How much stubble do I need to retain

It is important that stubble is managed so that the level is sufficient to realise the benefits. Retaining excess stubble can lead to machinery blockages and reduce crop establishment.

The three main benefits of retaining stubble are to:

  • Improve soil organic matter
  • Prevent soil erosion
  • Retain moisture

Improving soil organic matter

Increased biomass growth will provide more organic matter to return to the soil through stubble retention.

Improving soil organic matter is beneficial to soil and crops. It releases nutrients (nitrogen (N), phosphorus (P) and sulfur (S)) as it decomposes which may be available for plant growth. It also promotes good soil structure by holding soil particles together and improves properties such as water holding capacity, root growth and ease of cultivation.

The most effective way to build stable soil organic matter, nitrogen fertility and soil structure is by incorporating a well-grown pasture phase into the crop. Crop residues are generally carbon (C) rich but relatively nutrient (N, P, S) poor. In a continuous cropping system, where crop residues are the only source of carbon, sufficient nutrients (N, P, S) are needed to maintain or build the more nutrient rich humus.

Retaining stubble does not necessarily increase soil carbon. Instead it may simply increase the amount undecomposed plant material near the soil surface.

However, it is possible to improve soil carbon turnover and soil fertility, while retaining base levels of soil carbon.

Any practice which returns organic matter to the soil will improve soil carbon turnover. This includes retaining stubble and choosing rotations with crops that can be brown or green manured. Intense cultivation will accelerate carbon loss and therefore may limit the soil carbon turnover benefit from green manuring.

Some things to keep in mind when looking to improve soil organic carbon:

  • It takes a long time to either increase or decrease soil organic carbon levels
  • A one-off late strategic burn or cultivation to ensure the following crop is not compromised by a large stubble load will not significantly reduce the soil organic carbon concentration
  • It is expensive to apply enough nutrients to appreciably increase soil organic matter levels

Table 1. Farm practices that increase or decrease soil carbon turnover. Source: Mallee Sustainable Farming Soil carbon in Mallee farming systems.

Practice

Increase soil carbon turnover

Decrease soil carbon turnover

Retain stubble

 

Cultivation

 

Burning (including windrow burning)

 

Grazing

 

Increase crop biomass

 

Clay incorporation

 

Green manuring

 

Managed pastures in rotation

 

Weed-seed collection (e.g. chaff cart)

 

Preventing soil erosion

Stubble plays an important role in preventing soil erosion through:

  • Reduced runoff. Runoff can carry soil in suspension. Reduce runoff to minimise soil erosion.
  • Slowing wind speed. Reduce erosion by slowing the rate at which the wind travels across the soil surface (Figure 1).
  • Keeping the soil in place. Maintaining soil organic matter and soil surface cover can dramatically reduce the erodibility of the soil.

Increasing the level of prostrate groundcover reduces wind erosion

Figure 1. Increasing the level of prostrate groundcover reduces wind erosion. Source: Leys, Butler and McDonough 1994 via Mallee Sustainable Farming Stubble Management: A guide for Mallee farmers.

Retaining moisture

Standing stubble slows the evaporation rate of soil moisture through the creation of a microclimate and by reducing surface wind speed. Retained stubbles have a positive effect on soil structure that allows for better water infiltration, reducing evaporation and minimising runoff and the impact of raindrops on the surface.

However, it is not always possible to reduce the rate of surface evaporation with stubble ground cover, especially if rainfall is low and evaporation demand high. More than 8t/ha of stubble is needed to reduce evaporation and conserve moisture during summer.

In low growing-season rainfall environments such as the Mallee, stored soil moisture is of significant value in a drier year, but of little consequence in wetter years.

Weed management also plays an important role in retaining moisture.

While grazing stubbles is not discouraged, growers need to monitor stubbles closely to ensure they don’t fall below 70 per cent stubble cover on erosion prone land and at least 50 per cent cover in April (Figure 2).

Photo: Maintain at least 70 per cent groundcover on erosion prone land and at least 50 per cent cover in April

Figure 2. Maintain at least 70 per cent groundcover on erosion prone land (left) and at least 50 per cent cover in April (right). Source: Mallee Sustainable Farming Stubble Management: A guide for Mallee farmers.

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