Cultivated fallow for moisture storage?

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( Traditional rotation)




Cereal crop sown

Cereal crop sown

Cereal crop sown

Cereal crop sown


Cereal crop grows

Cereal crop grows

Cereal crop grows

Cereal crop grows


Cereal crop matures

Cereal crop matures

Cereal crop matures

Cereal crop matures


Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock


Weeds germinate naturally

Medic regenerates from seed
produced 18 months earlier.
No cultivation of the land required.

Land cultivated and sown to vetch or similar forage legumes.

Land cultivated and sown to grain legume such as lentils or
chick peas.


Weeds grazed. Low stocking rate.

Medic pasture grazed. High stocking rate.

Grazed or more often left for hay.

Grain legumes grow.


Land cultivated for fallow

Medic grazed. Pods produced for future regeneration.

Cut for hay.

Grain legumes mature.


Bare soil vulnerable to erosion.

Pods and stubble grazed.

Stubble grazed.

Grain legumes harvested.

Stubble grazed.


Cereal cycle begins again.

Cereal cycle begins again

Cereal cycle begins again

Cereal cycle begins again




Soil erosion is the main sustainability issue for farming in the cereal zone of
the WANA region. The impact of the four rotations on soil erosion is


The possible benefits of moisture storage still lingers on as an issue with
many farmers. This chapter shows how moisture storage (if it occurs) cannot justify the use of a long cultivated fallow.

Costs and returns are the major determinants of farmers profits. The cost of production for each rotation is examined both for small and large farmers.

Returns relate to the level of output and price. This chapter looks mainly at

For small farmers with few resources and financial reserves risk is
particularly import. A balance needs to be struck between high profits and

Each rotation has an inherent level of weed control. Other weed control
measures can be applied (see later chapters) but the natural ability of the
rotation to "clean" the land or otherwise is an important part of the decision
making process.

The amount of labour and the time it is used are an important aspect of each rotation.

This chapter looks at the capital requirements for each rotation but machinery is treated separately (see below)

Machinery is a special part of the general capital requirements. It is
particularly difficult for small farmers.

We have assumed that the starting point for most farmers is the growing of a cereal crop. We have examined the conflict between the requirement of the cereal crop and the new crop, new forage or pasture being introduced into the rotation.

Small farmers are resource poor. In this chapter we have selected the aspects of the above comparisons that would be appropriate for small farmers.

This chapter provide a framework for selecting a combination of the four
rotations and other variations.

The Zaghouan 4 rotation is not included in the comparison. It is an innovation from Tunisia that cleverly overcomes many of the problems of medic on small farms.


The theory

    The basic theory of storing moisture in the soil under a cultivated fallow was developed at the end of the 19th century.

It was developed as follows:-

    * The land was cultivated in the spring to kill weeds and form a layer of loose soil on the surface.

This was called a dust mulch.

Its purpose was to break the capillary tubes that exist between the soil particles.

It was thought that these tubes lifted moisture to the surface where it evaporated.

It was thought that moisture would be carried over (under the dust mulch) from one winter rainfall period through the dry summer to the next autumn where it would provide additional moisture to the cereal crop.

It is important here to distinguish this Mediterranean cereal fallow from fallows carried out in other regions.

Most other fallows are more limited.

There is the winter to summer fallow (for example cultivation used under vines or olive trees) used extensively in the Mediterranean.

There is the summer to winter fallow (used in some semi- tropical regions) used to grow crops during a period when evaporation is much lower.

    * Further scientific research showed that the capillary tubes (formed between soil particles) lifted insignificant amounts of moisture to the surface.

The retention of moisture depended on killing the vegetation not the dust mulch.

Plants removed most of the soil moisture.

They are much more efficient "pumps" than capillary tubes in the soil.

Unfortunately the dust mulch concept once established proved difficult to reverse and quickly produced the "dust bowl" in the USA and parts of Australia.

    * With the development of herbicides it is possible to produce an effective fallow without cultivating the soil at all.

The practice

     In the WANA region the cereal-fallow system became hugely popular after the middle of the 20th century.

The transfer of soil moisture from one winter over the hot summer to the next autumn was put forward as a major advantage of fallow.

    The fallow carried out on real farms in the WANA region falls far short of the theory.

