Date
2018/11/22
Duration
5 min read
Soil Quality ebook
Soil Quality: 3 Soil Organic Matter
Organisations
SoilsWest
Department of Primary Industries and Regional Development
Grains Research and Development Corporation
Liebe Group
Authors
Frances Hoyle
Daniel Murphy
Aim: To determine if the theoretical target storage of soil organic matter could be reached where sufficient material is added
Background and trial design
- Trial location: Buntine, Western Australia
- Soil type: Deep yellow sand
- Average growing season rainfall: 291 mm
Large amounts of sustained organic matter inputs to soil can result in accumulation of soil organic carbon—though in practice would take decades longer than this case study. In this study, the amount of chaff added to soil would be unrealistically high compared to normal stubble loads, and is not considered economically viable when using offsite inputs. The trial plots were situated on a deep yellow sand with no significant chemical or physical soil constraints that would impose limitations to crop growth or responses; surface pHCaCl2 6.3, subsurface (10–30 cm) pHCaCl2 5.2 .
The two treatments reported here include a control and a treatment where 100 tonnes of organic matter as chaff per hectare* has been added, with both tilled using offset discs prior to sowing.
*Five applications (2003, 2006, 2010, 2012 and 2015) of 20 tonnes of organic matter as chaff per hectare, or the equivalent of 48 tonnes carbon per hectare was added to soil and incorporated.
Summary results (2016)
- An additional 7.5 tonnes of carbon per hectare (16% of the material added) had accumulated associated with the addition of organic materials, with 77% of this located in the top 10 centimetres.

- Associated with the increase in soil organic matter, plant available nutrients also increased. Inorganic sulphur increased from 24 to 60 kilograms per hectare in the top 30 cm, nitrogen (as nitrate) from 11 to 21 kilograms per hectare, potassium increased by 135 kilograms per hectare while phosphorus remained unchanged.

- Increasing concentrations of nutrients with depth suggests that while changes in organic matter were evident primarily in the surface, soluble nutrients were leached to a greater depth.
- Grain yield responses were dependent on crop type and season, with most positive yield responses associated with cereal grain phases.
- Soil organic carbon increases were not ‘permanent’ and carbon stock declines are noted for treatments where the application of residues was stopped.
- Incorporating crop residues increased N2O emissions, but these remained low compared to international standards. Tillage decreased soil carbon stocks marginally in the surface, while burning stubble residues had no adverse short-term impact on soil organic carbon at this site.
- Doubling of the microbial population when organic matter was added suggests there is no limit to the number of microbes which can survive in the soil provided there is a sufficient food source. More food = more microbes.
- Average wheat yield increased by 12% (2003–2013) in treatments receiving added organic matter and was directly related to nitrogen uptake. Few yield benefits were observed in other rotational crops.

Where to?
Treatments having now received a total of 100 tonnes per hectare of organic matter as chaff remain below the soil’s natural capacity to store soil organic carbon which has been modelled** at 35 tonnes of carbon per hectare in the top 30 centimetres.
**Modelled using RothC based on the same rotation but with added organic inputs and by increasing water use efficiency from 46% to 60% to reflect measured changes.
Trial seasons varied markedly in climatic conditions and treatment responses. Increased organic matter in the 0–10 centimetre layer increased the soil’s water holding capacity by approximately 6%. Yield gains in the organic matter treatments, may thus be partly attributable to more water being available and greater nutrient availability – particularly in seasons where soil water was limiting. The importing of chaff to organic matter plots itself acts as a source of potassium and phosphorus but it is unclear if the plants are taking full advantage of this extra supply of nutrients due to other limiting factors such as rainfall.
References
ebook Soil Quality: 3 Soil Organic Matter
Hoyle F and Murphy D (2018).
Barton L, Hoyle FC, Stefanova KT and Murphy DV (2016). Incorporating organic matter alters greenhouse gas emissions and increases grain yield in a semi-arid climate. Agriculture, Ecosystems & Environment 231: 320–330.
Hoyle FC, D’Antuono M, Overheu T, Murphy DV (2013) Capacity for increasing soil organic carbon stocks in dryland agricultural systems. Soil Research 51: 657–667.
Janik L, Spouncer L, Correll R, Skjemstad J (2002) Sensitivity analysis of the Roth-C soil carbon model (ver. 26.3 Excel). National Carbon Accounting System Technical Report No. 30, Australian Greenhouse Office, Canberra, ACT.