It used to be that nitrate was often the only form of nitrogen (N) specified in soil test results. Now it’s common for soil test results to specify the quantity of ammonium-N as well.
Recently some unusually high ammonium levels – equal to or higher than nitrate levels – have been recorded in Queensland paddocks. At these levels how the numbers are incorporated into decision matrixes can make a big difference to fertiliser rates.
Should ammonium levels affect fertiliser recommendations?
The proportion of N present as ammonium is usually so small that including or excluding it from nitrogen budgets makes little difference. In most warm, well-aerated soils, ammonium levels are at least five to ten times lower than the nitrate concentration, and generally below 10 mg/kg. Higher concentrations are only sustained in soils with high organic matter.
How to include ammonium-N in fertiliser calculations
Soil test result < 10 mg/kg ammonium-N
- Consistency of approach is most important. Including ammonium-N increases the suggested N fertiliser rate, but seasonal yield potential has more impact than including or not including normal range ammonium-N in the N budget.
- Discount about 2–3 mg/kg of ammonium-N from crop N supply in an N budget. This represents the buffer or background ammonium-N, not likely to be able to be taken up by crops. An alternative approach is to use 50% of the ammonium-N test result in the nitrogen budget.
Incorporating a background ammonium-N discount, the mineral N budget supply equation becomes:
|N supply (kg/ha)||=||nitrate N (mg/kg) + ammonium N (mg/kg) – 3 (mg/kg)||x||Bulk Density (g/cm3)||x||depth increments (m)|
Soil test result > 10 mg/kg ammonium-N
Assess the feasibility of the soil having sufficient mineralisable N to sustain a high background level of ammonium-N. If this can not be established:
- Option 1: resample the soil, avoiding issues that might have led to an unusually high ammonium-N result (see below).
- Option 2: go with nitrate-N only and ignore ammonium-N
What can cause high ammonium-N levels?
Higher than expected ammonium-N results are sometimes seen in soil tests from soils that do not have high enough levels of mineralisable N to sustain long term high ammonium-N levels.
|Cause||Why?||How to avoid|
|Incomplete mineralisation||During decomposition of crop residues flushes of ammonium are produced as an interim step to nitrate accumulation. An ammonium flush might be observed soon after rain. Mineralisation can stall, not proceeding to nitrate production, if the soil drys during a mineralisation cycle.||Resample later in the season. Warm and moist soil conditions will complete the mineralisation cycles quickest.|
|Fertiliser band||Elevated ammonium concentrations can be found in fertiliser bands for months in some conditions.||Resample, avoiding the fertiliser band. If unsure wait until the crop emerges so that the position of the fertiliser band can be located relative to the seed row.|
|Subsoil acidity/ clay fixation||Some clay subsoils with low pH and/or high salinity hold high levels of ammonium, which appears to be unavailable to crops.||Disregard, this N will probably not be available to most crops. Only consider for crops tolerant to low pH and high salinity.|
|Sample conditions in transit||Moist soils with high carbon or quantities of crop residues will continue to mineralise, producing an ammonium flush during transit to the laboratory, if kept at high temperatures.||Resample, and ensure samples are kept below 5oC during transit to the laboratory, or dried (>40oC) for storage.|
|Contamination||On farms, numerous sources of ammonium and ammonia can contaminate soil samples. In gaseous forms, they are strongly attracted to moist soil.||Resample, isolating samples quickly into airtight containers or bags.|
Photo courtesy of the GRDC
This article is a summary of High ammonium in soil test: artefact or reality? Northern Nutrition Extension Team (DAF), Cox Inall Communications