|When plants (trees & shrubs) are cleared from a site, soil is exposed to sunlight and the eroding effects of wind and water. Soil aeration is increased and the rate of weathering increases.
Apart from erosion, the proportion of organic matter in the soil gradually decreases, through the action of microbes in the soil which use it as a source of energy ‑ unless the new land use provides some replacement.
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TYPES OF SOIL DEGRADATION
A number of major soil related problems occur in Australia these include:
- Loss of soil fertility (see lesson on nutrition)
- Soil compaction
- Soil acidification
- Build up of dangerous chemicals
No Dig Garden.
If an area is badly contaminated, your only way of growing plants might be to cover the ground with a black plastic sheet, and create a no dig garden.
Soil erosion, which is the movement of soil particles from one place to another by wind or water, is considered to be a major environmental problem. Erosion has been going on through most of earth's history and has produced river valleys and shaped hills and mountains. Such erosion is generally slow, but the action of man has caused a rapid increase in the rate at which soil is eroded (ie. a rate faster than natural weathering of bedrock can produce new soil). This has resulted in a loss of productive soil from crop and grazing land, as well as layers of infertile soils being deposited on formerly fertile crop lands; the formation of gullies; siltation of lakes and streams; and land slips. Man has the capacity for major destruction of our landscape and soil resources. Hopefully he also has the ability to prevent and overcome these problems.
Causes of Human Erosion
* Poor agricultural practices such as ploughing soil to poor to support cultivated plants or ploughing soil in areas where rainfall is insufficient to support continuous plant growth.
* Exposing soil on slopes.
* Removal of forest vegetation.
* Altering the characteristics of streams, causing bank erosion.
* Causing increased peak water discharges (increased erosion power) due to changes in hydrological regimes, by such means as altering the efficiency of channels (channel straightening); reducing evapotranspiration losses as a consequence of vegetation removal; and by the production of impervious surfaces such as roads and footpaths, preventing infiltration into the soil and causing increased runoff into streams.
With water erosion, soil particles are detached either by splash erosion (caused by raindrops), or by the effect of running water. Several types of water erosion are common in our landscapes. These are:
1. Sheet erosion ‑ where a fairly uniform layer of soil is removed over an entire surface area. This is caused by splash from raindrops, with the loosened soil generally transported in rills and gullies.
2. Rill erosion ‑ this occurs where water runs in very small channels over the soil surface, with the abrading effect of transported soil particles causing deeper incision of the channels into the surface. Losses consist mainly of surface soil.
3. Gully erosion ‑ This occurs when rills flow together to make larger streams. They tend to become deeper with successive flows of water and can become major obstacles to cultivation. Gullies only stabilize when their bottoms become level with their outlets.
4. Bank erosion ‑ this is caused by water cutting into the banks of streams and rivers. It can be very serious at times of large floods and cause major destruction to property.
The force of wind becomes strong enough to cause erosion when it reaches what is known as the 'critical level' and this is the point at which it can impart enough kinetic energy to cause soil particles to move. Particles first start rolling along the surface. Once they have rolled a short distance they often begin to bounce into the air, where wind movement is faster. The effect, of gravity causes these particles to fall back down to the surface where they either bounce again or collide with other particles. This process is known as 'saltation'.
Two other ways of wind borne particle movement occur. The first is 'free flight', which occurs where very small particles are entrained in air, which acts as a fluid, and are carried long distances. The other is called 'surface creep', where soil particles too large to bounce and are rolled downwind.
Control of Erosion
As erosion is caused by the effects of wind and water, then control methods are generally aimed at modifying these effects. Some of the most common control methods are listed below.
- Prevention of soil detachment by the use of cover materials such as plants (ie. trees, mulches, stubbles, crops).
- Crop production techniques (e.g. fertilizing), to promote plant growth and hence surface cover.
- Ploughing to destroy rills and contour planting to create small dams across a field, to retard or impound water flow.
- Filling small gullies with mechanical equipment or conversion into a protected or grassed waterway.
