ROTATIONS  COMPARED 

SUSTAINABILITY

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Erosion the main issue

    Sustainability includes many topic but as far as the cereal zone of the WANA region is concerned soil erosion by wind and water is the most serious threat to sustainable farming.
 

    The major causes of soil erosion in the cereal zone of the WANA region are:

    *  Excessive cultivation. Every time the soil is cultivated the organic matter is depleted and the soil structure declines.

    * Lack of plant cover and return of organic matter to the soil.

    Overgrazing is considered by some to be a major factor in the lack of plant cover but livestock numbers in the cereals zone of WANA are not high by the standards of similar zones in Australia. Rather than considering "over-grazing" as a problem one should consider "under-pasturing" as the major cause.
 

Chart 1

The chart below shows the effects of each rotation on the soil structure. It is a simplified version of a number of indicators of soil structure (organic carbon, water stable aggregates, etc.) conducted in many experiments

CULTIVATION AND SOWING reduces the soil organic matter and soil structure. This is indicated by - - - -

ORGANIC MATTER is added to the soil when pasture of vetch is grazed or when stubbles are return to the soil. This is indicated by + + + +
 
 

	Fallow -cereal rotation	Vetch or grain legume -cereal	Classic medic - cereal rotation	Zaghouan 4 rotation
AUTUMN First season	Cultivation and sowing of cereals. - - - -	Cultivation and sowing of cereals. - - - -	Cultivation and sowing of cereals. - - - -	Cultivation and sowing of cereals. - - - -
WINTER	Crop growing +	Crop growing +	Crop growing +	Crop growing +
SPRING	Crop growing +	Crop growing +	Crop growing +	Crop growing +
SUMMER	Stubble +	Stubble +	Stubble +	Stubble +
AUTUMN Second season	Sparse weeds. Little organic matter   +	Cultivation and sowing of vetch or grain legume - - - -	Medic pasture. Abundant organic matter       ++++	Medic pasture. Abundant organic matter       ++++
WINTER	Sparse weeds. Little organic matter   +	Crop growing +	Medic pasture. Abundant organic matter       ++++	Medic pasture. Abundant organic matter       ++++
SPRING	Cultivated fallow - - - -	Crop growing or vetch cut for hay +	Medic pasture. Abundant organic matter       ++++	Medic pasture. Abundant organic matter       ++++
SUMMER	Cultivated fallow - - - -	Stubble +	Dry Medic +	Dry medic +
AUTUMN Third season	Cultivation and sowing of cereals. - - - -	Cultivation and sowing of cereals. - - - -	Cultivation and sowing of cereals. - - - -	Medic pasture. Abundant organic matter       ++++
WINTER	Crop growing +	Crop growing +	Crop growing +	Medic pasture. Abundant organic matter       ++++
SPRING	Crop growing +	Crop growing +	Crop growing +	Medic pasture. Abundant organic matter       ++++
SUMMER	Stubble +	Stubble +	Stubble +	Dry medic +
AUTUMN Fourth season	Sparse weeds. Little organic matter   +	Cultivation and sowing of vetch or grain legume - - - -	Medic pasture. Abundant organic matter       ++++	Medic pasture. Abundant organic matter       ++++
WINTER	Sparse weeds. Little organic matter   +	Crop growing +	Medic pasture. Abundant organic matter       ++++	Medic pasture. Abundant organic matter       ++++
SPRING	Cultivated fallow - - - -	Crop growing +	Medic pasture. Abundant organic matter       ++++	Cultivated fallow - - - -
SUMMER	Cultivated fallow - - - -	Stubble +	Dry medic +	Cultivated fallow - - - -

Cereal - fallow rotation.

      *  Soil erosion

        With the cereal-fallow rotation the land is sown to cereals in the autumn and the crop is harvested in late spring or early summer.

In the following autumn the land is left to regenerate weeds. There is only a sparse cover of the ground during the second autumn and winter. The land is vulnerable to erosion by water whenever there is intense rain.

One should also remember that runoff is a loss of water. Rarely is the soil in the cereal zone saturated to the level of field capacity. Rain that soaks in rather than runs off is useful for crops or pasture.

