Managing soil organic carbon on Western Australian farms

Page last updated: Friday, 4 September 2020 - 4:29pm

Organic carbon has declined in most Western Australian agricultural soils, as a result of land clearing and agricultural practices. Increasing biomass production and retaining crop and pasture residues has the potential to slowly increase soil organic carbon.

This page provides management options for increasing long-term soil organic carbon levels in agricultural soils.

Management to increase biomass production can increase soil organic carbon (SOC) where carbon inputs are greater than losses.

To increase SOC, practices must increase organic inputs above current levels, and then sustain that level of input.

Organic amendments such as manure and compost can increase SOC levels – but growers should consider the economic cost and look for local evidence of benefits. An ever increasing number of products claim to increase organic matter in soil or provide agronomic benefits. On-farm strip trials are a great way of sorting out what works and what doesn’t in your own circumstance.

Table 1 provides some management options that may improve long-term SOC levels in agricultural soils (Sanderman et al. 2010, Hoyle et al. 2011).

Tabel 1 Management options for improving long-term SOM levels in agricultural soils
Activity Potential for change Notes Influence on profit outcome
Increase biomass of crops and pastures Low increase
  • Slow increase in carbon (less than 0.2tC/ha for each tonne extra grain yield)
  • Higher fertiliser inputs may increase decomposition rate.
  • Where harvest index maintained profit and organic matter should increase.
Retain crop and pasture residues on paddock Low increase
  • Higher nutrient cycling capacity and biological fertility.
  • Lower erosion risk.
  • Cost neutral long-term.
Burn crop and pasture residues Low decrease
  • Decrease in particulate organic matter and nutrient cycling.
  • Can be positive or negative.
  • Where high weed pressure exists, should increase yield.
  • Burning leads to loss in soil fertility, which could decrease yield.

Increasing rotational diversity

Low increase
  • Supports a more diverse microbial community.
  • Provides disease break options.
  • Can be positive or negative.
  • Lack of profitable break crop options in low rainfall may constrain potential for profit.
Increase rotational frequency Moderate increase -
  • Positive provided no water deficit occurs in cash crops.
  • Potential for green bridge to increase disease risk.
Add a pasture phase High increase
  • Mixed pastures provide best quality inputs and capitalise on longer growing season.
  • Well established perennials with high biomass production or deeper carbon allocation.
  • Perennial systems tend to be dominant in high rainfall areas.
  • Positive where good pasture establishment and weed and erosion risks are managed.
  • Dependent on stock/meat/wool prices.
  • Pasture cropping could influence moisture availability in subsequent crop.
Manage grazing intensity Low increase
  • Lower grazing pressure to decrease erosion risk and increase organic matter inputs.
  • In theory, rotational grazing increases productivity and residues turnover.
  • Dependent on stock/meat/wool prices.
Cover crop, green manure, pasture cropping Moderate increase
  • One-off cover crops etc. likely to have little impact.
  • Impact greater if able to support two or more crop/pasture phases in a year.
  • Pasture cropping where organic matter inputs are effectively extended (higher net primary productivity).
  • Significant cash flow deficit in year of implementation for green manures.
  • Viability dependent on opportunity and operational costs and subsequent seasonal conditions.
  • Carefully consider the area to be targeted.
Apply off-paddock organic amendments such as manures, compost and biochar Low increase
  • Carbon in manure and compost is more stable than fresh residues.
  • While possible to generate enough biomass to raise soil carbon, often external sources of organic matter (manure, compost) are required.
  • Amendments vary widely in properties and effect on crops and soils.
  • Negative in the short-term with high application cost, with little evidence of long-term profit outcome.
  • Economic outcomes likely to be constrained by rate of application and costs, including transport.
  • Agronomic responses vary widely and can be negative. Seek local trial data.
  • Consider any potential environmental risks.
Maintain low soil disturbance system Low increase
  • Small, if any benefit to soil organic matter. Long-term adoption can promote soil aggregation and slow decomposition rates.
  • Direct drilling decreases erosion risk and helps maintain soil structure, slowing decomposition.
  • Organic residues on the soil surface tend to provide only minor contributions to stable soil organic carbon fractions.
  • Effect on structured or aggregated soils greater than on coarse textured sand.
  • Increased reliance on herbicides can contribute to negative profit base.
Increase bare fallow in the rotation Moderate decrease
  • Fallow contributes to soil carbon losses because no additional biomass is generated and erosion risk increases.
  • While short-term economic responses to fallow can be positive, longer term profit outcomes are often negative.
Decrease erosion risk High increase -
  • Low cost practices available.
  • Likely to be positive though dependent on current losses and the intervention required.
Retirement of non-productive areas Low increase
  • All organic matter (minus natural loss) returned to the soil, with replanting of native species on degraded land.
  • There are direct and indirect costs in retiring agricultural land. Economic viability dependent on foregone opportunity costs, carbon price and market opportunities.
Revegetation and destocking of cleared areas Moderate increase
  • Large potential for carbon storage through the establishment of forests, trees and other perennial vegetation.
  • Less erosion potential.
  • Opportunity costs over the long-term should be considered. Economic viability dependent on carbon price and market opportunities.
Irrigation Moderate increase
  • Higher biomass production.
  • Increased frequency of crops and pastures supports higher soil organic carbon.
  • Irrigating when warm can increase decomposition rate.
  • Large on-farm impact.
  • Potential trade-off between higher carbon inputs and increased decomposition rates.

Contact information

Tom Edwards
+61 (0)8 9083 1151