- Terracing of slopes to reduce rates of runoff.
- Prevention of erosion in the first place by careful selection of land use practices.
- Conservation tillage methods.
- Armoring of channels with rocks, tyres, concrete, timber, etc., to prevent bank erosion.
- The use of wind breaks to modify wind action.
- Ploughing into clod sizes too big to be eroded, or ploughing into ridges.
High salt levels in soils reduce the ability of plants to grow or even to survive. This is can be caused by natural processes, but much occurs as a consequence of human action. Salinity has been described as the 'AIDS of the earth' and its influence is spreading throughout society; particularly in rural communities, where crop production has been seriously affected and caused economic hardship. Salinity problems have been grouped into two main types.
Dry land salinity is that caused by the discharge of saline groundwater, where it intersects the surface topography. This often occurs at the base of hills or in depressions within the hills or mountains themselves. The large scale clearing of forests since European settlement has seen increased 'recharge' of aquifers (where groundwater gathers in the ground) due to reduced evapotranspiration back to the atmosphere. The result has been a rise in groundwater levels, causing greater discharges to the surface.
Wetland salinity occurs where irrigation practices have caused a rise in watertables, bringing saline groundwater within reach of plant roots. This is common on lower slopes and plains and is particularly common on riverine plains. The wetland salinity problem is exacerbated by rises in groundwater flow due to dryland salinisation processes higher in the catchment.
Sources of Salt
Salts are a naturally occurring by product of the weathering of bedrock and soil materials. Salts can be accumulated in a number of ways, which may have varying importance from area to area. These include:
1. Cyclical movement
This is salt carried in evaporating ocean water that is later precipitated in rain.
2. Marine incursions
At various times in the geological past, large areas of the land were under sea level. Salt deposits may be remnants of these incursions.
3. In Situ weathering
The natural weathering of bedrock and soil resulting in the movement of salts through a soil profile.
4. Aeolian deposits
Much of the salt found in the eastern part of Australia (for example) is believed to be material picked up and transported by wind from salt pans, playa lakes, etc., in times of arid weather during the past, when saline groundwater evaporated leaving salt deposits.
Control Methods for Salinity
Many of the control methods for salinity are very expensive and require strong commitment by governments if they are to be undertaken. But it also requires regional community co‑operation, as such problems don't respect artificial boundaries. One of the major problems with salinity is that the area in which occurs may be a fair distance from the cause. Thus we have saline groundwater discharging on the plains as a consequence of forest clearing high in adjacent hills ‑ where salinity is not apparent. Many hill farmers are loathe to change their practices for the sake of someone far away, especially if they must suffer some economic loss as a result (eg. the cost of tree planting and the loss of cropping area).
Some of the main control methods are:
- Pumping to lower groundwater levels, with the groundwater being pumped to evaporation basins or drainage systems.
- Careful irrigation practices to prevent or reduce a rise in groundwater levels.
- 'Laser' grading to remove depressions and best utilize water on crop and grazing land.
- Use of saline resistant plant species.
- Revegetation of 'recharge' areas and discharge sites.
- Engineering methods designed to remove saline water from crop land.
- Leaching suitable soils (e.g. bowling greens, raised crop beds, etc.)
This is a problem becoming increasingly more common in cultivated soils. Soil acidification is the increase in the ratio of hydrogen ions in comparison to 'basic' ions within the soil. This ratio is expressed as pH, on a scale of 0 ‑ 14 with 7 being neutral. The pH of a soil can have major effects on plant growth, as various nutrients become unavailable for plant use at different pH levels (see lesson on nutrition). Most plants prefer a slightly acid soil, however an increase in soil acidity to the levels being found in many areas of cultivated land in Australia renders that land unsuitable for many crops or requires extensive amelioration to be undertaken.