In the spring the land is cultivated for the fallow. The cultivated soil is even more vulnerable to erosion by water and during the long dry summer by wind.

      *  Soil structure

     The structure of the soil is extremely poor when the land is fallowed. 

The cultivation of the soil for the cereal crop destroys organic matter in the soil and the structure declines. This happens with all rotations when cereals are grown.

Almost all the cereal stubble it eaten during the summer by livestock. Little organic matter is returned to the soil either directly or through animal droppings.

In autumn weeds regenerate in the cereal stubble but they are sparse. Little organic matter is returned to the soil.

The land is fallowed in the spring and the weeds are killed so there is no return of organic matter from from a spring flush of growth.

 The effect of the poor soil structure is to exaggerate the soil erosion described above. Water absorption is poor as the poorly structured soil easily caps over.

On light textured soils the poor structure leaves the soil more vulnerable to erosion by wind.

Cereal - vetch rotation

    *  Soil erosion

With the cereal-vetch rotation the land is vulnerable to erosion during cultivation and seeding for the cereal crop in the first autumn as described in the paragraphs above.

In the following autumn the land is cultivated and sown to vetch or forage peas.

The land is vulnerable to erosion during the period of land preparation and sowing until the vetch has formed a dense cover in early winter. 

The vetch continues to protect the soil in winter and spring. Usually the vetch is cut for hay in late spring.

The stubble, provided it is not too heavily grazed, provides protection to the soil in summer.

    * Soil structure

    Soil structure is a little better than the cereal - fallow rotation particularly if the vetch is grazed rather than cut for hay but not as effective in improving soil structure as the cereal- medic rotation.

The vetch adds more organic matter to the soil than the sparse cover of weeds that regenerates naturally after the cereal crop.

The vetch produces an abundant spring flush of growth but this is usually removed with the hay.

Compared to medic, organic matter is lost when the land is cultivated and sown for the vetch. If the vetch is cut for hay organic matter is removed and the manure is rarely returned.

 Cereal - grain legume rotation

    * Soil erosion

    With the cereal - grain legume rotation (chick peas or lentils) the land is vulnerable to erosion during the cereal phase of the rotation as above.

In the second autumn the land is cultivated and sown to grain legumes.

Weed control and seed bed preparation needs to be better than for vetch. More cultivations are carried out leaving the land vulnerable to erosion for a longer period. A level seedbed is need for mechanical harvesting.

    The grain legumes do not form such a dense cover of the land as vetch or medic until late in the winter or spring.

    Harvesting destroys the protective capacity of the stubble. The land is vulnerable to erosion during the summer.

    * Soil structure

    The return of organic matter is a little better than the cereal-fallow rotation but not as good as either vetch or medic.

The increased cultivation of the soil to prepare a good seedbed will destroy more organic matter.

The level land is vulnerable to wind erosion in summer after harvest.
 

  Cereal - medic rotation

    *  Soil erosion

With the cereal - medic rotation the land is sown to cereals in the autumn and harvested in late spring as above. 

In the following autumn (once the medic system has been established) the medic regenerates to a dense pasture without any further cultivation of the soil.

The medic provides excellent protection to the soil from intense rainfall during autumn, winter and spring.

In the spring the land is not cultivated for a fallow and the pasture continues to protect the soil through spring.

The dry pasture stubble provides protection through the summer until it is cultivated in the following autumn for another cereal crop.

    * Soil structure

    The structure of the soil is good.

There is a similar depletion of the organic matter during the cereal phase of the rotation.

The medic pasture provides abundant feed for livestock and organic matter is returned to the soil through animal manure during winter, spring and summer.

Some dry medic pasture residues are incorporated into the soil in the following autumn.

There is less pressure on the cereal stubbles as a source of feed for livestock and some of the stubble can be incorporated into the soil.

    The greatly improved soil structure means more rain is absorbed. Plant growth is increased. Runoff is less and erosion is reduced.

    Zaghouan 4 Rotation

    * Soil erosion

    The Zaghouan rotation includes a fallow every fourth year.

