Nitrogen monitoring tools
How to use these tools
Monitoring is an essential component of developing an Environmental Management System (EMS).
The community is increasingly concerned about water quality resulting from nitrogen (N) loss from agricultural land. Excessive nitrogen can stimulate the growth of aquatic plants in our rivers and lakes. Some plants choke streams, others such as blue green algae, can be toxic to both humans and stock. We need to limit nitrogen export from our farms. The N tools outlined below will allow you to assess whether your farm meets community expectations of minimal N risk loss. It is suitable only for cropping enterprises, as measurement of deep soil N is not routine in grazing enterprises.
There are three tools to assess the risk of N loss
- Risk of N leakiness: This tool uses deep soil N testing, plus leakage risk, to assess the risk of N leakage.
- N Budget: Whilst this tool has been used for production purposes only, it can be used for both production and environmental risk assessments.
- Deep N Discrepancy: Recommended where deep-soil-N tests are failing to predict crop responses.
Risk of Nitrogen leakiness
Nitrogen leakage results in both negative environmental impacts (it reduces water quality in streams and/or groundwater, and is the major cause of soil acidification under agriculture) and a loss of profit. The main risk of N loss is under annual crops and pastures because these use less water and N than perennial plants.
Many cropping farmers routinely use deep-N soil testing to assess whether there is enough N in the soil to achieve grain yields and protein content. This deep-N testing, combined with an assessment of the amount of water lost below the root zone (called leakage) can also be used to assess whether there is a large risk of N leakiness occurring. If N leakiness is calculated then there is high risk that your farm management practices might be affecting the environment beyond your paddock or farm. Conversely, if you do not calculate N leakiness, then you have some simple justification for proving to others that your farm management practices are not harming the environment.
Four steps are outlined on the next page to allow you to assess the risk of N leakiness and therefore environmental risk.
Steps to assess risk of N leakiness
- Step 1. Record paddock names and years which are to be tested for deep soil N.
- Step 2. Sample for deep-soil-N test: (see Table 1 for instructions).
- Step 3. Assess N leakiness risk: Risk of N leakiness is assessed by using the amount of N from the deep-soil-N test and the risk of leakage. To calculate leakage you need to use the Water Monitoring tools, also available on this website. Follow steps 1-7 in the Water Tool B 'Frequency of leakage on dryland paddocks' calculation) and then use Table 2 to assess whether your risk of N leakage is 'very high', 'high', 'medium' or 'low'.
- Step 4. Repeat for other paddocks where deep-N tests have been taken and build up a picture of the N leakiness of particular paddocks.
Table 1: Instructions for deep-soil-N testing
Deep-soil-N testing (sampled to 60 cm depth) indicates (at the time of the test) the amount of N available to the crop. It is the sum of N that could have broken down (called mineralised N) from the plant litter remaining from the last two or three seasons as well as more recent inputs of fertiliser N. Many grain farmers use deep soil N testing to assess N fertiliser requirements. The choice of 60 cm depth for mineral N sampling is a compromise between recovering all the N available to crops (crops can often take up N from depths of 1-1.3 m) and the speed and ease of sampling.
Time of sampling
To allow comparison of results between different paddocks, and also to allow you to compare the differences in N availability between years, we recommend that you do a deep-N test in June-July (where practical). On paddocks that commonly get very wet, if July sampling is impractical, then soil sample at around sowing time. You can add 300 g N/ha/day each day between your date of sampling and July 1 to give the July mineral N figure.
Take at least 5 auger holes/paddock to 60cm depth and combine samples. If you wish to see where most of the N is located you can separate samples into different depth intervals (eg. 0-30 cm, 30-60 cm or 0-10 cm, 10-60 cm) but this will cost extra to test. Sample only representative areas of the paddock; that is, the main soil type(s) and avoid headlands, stock camps, trees and old fencelines. If soil types are markedly different and you can tailor N applications, sample soil types separately. Use a zig-zag diagonal sampling transect.
Use a reputable soil-testing laboratory to get samples analysed. More information on this can be found in the Soil Fertility Monitoring Tool.
Interpretation of results
Results need to be expressed as kg N/ha. If the results are expressed as parts per million (ppm), you will need to multiply the ppm value by the depth interval (x 6 for the full 60 cm depth) and by the soil bulk density (a typical value is 1.5). Example: if soil mineral N concentration of a 0-60 cm sample is 11 ppm, then kg N/ha is 11 x 6 x 1.5 = 99 kg N/ha.
Table 2: Risk assessment of N leakiness
|Deep soil N (kg N/ha)||Leakage (calculated from frequency of leakage tool –Water Tool B)||N leakage risk|
|>150||Leakage (calculated from frequency of leakage tool Water||VERY HIGH|
|150||No leakage although soil was calculated to come to within 20 mm of |
|150||Leakage unlikely and calculations indicated that soil did not come |
within 20 mm of leakage.
|80-150||No leakage although calculations indicate that soil came to within 20 |
mm of leakage occurring.
|80-150||Leakage unlikely as the soil did not come within 20 mm of leakage.||LOW|
|Less than 80||Leakage calculated.||MEDIUM|
|Less than 80||No leakage although calculations indicate that soil came to within 20 |
mm of leakage.
|Less than 80||Leakage unlikely and calculations indicate that the soil did not come |
within 20 mm of leaking.