    * Fallowing is often carried out too late.

Farmers are short of feed for their livestock and are reluctant to plough the land in winter.

Instead they leave the cultivation for the fallow until spring when the soil profile is already beginning to dry out.

If the soil is not wet to field capacity at the time of cultivation then any carry-over of moisture will be limited.

    * Fallowing is carried out with deep ploughs.

These open up a large volume of soil.

The land is left open and allowed to dry as wind enters the soil.

The cultivated and open 20 to 25 cm of soil becomes dry. Any moisture that may be stored below that level is not readily accessible to germinating cereal seedlings in the following autumn.

    * Summer weeds can grow on fallow.

These weeds tap into the underground moisture.

Summer weeds are usually unpalatable to livestock and are difficult to control without further cultivation or the use of herbicides.

This means additional cost and soil erosion.

If the summer weeds are not controlled they will not only use all the stored moisture (described as X below) but will actually reduce the moisture for the next cereal crop.

    How can summer weeds remove more than the available moisture? 

Moisture is held in the soil under tension.

This means that the more moisture that is removed from the soil the greater the tension of the remainder and the more difficult it is to remove.

When the cereal crop or pasture dries off in the spring there is still moisture in the soil but it is held there with a high tension and is not accessible to these plants.

If summer weeds establish themselves (and this is more common on a fallow) they will remove more of the moisture. Summer weeds have the ability to take even more moisture held at a higher tension.

When the rains come in autumn the first requirement is to raise the soil moisture tension level to the value where the cereal crop can access any of it.

    Let us say that cereals can access soil moisture at level X in the soil.

    Cereals and pasture dry off in spring when level X is reached. The soil is now "dry" but in reality still holds some moisture at higher tensions.

    Level X stays in soil over summer - soil apparently dry.

    In autumn rain over X is available to cereal crop.

    If we put summer weeds into equation.

    Summer moisture level drops to X - Y where Y is the additional pulling power of summer weeds compared to winter growing plants.

    In autumn first the Y amount of moisture deficit has to be filled. When that has happened moisture is available to the cereal seedlings.

    In summary a poor fallow with summer weeds will not carry over any stored moisture but will in reality reduce the amount to an even lower level than that without fallow.

Does it work?

    Experiments on the effectiveness of fallow in the storage of moisture are difficult to conduct as fallow has three major advantages. These are not easy to separate.

    * Fallow controls weeds and weed seeds.

Fallow is a most effective means of controlling weeds in the spring when they are about to produce seed.

The reserves of weed seeds in the soil are reduced and there are fewer weeds in the following autumn. Over many cycles of cereal-fallow the seed population is greatly reduced.

    * Fallow begins the process of breakdown of organic matter in the soil.

This mobilises the soil nitrogen (if the soil is fertile) and reduces the effect of some soil borne fungus diseases that affect cereals.

    * Moisture storage

Finally there is the controversial question of moisture transfer from one winter to the following autumn.

Table 1

    This show the mean yield of wheat in some long term rotational trials carried out at the Waite Institute in Adelaide, South Australia.

The relevance of these trials to the WANA region is that Adelaide has a Mediterranean climate very similar to Tunis in Tunisia.

This means it represents the better rainfall part of the cereal zone ( above 500 mm). This may account for the length of time for some exploitative rotations such as fallow to exhaust the organic matter in the soil.

Mean yield of wheat




Period 1926 to 1951

Period 1952 to 1983

Continuous wheat






Wheat - grain legume (peas)



Wheat - legume pasture - legume pasture (3 years)

Not included


Wheat - legume pasture - legume pasture - fallow
(Similar to the Zaghouan 4 rotation) 

Not included


    It can be seen from this  experiment that:

    + Fallow rotations produced the two highest yields.

The wheat-fallow rotation during the first 25 years and the wheat - legume pasture - legume pasture - fallow rotation.(Zaghouan 4)

    + Fallow rotations can also crash

The fact that the wheat-fallow yield fell so sharply during the second 25 years and the fact that fallow after legume pasture produced the highest yield seem to indicate that the major effect of the fallow was to mobilise soil nitrogen rather than store moisture.

    Modern equipment has made it possible to measure the moisture effect separately from yield data.

Dahmane in Tunisia has carried out an extensive series of experiments and shown that fallowing in the spring can store soil moisture for three months or so.