Obviously the erosion potential is half that of the cereal-fallow rotation. The cereal -fallow rotation has a fallow every second year.

In fact the erosion potential is much less than half because of the excellent soil structure - see below.

    * Soil structure.

    In the Zaghouan rotation there are two full seasons of medic and a further autumn - winter season. There is a considerable build up of soil organic matter. The soil structure is considerably improved.
 

Using the chart in the cereal zone.

    Choosing a suitable rotation requires a balance between the slope, the soil texture and the the rainfall.
 

Soil texture and slope	Cereal zone 500 mm to 350 mm	Cereal zone below 350 mm
Light textured sandy soil. Slope or plain.	Avoid all fallows as sandy soil is vulnerable to wind erosion during summer. Vetch and grain legumes can be grown in rotation with cereals but there should be some pasture breaks to restore soil structure.	All fallow should be avoided as wind erosion in summer is serious. Also avoid grain legumes as their stubble provides little protection. Avoid Zaghouan 4. because of the fallow year. Medic - cereal or better still use a longer rotation of medic - medic - cereal.
Loamy soil plain.	Wider range of rotations but some medic is important to restore soil structure.	Medic-cereal rotation provides better protection.
Loamy soil slope.	Water erosion now becomes an issue and soil structure most important. Medic-cereal or Zaghouan 4 provides the best soil structure. Grain legumes can be added on to Zaghouan 4. (See note below)	Medic-cereal rotation or an even longer medic phase on steep slopes such as medic - medic - cereal.
Heavier clay soils plain.	Ideal for a wide range of intensive rotations but soil structure cannot be ignored.	Wide range of rotations theoretically possible. Risk of crop failure due to low rainfall will influence the decision more than the erosion issue.
Heavier clay soils slope.	Water erosion can be serious on poorly structured soils and intensive rotations. Use medic rotations.	Even though rainfall averages are lower erosion can be serious because of individual rainfall events. Use medic rotations to protect soil.

    Note:  "Grain legume add on."  Grain legumes can be grown after cereals in the Zaghouan 4 rotation which becomes a Zaghouan 5. The medic pods are broadcast over the grain legume crop to restore the medic pasture.
 

Effects of  soil erosion.

On farms

    The obvious effect of water erosion is the formation of deep gullies and wind erosion the formation of sand drifts. Both these glaring symptoms occur frequently in the WANA region and national governments generally take remedial action. They are the end of a long process that is causes much less obvious damage.

    Wind erosion and water erosion remove the surface layers of soil which contain a high proportion of the soil nutrients.

Soil fertility is reduced. Crop yields are lower long before the gullies and drifts appear.

Taking the process back a further stage run-off occurs because of poor soil structure. The poorly structured soil caps over. The rain runs off rather than soaking into the soil.

The rain is lost to the crop. Yields are lower because crops have less soil moisture.

The solution to run-off problems is not physical barriers to lead the water down the slope without creating gullies. These physical solutions may be an added safety measure for very severe rainfall events but the best solution is to improve soil structure. Better soil structure will lead to better rainfall absorption and less run-off.

On the environment

    One of the most striking environmental effects of wind erosion is dust. This pollutes the WANA environment at two levels.

One is the local level in and around towns and villages where the intense grazing of livestock on purchased feed creates a dust bowl as sheep are still taken out to graze on local roadsides and waste ground.

Local control of livestock is essential to prevent this pollution.

If good pastures were available there would not be an incentive to feed animals on purchased feed.

The effectiveness of this concept was demonstrated at the Australian town of Broken Hill. Broken Hills in in the state of New South Wales and is a mining centre located in the arid zone. When an area of 15 km around the town was closed to livestock the quality of the air and freedom from dust improved dramatically over a decade.

On a wider scale the cultivated fallow creates the base for larger dust storms.

Again the comparison with South Australia is interesting. Severe district or complete cereal zone dust storms were a regular occurrence during the 1930s when fallowing reached its peak.

As fallow was replaced by medic pasture the incidence of dust storms declined and now they occur every few years rather than a number of times each year.

