Date

2018/11/22

Soil Quality ebook

Soil Quality: 3 Soil Organic Matter

Organisations

SoilsWest

Department of Primary Industries and Regional Development

Grains Research and Development Corporation

Authors

Frances Hoyle

Daniel Murphy

Nutrient supply from organic matter

Biotic and abiotic factors determine nutrient availability from decomposing soil organic matter. Nitrogen supply to plants results from mineralisation and nitrification of organic compounds containing organic nitrogen that then determine ammonium (NH4+) and nitrate (NO3) supply.

Phosphorus exists within organic and inorganic compounds and moves between pools (microorganisms, organic matter and plants) via mineralisation, immobilisation and redistribution of phosphorus. A large proportion (up to 80%) of soil organic phosphorus remains unavailable to plants. Plant responses to phosphorus are dependent on background soil levels, adsorption and depending on plant type may be influenced by mycorrhizal associations.

Sulfur is linked to the composition of parent material and is present in nearly all soils. Organic sulfur is the primary source of sulfur in soils and when mineralised is converted to plant available sulfate (SO42-). In aerated soils organic matter requires a carbon to sulfur ratio of less than 200:1 to increase sulfur supply. Clay and gravel soil generally have higher sulfur than sandier soils, particularly in high rainfall areas where sulfur can leach.

Excluding fertilisers, potassium is supplied by mineral weathering and mineralisation of organic inputs. Potassium is released relatively quickly and contributes to non-exchangeable, exchangeable and solubilised potassium pools. Supply is more closely linked to soil type and clay complex than soil organic matter, with the exception of sandy soils.

Carbon to nitrogen ratio (C:N)

The humus fraction, being the largest component of soil organic matter dominates soil nutrient supply. Humus breaks down slowly at a relatively constant rate and thus provides a relatively stable source of nutrients. Fresh or recently decomposed plant residues have a variable influence on nutrient availability as they break down, depending on their composition.

Poorer quality residues may cause soil nitrogen to be immobilised. For example, wheat straw has a wide C to N ratio (approximately 80 units of carbon:1 unit of nitrogen) and insufficient nitrogen for microbial demands to decompose the residue. As a result, wheat and other cereal residues are frequently associated with immobilisation of existing plant-available nitrogen in soil. This can lead to nitrogen deficiency during periods of high crop demand, particularly early in the growing season. Legume residues (often 20–30 units of carbon:1 unit of nitrogen), by comparison, often release surplus nitrogen, which then becomes plant available.

Increasing soil disturbance and mixing of residues, smaller residue size and warm, moist climates all serve to increase the rate at which organic matter will decompose.

Calculating the impact of incorporating organic residues on soil nitrogen supply

Scenario: A grain grower retains three tonnes per hectare wheat stubble, with a C:N ratio of 120:1 and wants to estimate the likely impact of incorporating the stubble on soil nitrogen levels in the paddock.

Step 1: The amount of carbon present in the plant residues added to the soil.

  • 3000 kg (3 tonnes) of stubble x 0.45 carbon content = 1350 kg of carbon in plant residues

Step 2: The amount of nitrogen present in the plant residues added to the soil.

  • The stubble contains 1350 kg of carbon per hectare and has a C:N ratio of 120:1
  • 1350 kg carbon ÷ 120 = 11.25 kg nitrogen per hectare in organic matter

Step 3: Allow for 30% of the carbon being used by microbes to grow, with the remaining 70% respired as carbon dioxide.

  • The amount of carbon used by microbes is 1350 kg carbon x 0.3 = 405 kg carbon per hectare

Step 4: The amount of nitrogen microbes require.

  • Given that microbes have a C:N ratio of 6:1 they therefore require 1 kg of nitrogen for every 6 kg of carbon to grow. 405 kg carbon ÷ 6 = 68 kg nitrogen per hectare

Step 5: Compare the two nitrogen values.

  • The fresh organic matter contained 11 kg of nitrogen and the microbes require 68 kg of nitrogen to grow
  • The nitrogen balance = 11 – 68 = – 57 kg nitrogen per hectare.

A NEGATIVE balance indicates that nitrogen in the organic matter was LESS than the nitrogen required by the microbes. This nitrogen deficit will be sourced from the soil, making it unavailable for plants. In this case, a grower should consider fertiliser strategies that will ensure sufficient nitrogen is available to plants early in their growth.

If the nitrogen balance had been POSITIVE a surplus of nitrogen would then become available to plants because there would be MORE nitrogen than required for microbial use. If this was the case, growers may consider split applications of nitrogen to save input costs and minimise losses of nitrogen via leaching.

Knowing the contribution of organic matter to your soils nutrition can help inform more profitable fertiliser management strategies.

References

ebook Soil Quality: 3 Soil Organic Matter

Hoyle F and Murphy D (2018).

Hoyle FC and Murphy DV (2011). Influence of organic residues and soil incorporation on temporal measures of microbial biomass and plant available nitrogen. Plant and Soil 347: 53–64.

Hoyle FC, Baldock JA and Murphy DV (2011). Soil organic carbon – role in rainfed farming systems: with particular reference to Australian conditions. In: P Tow, I Cooper, I Partridge, C Birch (Eds) Rainfed Farming Systems Springer, Netherlands, pp 339–361.

Calculating the impact of incorporating organic residues on soil nitrogen supply:

Hoyle F (2013). Managing soil organic matter: A practical guide. Grains Research and Development Corporation. Canberra.

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