APPENDIX 2. Australian Soil Resources Information System
Total nitrogen
Nitrogen is a part of all living matter and is essential for plant growth. Nitrogen stimulates above-ground plant growth and is required to maintain high yields.
Most soil nitrogen is associated with organic compounds such as proteins or fertiliser inputs.
Soil nitrogen is presented here as weight percent.
How does nitrogen vary and what is it related to?
Soil nitrogen varies with depth. Levels are highest in the topsoil and generally decrease exponentially with depth. Organic nitrogen commonly ranges between 0.2% and 0.5% (percent by weight or grams of nitrogen per 100 g of soil) in cultivated topsoils. It can reach > 2.5% in peats.
How and why does nitrogen vary across Australia?
Total nitrogen varies mainly as a function of climate and land use. Highest levels (> 0.2%) are found on forest and cultivated soils; lowest levels (< 0.1 %) are found in rangelands.
Mapping of nitrogen can be used to identify areas where natural fertility of soils is low and fertiliser inputs would be required for crop production. Nitrogen measurements are difficult to interpret, without information on the types of nitrogen present and their relevance to crop nutrition.
Level of uncertainty
The certainty in the nitrogen model depends on the strength of the organic carbon model (for topsoil) and the relationship between nitrogen and organic carbon. The uncertainty surface for nitrogen is calculated from that for organic carbon, downgraded to reflect the fact that nitrogen is not estimated directly but rather through this relationship. There is some suggestion that this carbon-nitrogen relationship may over-predict nitrogen at the low end.
Very few measurements of total nitrogen were available for soils in New South Wales and South Australia; most measurements were from Queensland and Western Australia. The modelled estimates are thus best for Queensland, Western Australia and Tasmania, and poorest for New South Wales, eastern Victoria and the Northern Territory.
Error diagnostics
| Error diagnostic | Topsoil |
| Number of points used | 4746 |
| R2 | 0.75 |
Table A9 Nitrogen (%) in topsoil (derived from carbon-nitrogen relationship) by percent of land use type.
| Extremely low | Very low | Low | Moderate | Moderately high to high | ||
|---|---|---|---|---|---|---|
| 0 - .05 | 0.05 - 0.1 | 0.1 - 0.2 | 0.2 - 0.3 | > 0.3 | ||
| Conservation and natural environments | 13 | 41 | 29 | 10 | 8 | 54 814 100 |
| Production from native environments | 26 | 47 | 20 | 5 2 | 184 | 376 200 |
| Cropping | 6 | 65 | 26 | 3 | 1 | 22 241 100 |
| Grazing modified pasture | 5 | 37 | 46 | 11 | 1 | 18 482 600 |
| Horticulture | 6 | 35 | 43 | 13 | 4 | 351 300 |
| Irrigated cropping | 2 | 73 | 23 | 2 | 0 | 948 800 |
| Irrigated modified pasture | 0 | 69 | 25 | 4 | 2 | 1 080 000 |
| Total area* | 282 294 100 |
* Area of river basins containing intensive agriculture.
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