Matching plant demand with fertiliser N supply is important for a number of environmental and economic reasons. Nitrogen is lost through volatilisation, leaching and denitrification. By ensuring that the plant N use matches the fertiliser N you are minimising the chances of excess N reducing water quality and contributing to acidification.
More efficient use of nitrogen fertiliser reduces both greenhouse gas emissions and on-farm costs. Nitrous oxide, which can be emitted from N fertiliser applied to agricultural soils, is a potent greenhouse gas, having a global warming potential 310 times that of carbon dioxide. Using monitoring tools such as N budgeting will ensure that fertiliser applied to pasture and crops will provide maximum production benefit with minimal environmental impact.
- Step 1. Record paddock name
- Step 2. Calculate target yield: One good way to calculate a target yield is to use the French-Schultz approach:
Where growing season rainfall is the total received between April - October.
Wheat potential yield = 20 x (growing season rainfall in mm – 110)
Canola potential yield = 10 x (growing season rainfall in mm – 110)
Barley potential yield = 18 x (growing season rainfall in mm – 90)
Oats potential yield = 22 x (growing season rainfall in mm – 90)
Triticale potential yield = 18 x (growing season rainfall in mm – 90)
Grain legumes potential yield = 12 x (growing season rainfall in mm – 130)
- Step 3. Calculate N budget: Using Table 3 (and Table 4 to correct for the extra N that becomes available over the growing season).
- Step 4. Apply correct amount of fertiliser: Urea contains 46% N, ammonium nitrate 34% N, di-ammonium phosphate (DAP) 18% N, mono-ammonium phosphate (MAP) 11.3% N, and sulphate of ammonia 21% N. But note that sulphate of ammonia is highly acidifying and needs between 4-7 kg lime per kg N applied to balance the acidity; see Acidity Tools for further details.
Table 3: Nitrogen budget based on an autumn-winter deep soil N test to 60cm depth
|Target yield||4.0 t/ha|
|Multiplied by||Target protein||X 13%|
|Multiplied by||Correction factor||X 2.34|
|Equals||N-demand||122 kg N/ha|
|Measured soil mineral N in the top 60 cm at |
sowing – autumn - winter deep-soil-N test.
|100 kg N/ha|
|Add||Estimated production of mineral N |
(mineralisation*) during crop production –
see Table 4.
|80 kg N/ha if low |
|Equals||Gross N supply||180 kg N/ha|
|Less||Assume 50% is not taken up by the crop***||- 90 kg N/ha|
|Net N-supply||90 kg N/ha|
|Subtract soil supply |
from crop demand
|Net extra N required||122 - 90 = 32 kg |
|Crop demand: |
multiply "Net extra
N required" by 2
|Fertiliser N needed (assuming 50% |
efficiency of N recovery)***
|32 x 2 = 64 kg |
*Mineralisation of N, that is N made available to plants by the breakdown of plant residues and organic matter, is fastest in warm, moist conditions. Most rapid mineralisation occurs after summer and autumn storms and is slowest in dry summers and also in mid-winter. Mineralisation continues during crop growth but is difficult to measure because the N is taken up by roots as fast as it is formed. It occurs at rates of up to 1 kg N/ha/day in moist, warm (≈16°C) topsoil in spring, and at about 0.2 kg N/ha/day in cold, wet soils in winter (John Angus, Mark Peoples, personal communication). Mineralisation rates in soils with retained stubbles can be 1-1.5 kg N/ha/day following rain while the soil remains wet, dropping to zero once the soil has dried out.
*** The 50% efficiency of N recovery by plants is based on many studies which show that plant tops on average take up at least half of the applied N. This does not necessarily mean that the remaining 50% is all lost. Roots can take up about 30% of the N, and thus some of this residual N will eventually become available to the next generation of plants once the roots decay. True losses of N occur from:
- Volatilisation (gaseous losses of N) can be high in warm conditions, particularly from top-dressed urea with no follow-up rainfall to wash in the urea granules, and on alkaline soils).
- Denitrification (commonly less than 5-10 kg N/ha) is only likely if soils are warm and waterlogged and have a high amount of nitrate present and a source of carbon for microbes. Denitrification is the conversion of nitrate-N to gaseous N, and so is a loss of N which would have been available to plants.
- Leaching (loss of N in water – below the root-zone or laterally) as nitrate-N in wet years.
The amount of N fixed in different environments is determined by legume content and herbage yield. As a rule-of-thumb, 20-25 kg N in plant tops can be expected to be fixed on average for every tonne of legume dry matter produced. Assuming legume dry matter is not measured, Table 4 can be used to estimate N availability during the growing season.