Causes of Soil Acidification
Acid soils can be naturally occurring, however, a number of agricultural practices have expanded the areas of such soils. The main causal factor is the growth of plants that use large amounts of basic ions (e.g. legumes); particularly when fertilizers that leave acidic residues (such as Superphosphate) are used. Soil acidity is generally controlled by the addition of lime to the soil, by carefully selection of fertilizer types and sometimes by changing crop types.
Compaction of soils causes a reduction in soil pore space. This reduces the rate at which water can infiltrate and drain through the soil. It also reduces the available space for Oxygen in the plant root zones. For this reason, some of the major consequences of compaction are poor drainage, poor aeration, and hard pan surfaces which cause runoff. Compaction is generally caused by human use of the soil (ie. foot traffic on lawn areas or repeated passage of machinery in crop areas). Repeated cultivation of some soils leads to a breakdown of soil structure and this also increases the likelihood of compaction. Compaction can be prevented by farming practices that minimize cultivation and the passage of machinery. These include conservation tilling, selection of crops that require reduced cultivation, and use of machinery at times less likely to cause compaction (i.e. when soils aren't too wet or when some protective covering vegetation may be present). For heavily compacted soils deep ripping may be necessary.
Although not as large a problem as some of the other types of soil degradation, the presence of chemical residues can be quite a problem on a local scale. These residues derive almost entirely from long term accumulation after repeated use of pesticides, etc., or of use of pesticides or other chemicals with long residual effects. Some problems that result from chemical residues include toxic effects on crop species and contamination of workers, livestock and adjacent streams. Control is often difficult and may involve allowing contaminated areas to lie fallow; leaching affected areas; trying to deactivate or neutralise the chemicals; removing the contaminated soil; or selecting tolerant crops.
IMPROVING DAMAGED SOILS
Before deciding how to, or even whether to improve a soil, you need to know whether a soil is good, bad or whatever.
Drainage can be tested easily by observing the way in which water moves through soil which is placed in a pot and watered. However, when soil is disturbed by digging, its characteristics may change. Another way, to get a more reliable result, is to use an empty Tin Can. With both the top and bottom removed it forms a parallel sided tube which can be pushed into the soil to remove a relatively undisturbed sample. Leave a little room at the top to hold water, add some to see how it drains and then saturate the soil and add some more water to the top. You will often note slower drainage in saturated soil.
Soil nutrition is (to some extent) indicated by the vigor of plants growing in a soil.
Soil structure usually changes from the surface of the soil as you move deeper down into the earth. One reason for this is that surface soil usually contains more organic matter than deeper soil. Surface layers frequently drain better ‑ the drainage rate decreases as you get deeper. This natural change means that water moves quickly away from the surface of soil but slows down it's rate of flow as you get deeper. Bad cultivation procedures can damage this characteristic of a gradation in structure through the soil profile, by destroying the structure at the surface. Such a situation can be very bad!!
By contrast, good cultivation procedures will improve soil structure and increase the depth in the profile to which structured soil extends.
The improvement of soil structure may use two approaches. First, where the soil has not been badly leached ‑ the addition of organic material, use of crop rotations (with Legume cover crops to fix Nitrogen) and proper (not excessive) cultivation. This will normally give the best long term results. However, where soils have been leached and have become very acid, or very alkaline, the use of soil ameliorants such as Lime and Gypsum may be required. These act, not only to adjust soil pH, but replace Sodium ions in the soil with others (principally Calcium and Magnesium). These help 'flocculate' the clay into larger particles and so produce some initial structure that will allow the soil to drain better and be worked as above.
There are several ways to improve soils, and these include:
- Adding sand to clay soils to improve drainage.
- Adding clay or organic material to sandy soil to improve its ability to hold water.
- Adding organic matter to a sand, while improving water holding capacity, will not affect drainage to the same degree as the addition of clay will.
- Adding sand or organic matter will help break up a clay soil, making cultivation easier. Although the two will act in different ways.
- Adding organic matter will usually improve the nutritional status of any soil.
- Use of soil ameliorants ‑ Lime, Gypsum, Sulphates.
- Crop rotations and correct cultivation.
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