Historically, stubble has been burnt because it improves weed control and creates easier passage for seeding equipment. However, the practice of burning stubble has declined throughout Queensland cropping regions due to concerns about soil erosion and loss of soil organic matter. Instead of being burnt, stubble is now more commonly retained, which has several advantages for soil fertility and productivity (figure 1).
Figure 1:Cereal paddock where stubble has been retained, reducing erosion risk and improving fertility.
One of the main benefits of stubble retention is reduced soil erosion (figure 2). Retaining stubble decreases erosion by reducing the raindrop energy at the soil surface and decreasing run-off. In order to protect the soil from erosion, crops need to be managed so that at least 30 – 40 % ground cover is maintained throughout the year, but especially during the summer months when there is a greater chance of high-intensity rainfall. The amount of cover produced by crops will vary according to seasonal conditions and crop variety. However, as a general rule of thumb, a 1.5 t/ha grain yield should typically provide 90 % stubble cover. This cover may decrease over the fallow period, however, depending on whether the site is subsequently burnt, grazed or cultivated.
Figure 2: Soil loss observed depending on the percentage of surface cover from sites on the eastern Darling Downs (Freebairn, 2004).
Another advantage of retaining stubble is that it increases soil water content by decreasing run-off, and increasing infiltration (figure 3). The greater the amount of stubble cover, the greater the potential benefits to soil water storage. Stubble coverage of 30 % is considered the minimum level required for reducing the effect of soil water runoff. However, stubble coverage of 50 % or greater will further benefit for soil moisture storage and erosion control.
Figure 3: The influence of different amounts of ground cover from retaining wheat stubble on time to runoff (blue) and water infiltration (orange) (Thomas et al., 2008).
Retaining stubble increases the input of carbon to soil. Stubble is approximately 45 % carbon by weight and therefore represents a significant carbon source. Microorganisms in soil require organic carbon to obtain the energy they need to live. When stubble is retained, the greater inputs of organic carbon increase the number and activity of microorganisms in soil (see Organic carbon pools – Queensland and Microbial biomass – Queensland factsheets).
Stubble burning, grazing and cultivation are the main management practices with the potential to reduce stubble cover. A single tillage operation using a chisel plough, for example, can reduce stubble coverage by 30 – 40 % (table 1).
It is recommended that stubble cover be maintained as long as possible in the fallow, and that planting and fertilising machinery be adapted to minimise disturbance. Where cultivation is required in order to control herbicide resistant weeds, this should be carried out as a one off operation.
Table 1: Estimated reduction in wheat or barley stubble cover from different tillage operations (reproduced from Measuring stubble cover: Photo standards for winter cereals).
RESIDUE BURIED BY EACH TILLAGE OPERATION (%)
Old (brittle) stubble
60 – 80
80 – 90
30 – 40
40 – 60
20 – 30
30 – 50
High levels of stubble may increase soil moisture storage, increasing the yield potential (and nitrogen requirement) of the crop. However, stubble may also decrease the level of available soil nitrogen, which can reduce yield potentials. Where stubble retention is in place, it is important that nitrogen fertiliser and/or legume rotations be used to ensure adequate nitrogen is available for crop growth (see Nitrogen factsheet).
Carey B and Grodecki A (2010) Land Manager’s Monitoring Guide: Ground cover indicator, Department of Environment and Resource Management, Queensland Government. (online)
Freebairn D (2004) ‘Some observations on the role of soil conservation structures and conservation’, Journal of the
Australian Association of Natural Resource Management 7(1): 8-13.
Measuring and managing stubble cover: Photostandards for cereals Department of Employment, Economic Development and Innovation, Queensland Government.
Thomas GA, Orange DN and King AJ (2008) Effects of crop and pasture rotations and surface cover on rainfall infiltration
on a Kandosol in south-west Queensland. Australian Journal of Soil Research 46: 203-209.
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.