Table 4: Estimated within-growing-season mineralisation rates in southern NSW cropping soils (based on estimates of Mark Peoples, CSIRO Canberra).
|Fertility status of soil||Nitrogen (kg N/ha) which |
becomes available during the
|Low e.g. continuously cropped, low use of N fertilisers, N |
deficiency common in crops, < 0.08% topsoil total N.
|Medium e.g. crop-pasture rotation, 2nd or 3rd crop into the rotation, |
moderate use of N fertilisers, 0.08-0.12% topsoil total N.
|Moderate-high e.g. 1st crop after pasture, moderate clover pasture |
contained at least 20-30% clover, moderate use of N fertiliser, >
0.12% topsoil total N.
|High fertility e.g.1st or 2nd crop after winter-cleaned pasture of high |
legume content (>50% legume content), > 0.12% topsoil total N.
The N benefits from subterranean clover or annual medic pastures rarely last beyond 2 years. Nitrogen benefits from lucerne can occur 2-3 years after lucerne removal (due to slower breakdown of lucerne residues than for annual legumes). However, if lucerne is removed only shortly before cropping, N fertiliser may be required for the first crop because it can take several weeks for the N in legume residues to break down and become available to other plants. N deficiency is unlikely to be a problem for spring-removed lucerne.
Deep N Discrepancy Test
Sometimes, the deep-soil-N test gives a high value that is not reflected in the yield or protein content of the crop. Is this as a result of a faulty deep N test or could there be other reasons for the lack of response? Establishing the reasons are important from both profitability and environmental perspectives. Table 5 below lists the most common reasons for why high N tests do not always agree with higher yields and protein contents, with some suggested solutions. Any N lost through leakage from paddocks has both a direct economic cost and an environmental cost in terms of soil acidification and reduced water quality.
- Step 1. Record paddock name
- Step 2. Identify each paddock where the deep-N test was high (150 kg N/ha or higher) and where yield and protein results were lower than expected from the test.
- Step 3. Assess whether paddock was sampled well. If you think that the paddock was not properly sampled, re-test next season with more careful sampling. See Table 1 'Sampling instructions'.
- Step 4. If the paddock was correctly sampled, identify any other likely causes of the failure of deep N test. See Table 5.
- Step 5. Take action to minimise risks.
- Step 6. Still no explanation? If you believe that results still cannot be explained, call an experienced agronomist to help you.
Table 5: Common reasons why high deep-N tests fail to reflect grain yield and protein contents.
|Possible reasons for poor crop yield despite good deep N test results||Situations in which this risk is greater||If risk is identified, action needed to minimise it happening again|
|Risk of rhizoctonia||Higher risk following pasture, more common on light soils, where minimum tillage is practiced in crops with poor vigour.|| |
|Risk of other diseases e.g. take-all, common root rot, crown rot, eyespot lodging, Pratylenchus|| |
|Crop sown late||Late-sown crops can have poorly developed roots. Crop roots may not have reached the deep N needed during grain filling. This problem is made worse if the year prior to the crop was wet and the soil is light textured, resulting in leaching of N deeper into the soil.|| |
|Previous annual species||N leakage risk is higher when annual species were present in previous years.|
|Acidic soil – topsoil and/or subsoil or sodic soil|| || |
|Low soil P||Soil P test results unknown|| |
Breaches of the Water Act
Breaches of the Water Act may occur if appropriate approval has not been sought from the local Water Authority when soil sampling, installing piezometers or neutron probe access tubes. In general, if any soil sample, piezometer, soil moisture tube, is greater than 3 metres deep, or if it intercepts the groundwater, it may have to be registered with the relevant water authority. Individual water authorities could have different approaches to registration. Breaches of the Water Act are an offence that involves a financial penalty.
Registration requires a series of steps that generally involve:
- contacting the local water authority and applying for installation licence
- this licence will require maps of locations, depth, purpose and information regarding near by infrastructure. A dial before you dig, that locates underground infrastructure, power etc may also be required. If the person has never registered a bore, or a hole in the ground so to speak, then it is strongly advised they contact the water authority to determine the steps involved.
- the cost to register - roughly $400 for first hole and an additional $50 for second etc, but this may vary with the water authority.
The registration process exists for several reasons. For example:
- in an attempt to prevent pollution by aquifer leakage, this is why soil sampling is included in the registration process if groundwater is intercepted.
- to provide an identification number and location of groundwater bores, and generate funds to run the groundwater data bases.
- to ensure no illegal groundwater extraction.
The registration process also means that only a registered driller can install the holes or collect the soil samples. If you unsure of the groundwater depth in your region, then using a registered driller removes any risk.
There are generally no exceptions to the rule, however there are variations in how the Act is governed by water authorities, so contact with water authorities is essential. Also, as a low cost risk management strategy, it is recommended to contact the Dial Before You Dig website or hotline (phone 1100) and ascertain the location of any infrastructure (power, telecommunications, gas etc.).