Benefits of green and brown manuring
This activity is best suited to cropping soils with loamy to clay surface textures. Clay soils can sequester more carbon as soil organic carbon (SOC). Green or brown manuring solely for carbon credits is not viable because there isn’t an approved methodology for generating carbon credits from this activity, but green or brown manuring could be one component of a farming system.
- improved soil fertility (largely observed in leguminous green manures) achieved by building SOC and nutrient status, and increasing buffering capacity to moderate changes in pH
- reduced weed burdens, particularly when herbicides are not an option or effective, or a break is required
- improved soil structure and providing a protective cover for the soil surface: this increases water infiltration and retention, reduces wind and water erosion risk, and reduces the impact of extreme temperatures.
Risks with green and brown manuring for carbon storage
- There is limited data that clearly quantifies the change in SOC linked solely to manuring practices. Good data is needed to calculate the potential carbon sequestration value.
- More-specialised machinery, such as stubble rollers and mulchers, may need to be purchased.
- There will be a revenue loss from not cropping the paddock in the year of manuring: this may be offset in future years if improved soil quality increases the performance of subsequent crops.
- Green manuring can result in increased methane production through anaerobic decay.
- Green manuring over summer (an opportunistic activity) will reduce soil moisture for the following crop: this loss may be offset by the increased SOC and nutrient level.
- These practices will likely to be part of rotational cropping management for the paddock, making it difficult to isolate the component(s) of the farming system affecting SOC change.
Green manuring incorporates green plant residue into soil with cultivation, commonly with an offset-disc plough. Green manuring aims to kill weeds and control seedset while building soil organic matter and nitrogen status. More than one tillage pass may be required for a successful kill, and cultivation may lead to losses of soil organic matter and cause soil structure damage.
In Western Australia, pulses are preferred over cereal crops for green manuring because they add more nitrogen to the soil. Green manuring pulse crops carry the risk of increased nitrous oxide (N20) emissions, which may discount increases in soil carbon sequestration. In some locations it may be possible to grow a summer crop – for example, broad-leafed plants such as sunflower and safflower, or grasses such as sorghum and millet – for green manuring, especially on sandplain soils and in higher rainfall areas. However, high water use over summer and high carbon to nitrogen (C:N) ratios, which may tie up nitrogen, can depress subsequent yields.
Brown manuring is a ‘no-till’ version of green manuring, using a non-selective herbicide to desiccate the crop (and weeds) at flowering instead of using cultivation. A follow-up treatment may be required to control survivors. The plant residues are left standing, helping to retain surface cover and soil structure. Soil organic matter is increased.
A variation on brown manuring is mulching, where the crop or pasture is mowed, slashed or cut with a knife roller and the residue is left lying on the soil surface. This mulch reduces soil moisture loss through evaporation. Mulched residues break down more rapidly than for normal brown manuring because of the increased contact with soil and smaller pieces.
Maintaining SOC after manuring
The content of SOC in rain-fed Australian farming systems is influenced by soil type, climate and management:
- soil type: clay/loam soils can support higher concentrations of organic carbon than sandy soils
- climate: higher rainfall supports more biomass leading to higher SOC (areas receiving less than 400 millimetres per year have limited ability to generate SOC; high temperatures will lead to faster rates of biomass and SOC decomposition, with the release of carbon dioxide, methane and nitrous oxide
- management: soil disturbance through tillage will encourage SOC loss. Minimum tillage and no-till minimise SOC loss.