Role of organic matter in soil and its contribution to crop nitrogen nutrition

Tom Miles

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,
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 ( 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 * 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