What is it
Nitrogen is an essential major nutrient required by all living organisms. Nitrogen comes in many forms – as dinitrogen gas (N2 in the atmosphere), ammonia and gaseous oxides of nitrogen, ammonium, nitrite and nitrate salts, and organic forms such as proteins and amino acids. Animals obtain their nitrogen by eating plants or other animals. Plants obtain their nitrogen from soluble forms in soil, or from the atmosphere by a symbiotic relationship with specialist micro-organisms that can “fix” the nitrogen gas from the atmosphere. Virtually all the nitrogen in soil has been accumulated by fixation by microrganisms and their subsequent death and incorporation into organic matter. Very little of the N in soil comes from weathered rocks as most common minerals contain negligible amounts of N.
Total N give a measure of the total N stocks of a soil. Usually only a small fraction of the total N is immediately available for plant uptake (soluble inorganic N), while a variable proportion of the total N is potentially mineralisable to inorganic (see mineralisable N).
Because the N content of a soil represents the end product of years of biological fixation and accumulation, N contents tend to be low in young soils (sand dunes, fresh pumice), and greatest in those where there has been substantial organic matter accumulation and nitrogen fixation (long-term grass-clover pastures). ). Total N is generally related to the amount of carbon in the soil so soils with higher carbon content often have higher N content as well. The ratio of total C to total N (the soil C:N ratio) gives an indication of the quality of the organic matter to supply N and is useful, along with total N and mineralisable N, in evaluating the N content of the soil.
How to interpret it
In general, adequate levels of total N indicate the soil is in a good biological condition. However, low N soils are adequate for growing species with low N demand, such as pines. High total N contents increase the risk that N supply may be in excess of plant demand, and ultimately lead to leaching of nitrate. Since total N is largely dependent on the amount of soil carbon, the N content of the soil is best considered in conjunction with the carbon to nitrogen ratio and mineralisable N. The rationale for setting the boundaries between the various levels is similar to mineralisable N content.
Pastures and Horticultural uses
High - there is the risk that the mineralisation of organic N may be in excess of
plant demand. Soil C:N 10 or less. Excess mineralised
N, in the forms of ammonium and nitrate, has the potential to leach and contaminate
water resources.
Ample - the amount of mineralisable N is sufficient to meet plant demand and these
levels are at the higher range of soils in the 500 Soils data set. C:N ratio 10-12
Adequate - The amount of mineralisable N is sufficient to meet plant demand, and
these levels are typical of soils in the 500 Soils data set. C:N ratio
12-14.
Low - The amount of mineralisable N is low and unlikely to satisfy plant demands.
It may be that total organic matter contents of the soil are depleted (such as after
a period under cropping), or the N content of the organic matter is low because
there are few legumes (a common characteristic of native forest soils). C:N ratio
15–25
Very low - indicates a soil with very low organic nitrogen reserves.
Such soils are typically very young (stabilised dunes, pumice, sub-soils exposed
through land slips), or have been severely depleted through long term cropping with
inadequate organic N returns. Soil C:N ratio > 20. A period under
a legume crop or grass-clover sward will be needed to build up organic matter levels
and nitrogen reserves.
Plantation Forestry and Indigenous ecosystems
High - there is the risk that the mineralisation of organic N may be in excess of
plant demand. Soil C:N 12 or less. Excess mineralised
N, in the forms of ammonium and nitrate, has the potential to leach and contaminate
water resources.
Ample - the amount of mineralisable N is sufficient to meet plant demand and these
levels are at the higher range of soils in the 500 Soils data set. C:N ratio 10-15
Adequate - The amount of N is sufficient to meet plant demand, and these levels
are typical of soils in the 500 Soils data set. C:N ratio typically
15–17
Low - The amount of N is low and unlikely to satisfy plant demands. It may be that
total organic matter contents of the soil are depleted, or the N content of the
organic matter is low because there are few legumes (a common characteristic of
native forest soils).
Very low - indicated a soil with very low organic nitrogen reserves.