Soil carbon

A level of 2% or higher of carbon is the level which prevents degradation in most soils. High levels of carbon (as organic matter) increase the nutrients available to plants, improve soil resilience and water holding capacity. The level of soil carbon explains about 60% of the variation in soil structure.

Putting production figures onto levels of soil carbon has proved extremely difficult because carbon levels are determined over a long period. Experiments need to be 10 to 20 years in length to reflect the long term changes in soil carbon within various rotation. The long term rotational trials from the Waite Institute in Adelaide that are refered to in more detail in Zaghouan did not measure soil carbon but there is little doubt that it was a contributing factor in yield variation within a single rotation over the long term.

 Cultivation methods.

    While the cultivation methods are not strictly related to the rotations they are part of the package surrounding the new rotations.

    * Cereal - fallow.

    This is the common rotation in the WANA region and is practised with deep ploughing. The deep ploughing cultivates more soil. More soil is depleted of organic matter. Deep ploughing is often carried out with discs or mouldboards. These will bury or mix surface trash. Surface trash is more useful in protecting the soil against erosion if it is left on the surface.

    * Other rotations

    The cereal-medic requires shallow cultivation with tined implements. Deep ploughing buries the pods to a depth where they are unable to regenerate. Shallow cultivation with tined implements cultivates less soil and leave the trash on the surface.

    The cereal - vetch and cereal - grain legume rotations will use shallow cultivation because it is fast.

 Herbicides.

       Herbicides can be used to replace all or some of the cultivations of the land.

Less organic matter is destroyed and the structure does not decline as much. The soil is less vulnerable to erosion. 

In economic and management terms herbicides can only be used with shallow cultivation.

A shallow cultivator with tines can prepare a seed bed in a single operation or a seeder with suitable tines can sown the ground direct. Herbicides can replace the cultivation carried out to control weeds. Part of the cost of the herbicide is offset by the savings in cultivation.

With deep ploughing direct seeding is not possible. Many cultivations are required to break down clods and level the seed bed.

These cultivations (or most of them) are needed even when herbicides are applied. The cost is high. Seeding is delayed and the returns are lower.
 

See Training Kit No 1.6 for photos of erosion and soil structure.

Carbon sinks

There is a growing market for Carbon sinks to offset polluting greenhouse gases (mainly carbon dioxide). Forests in Australia are being planting with subsidies from Japanese power companies.

At present these carbon sinks are limited to forestry projects but in future it may be possible for farmers to receive a subsidy for soil carbon. Establishing a permanent medic pasture will greatly increase the soil organic carbon.

Energy use on the farm

Currently 37% of the energy used in agriculture in Britain goes directly into the production of nitrogen fertilisers. The rise in oil prices over the last few years has been reflected in higher prices for nitrogen fertilisers.

Nitrogen fertilisers require a large amount of energy - current from fossil fuels - and they also release nitrous oxide in their manufacture and use. Nitrous oxide is a 100 times more polluting as a greenhouse gas than carbon dioxide. While the quatities of nitrous oxide are small in comparison to carbon dioxide the effect is significant.

I have not been able to find the figures for energy used in fertilisers in WANA but I am sure nitrogen fertiliser would account for a similar high proportion of the energy put into agriculture.

The effectiveness of this fertiliser is low because of the variable rainfall in spring.

Nitrogen from legumes is a most cost effective alternative. The nitrogen from medic pasture in the medic-cereal rotation or the Zaghouan 4 rotation is the most effective as it is returned to the soil not removed in the form of grain or hay.

Large amounts of energy are used to fuel tractors for cultivation and seeding. A switch from deep ploughing to shallow cultivation will make considerable savings.

Methane

Ruminant animals such as cows and sheep produce considerable quantities of methane as part of their digestive process. Methane is a polluting green house gas.

Increasing the number of sheep through the replacement of the fallow with medic pasture will increase the output of methane.

The advantage of the medic pasture is that it allows the farmer to have a more efficient flock.

The lambing percentage is higher.

The death rate is lower.

The growth rate for lambs is higher.

The production of methane may be greater with more livestock but the production per unit of output (mostly meat but also milk) is lower.

See also Profit from livestock

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