
Soils are considered to be a non renewable resource. When a soil is destroyed it is often considered to be gone forever.
Organic matter is a renewable resource and can be built up again. Often degraded soils can be recovered by building up organic matter and preventing total collapse of soils.
Appendix
Soil carbon sinks
The loss of soil carbon (soil organic matter) leads to soil structure breakdown, land degradation and erosion. Decomposition of organic matter releases carbon dioxide into the atmosphere, increasing greenhouse gases.
Carbon sinks can be built up in soils, reducing land degradation and global warming. The Federal Government in Australia passed laws (June 2011) enabling farmers to create carbon credits from soil carbon sinks and to undergo carbon trading.1
Encouraging the growth of plants in urban soils has many advantages. Flooding is reduced Transpiration Benefits For Urban Catchment Management, and Urban Catchments Enhanced By Green Corridors. Air temperatures are reduced in urban areas by plants Cool Trees and urban creeks Rehabilitation Of Urban Creeks will create cool spaces in the suburbs.
Soil carbon in urban environments is described in the articles Soil Organic Matter and Carbon Cycle in Soils.
Peak Soil is a warning of the dangers of reduced farm productivity and possible food shortages in the future. It is now realized farm productivity, soil carbon and climate change are all linked. Reduction of soil carbon is an important factor leading to soil degradation and also contributes to climate change. Farm management can increase soil carbon and city dwellers can also increase soil carbon in parks and gardens.
Increasing soil carbon should become an aim of all farmers and backyard gardeners.
1http://www.carbonfarmersofaustralia.com.au/CarbonFarmers/index.html