It is therefore most effective for vines and olives in areas of low rainfall.

He found no significant additional moisture was carried over to the following autumn.

It must be remembered that these experiments were conducted with a "good" fallow (fallow carried out early and soil worked down - not left open) not the common fallow found on most farms in the WANA region.

 Other experiments conducted in Australia and in the WANA region have shown (contrary to Dahmane's results) that there can be a slight carryover of moisture from one winter to the following autumn.

This is more likely to occur in high rainfall areas where the soil profile in winter is completely wet.

More soil moisture is stored in heavier soil types.

The benefit, on the other hand, for the cereal crop is greatest on light soils and in low rainfall areas.

The model makers

    Another approach carried out by ICARDA looked at the possible benefits from the point of view of seasonal combinations.

Assuming that a transfer of moisture from one winter to the next autumn can take place it would only be effective in a few combined seasons.

First winter.

Second autumn - winter

Potential value of stored soil moisture

Good winter rain. Saturated soil profile.

Good rainfall.

Fallow moisture not needed.

Good winter rain. Saturated soil profile.

Very dry season - drought.

Even carry over moisture not enough to save crop.

Good winter rain. Saturated soil profile.

Below average.

Fallow moisture can have some value.

Poor winter rains. Soil profile not completely wet.

Carry over not possible.

No value and no moisture available from first season.

    The ICARDA model was run over 20 years of rainfall records and it was concluded that this combination of seasons (wet winter followed by medium dry winter)  does not occur frequently enough to use a moisture storing fallow for increasing cereal yields.

The long term impact

    The short term effect of the fallow is excellent:

    * It controls weeds.

    * Reduces soil borne disease.

    * Mobilises soil nitrogen and altogether increases cereal yields.

    That is why it was introduced in the 1950s in the WANA region.

    Over the longer term:-

    * The fallow destroys the natural pasture that regenerates after the cereal crop.

The natural pasture becomes a sparse cover of unpalatable weeds.

There is no restoration of soil fertility through regenerating legumes.

There is no more soil fertility to mobilise.

The length of time needed to totally mobilise the soil fertility varies.

In the above experiment conducted in Adelaide the cereal-fallow rotation performed well (in strictly yield terms) for 25 years before collapsing to one of the lowest yielding rotations.

The rate of exhaustion is a factor of the soil type and rainfall. Light textured soils in low rainfall areas will collapse faster.

In the WANA region the fallow has been used for more than 25 years. In has effectively collapsed.

    * The bare soil is vulnerable to erosion by wind and water.

Erosion will reduce soil fertility and reduce yields.

It is difficult to measure because it also depends on local conditions of slope.

    * Fallow destroys soil structure

The destruction of the soil structure means water runs off rather than being absorbed. Less is available to the crop.

The economic impact

    The long term economic impact of fallow has been a total disaster.

Fallowing has destroyed fodder production in the cereal zone of the WANA region.

In the 1950s when fallowing became more widespread, cereal prices were high relative to livestock but over the last 50 years returns from cereals have declined and returns from livestock have increased.

The cost of destroying the pasture is no longer acceptable.

The question now is whether there is a need to grow cereals at all or whether it is more profitable to grow only pasture.

 The feed has been destroyed for no purpose.

Once the fertility was mined from the soil the yields after fallow declined. They have only been stabilised in the region by the improvement of cereal varieties and the addition of nitrogen fertiliser.

 Using the land for two years to produce a single cereal crop is a most expensive means of producing grain.

The return of the fallow

    The Zaghouan 4 rotation brings back most of the advantages of fallow and few of the costs.

The soil fertility is recharged under a medic pasture for 2.5 years.

The land is fallowed.

Weeds are controlled.

Soil nitrogen is mobilised.

Soil diseases are reduced.

A cereal crop is grown and then the land returned to medic using pods.

It is a system suitable for small farmers in particular as the mechanisation needs are also reduced.

The cost of the fallow season is much less than the fallow in the cereal-fallow rotation.

Admittedly the spring output of the medic pasture is destroyed in the fallow season but there is still good autumn and winter production of medic.

With the cereal-fallow rotation the accumulative effect is to reduce the seed reserves to such a low level that virtually nothing regenerates after the cereal.