Role of organic matter in soil and its contribution to crop nitrogen nutrition
Jeff Baldock, Jan Skjemstad & Evelyn Krull, CSIRO Land and Water, Adelaide, SA, jeff.baldock@csiro.au
Ph: (08) 8303 8537

Composition of soil organic matter

* Soil organic matter is composed of a wide range of different materials with different chemical and physical properties and different extents of decomposition.
* Four biologically significant fractions are now recognised:
o Crop residues (>2 mm) in and on the soil surface
o Particulate matter - small (<2 mm) pieces of plant residues
o Humus - organic matter of unrecognizable structure (molecules)
o Recalcitrant organic matter (biologically stable) - charcoal
* The amount of each type of organic matter varies significantly across soil types and can be altered by management practices.

Roles of organic matter in soil

* Soil organic matter contributes to a variety of biological, chemical and physical properties of soils.
* Chemical - cation exchange, pH buffering, reduces effects of sodicity
* Physical - water retention, soil structural stability, soil temperature
* Biological - energy for microbes, provision of nutrients and resiliency
* Each fraction of soil organic matter contributes differently to various soil properties.

Role of organic materials in defining plant-available N

* Significant quantities of organic N exist in soils as either residues from previous crops or pastures or soil organic matter; however, this N must be mineralised to become available to plants.
* Previous crop/pasture residues can have a large impact on plant-available N
* Residues with a C/N >40 (e.g. most cereals) deplete available N
* Residues with a C/N <40 (e.g. pulses and legume pastures) release available N
* Mineralisation of N from soil organic matter has been treated very simply and often equated to a given fraction of the total nitrogen present
* Soil organic N is found in several different forms, which are not equally able to release plant-available N. Understanding the dynamics of each type of N should lead to better predictions of mineralisable N.

See GRDC presentation
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