Stubble management options include burning, physically removing stubble, or retaining it in the paddock. Each of these systems has advantages and disadvantages. Although physical removal of stubble (eg for animal feed) is sometimes an option, this article will focus on stubble burning and stubble retention.
Advantages of Burning
- Quick and easy
- Improved weed, insect and disease control.
Disadvantages of Burning
- Loss of nutrients
- Loss of carbon
- Loss of soil microbes and critters
- Reduction in soil structure (soil aggregate stability)
- Increase in erosion (wind and water)
- Can increase acidity over time.
Unburnt soils contain nearly double the amounts of carbon and nitrogen and much more P than burnt soils. Unburnt soils have double the microbial biomass, and CO2
respiration is 3 times higher in unburnt soils (Table 1).
Table 1: Stubble nutrients and amounts lost from a hot burn
(for a wheat crop - yielding 5 t/Ha, produces 7.5 t stubble per Ha)
|Amount nutrients in stubble (kg/Ha)|
|Amount lost during a hot burn (kg/Ha)|
|Percentage lost (%)|
|Amount fertiliser to replace lost nutrients (kg/Ha)|
Muriate of potash
Taken from Australian Farm Journal December 2003
Surface Retention of Stubble
Surface retention of stubble is used to protect soil from loss due to wind and water erosion, and eventually the stubble decomposes adding organic matter to the soil.
Surface stubble retention works best with low stubble loads (2-3 t/Ha); in these cases little stubble remains at the start of the next season to cause problems such as nitrogen tie-up, increased pest pressures (eg slugs and mice) or difficulties with ground preparation. Surface retention of stubble usually works best in dryland cropping systems in low rainfall areas.
As stubble loads increase, pest pressure also increases (eg snails, slugs and mice). Direct drilling with straw present carries the greatest risk of crop failure (mainly due to poor establishment, or attack by slugs or snails). For example surface-retained wheat stubble can reduce growth and yield of canola by 25%. It is not known exactly what the cause of the reduction in vigour is, but it is suspected to be due to any (or all) of the following:
- Nitrogen is tied-up by microbes as they decompose the stubble and is not available for plants
- Toxic by-products of decomposition affect seedlings
- Shading of seedlings by the layer of stubble.
However, changes to sowing methods have been found to lessen the growth rate reductions in canola (for example, using wider rows, narrow points and press wheels, which move stubble 10-20 mm off the seedling row and into the inter row). A newly developed planting system, known as "stubble star" has resulted in a 30% increase in emergence compared with a conventional sowing system.
Converting to stubble retention can be difficult initially, and it may take a few years before things improve.
Cultivation leads to an increase in the decomposition rate of organic matter. Cultivation can cause a slight degradation of soil structure compared to soils with surface retained stubble, but soil structure is often substantially better than in traditionally tilled soils where stubble has been burnt.
In situations where high surface stubble loads can cause problems such as affecting crop emergence or increased pest pressure, stubble incorporation may be a viable option. However, increasing the tillage of soils results in reduced nitrogen and organic matter.
Ploughing stubble in can increase the C:N ratio meaning there is less nitrogen available for the crop as micro-organisms use it to break down the crop residues. However, this hasn't been found to effect yields or sap N at crop maturity. Applying N to systems with incorporated stubble doesn't effect the organic carbon levels but does increase microbial activity. Extra urea may be required to offset reduced nitrogen uptake efficiency, where there are low levels of soil nitrogen.
Improved soil structure resulting from stubble retention and reduced tillage has been implicated in increased nitrate leaching, however the nitrates may not always be leached beyond the root zone of the crop. Tillage before autumn is thought to increase nitrate leaching, by enhancing mineralisation at a time when plant nitrogen demand is low.
Maintaining Soil Carbon
How much stubble needs to be retained to maintain soil carbon levels?
This depends on:
- Soil type
- Original soil C levels.
Soil carbon will only change very slowly; most of the carbon in crop residues is used by micro-organisms breaking down the stubble (approx 70% is transpired as CO2
). So only 30% of the carbon from the stubble will remain in the soil, and even this may break down with time. Maintaining and/or increasing soil carbon requires an ongoing, long-term program of stubble retention. Retaining the organic carbon you already have in your soils is very important, and is easier than trying to replace organic carbon that has been lost through poor management practices.
The increase in soil fungi and bacteria, as well as the increase in organic matter that result from stubble retention and the decomposition of organic matter, also helps to maintain and improves soil structure.
To Fallow or Not to Fallow?
During a fallow period when no plant roots are contributing to the soil organic matter pool, microbial activity is still metabolising soil organic matter into CO2. Consequently, it is the fallow period associated with annual cropping that is the main cause of long term reductions in soil organic matter.
What to Do?
Each stubble management method needs to be examined in relation to the cropping system in question to determine the best approach. Issues to consider include:
- How the soil is affected
- How convenient/efficient the system is
- How it affects yield and/or plant establishment
- How much does it cost
- What environmental impacts does it have
- Does it introduce or exacerbate other problems (eg weeds or slugs)?
While zero tillage is not really an option for paddocks with heavy stubble loads, soil structure can still be improved by combining minimum tillage with stubble retention, compared with using traditional tillage (many passes) and burning stubble.