Soil C:N ratio > 25. Such soils are typically very young (stabilised dunes, pumice,
sub-soils exposed through land slips) that have had little opportunity to accumulate
organic N.
How to improve it
Increasingly, high soil N is becoming a problem as organic nitrogen is converted to inorganic nitrogen (first ammonium and then to nitrate) by microbial activity. Nitrate, in excess of plant demand can be easily leached from the soil and has detrimental effects on lakes and streams. Pastures with high N usually indicate over inputs of N over long periods of time or repeated incorporation of low C/N material (manure, etc.). In such cases lowering (or stopping) incorporation of the material will help to alleviate the problem. Nutrient budgeting is the best way to ensure pasture or crops are getting adequate N without applying excess fertiliser. In dairy pasture, cow urine spots present a particular problem because they contain large amounts of N in a small area. New technologies such as nitrification inhibitors may prove useful in decreasing N leaching in dairy pasture, but results on different soil types are still being evaluated. Although not directly related to soil quality, fencing off waterways and creating buffer zones to keep stock out and allow uptake of N can also help to minimise N leaching into surface waters.
When low, total N can be increased by the accumulation of organic matter with low C:N ratio, typically from a legume crop. In pastures this crop is usually clover, under arable cropping a rotation including lucerne, peas or beans will contribute to total N. On low N status soils, a crop of lupins may be planted before the tree seedlings, while the presence of gorse and broom, although often considered weeds, is beneficial for increasing the N stock and secondary forest succession.
Nitrogen addition is only rarely needed for indigenous ecosystems, although gorse can be a useful nurse plant for the establishment of secondary forests species. In contrast, exotic legumes may disrupt primary succession sequences, and, in general, exotic legumes should be discouraged in indigenous ecosystems.
Technical details
Total soil N is measured in the laboratory using air-dry ground soil. Traditionally the soil was digested using strong acids (Kjeldahl method), but increasingly, the high temperature combustion method is used. In the traditional Kjeldahl method the soil is boiled with nitric and sulphuric acids to convert the organic N to inorganic forms, which are analysed as ammonium. In the combustion method the soil is heated to high temperature in the presence of catalysts, the N converted to gaseous forms which are then reduced, ending up as di-nitrogen gas measured by volumetric or gas chromatography methods. These various stages of high temperature combustion and reduction are usually automated within a dedicated instrument providing direct read-out, and which can often measure total C and H in the same sample.
Reference material
Di HJ, Cameron KC 2002. Nitrate leaching and pasture production from different nitrogen sources on a shallow stoney soil under flood-irrigated dairy pasture. Australian Journal of Soil Research 40: 317–334.
During, C. (1984) Fertilisers and Soils in New Zealand Farming, Third Revised Edition Ed., 1-354, Wellington, P.D. Hasselberg, Government Printer.
Ledgard,S.F. and Steele, K.W. (1992) Biological nitrogen fixation in mixed legume / grass pastures. Plant & Soil 141, 137-153.
Page, A.L., Miller, R.H., and Keeney, D.R. (1982) Methods of Soil Analysis. Part 2. Chemical and Biological Properties, Second Edition Ed., 1-1159, Madison, WI., American Society of Agronomy, Inc., Soil Science Society of America, Inc.
Roberts, A.H. C and Morton, J.D. Fertiliser Use on New Zealand Dairy Farms. Roberts, A. H. C and Morton, J. D. Revised, 1-37. 1999. Auckland, New Zealand, New Zealand Fertiliser Manufacturers Association.
Shepard M, Wyatt J, Welten B, Ledgard S 2010. Form of nitrogen leaching from dairy cow urine and effectiveness of dicyandiamide: not all soils are equal. In: Gilkes RJ & Prakongkep N eds. Proceedings of the 19th International World Congress of Soil Science: Soil Solutions for a changing world. Brisbane, Australia, August 2010.
Sparling, G.P., Rijkse, W, Wilde, R.H., van der Weerden, T, Beare, M. H., and Francis, G. S.(2000). Implementing soil quality indicators for land. Research Report for 1998/1999. Landcare Research Contract Report: 9900/108, Hamilton, Landcare Research.