Soil mineral nitrogen testing: Practice and interpretation

Obtaining a reliable estimate of the Soil Nitrogen Supply (SNS) can be an important step in optimising nitrogen fertiliser doses, or quantifying potential losses to the environment. Where high or uncertain amounts of soil nitrogen are present, direct measurement of available Soil Mineral Nitrogen (SMN), as nitrate or ammonium, has been advised in preference to predictions based on previous crop, rainfall and soil type. However, a lack of confidence in test results, due to variation in the values indicated by analyses performed at different laboratories, and failure to meet expectations as to their accuracy as predictors of optimum nitrogen fertiliser dose, mean that this potentially useful tool could be under-used. The aim of this review was to examine how SMN analysis has evolved since its development, to identify possible causes of error and variation, and to re-define how best to utilise the technique.

Research in Germany in the 1970s found large differences between soils in the amount of SMN, even following the same previous crop. The highest accumulation typically occurred in early spring, with evidence that wheat was able to utilise this to at least 100cm depth. Soil nitrogen within rooting depth was found to contribute to crop requirement as effectively as applied nitrogen fertiliser.

Subsequent studies in the UK have identified significant seasonal variation in SMN, linked to soil type, previous crop, fertiliser use and weather, confirming direct measurement to be important. Current guidance is to test medium or heavy soils in autumn or spring, but for high rainfall areas or light soils to test in late winter or spring. Published research suggested that autumn sampling provides a better guide to optimum applied nitrogen dose. However, consultation revealed that spring sampling is considered by most to be preferable, as this removes the uncertainty of winter losses. The most appropriate time will depend on the purpose for which the information is being obtained, and the likely balance between net mineralisation (from crop residues or organic matter) and losses (due to leaching or denitrification). Errors in sampling or analysis were considered the most likely cause of very large differences that have been found when testing at different times in the spring.

Research has shown that SMN is present throughout the 0-90cm soil profile (or deeper), with at least half of the total at below 30cm depth. However, the proportions that are present in each layer can vary considerably. Consultation revealed that sampling to 60cm depth was considered essential, but views differed on the value of sampling to 90cm, even in the spring (as current guidelines suggest). SMN at 60-90cm depth has been found to be closely related to the amount present at 0-60cm, with prediction of optimum applied nitrogen doses not improved by directly measuring this. For manual sampling, a minimum of 10 replicate cores is recommended for homogenous sites. Areas known to have differing soil types or field histories should be sampled separately. The introduction of mechanical sampling has allowed a higher sampling intensity of 15-25 cores per 10ha field to be used. Careful mixing and sub-sampling is necessary to ensure a representative sample for analysis. Most laboratories advise that, for SMN testing, samples are analysed as soon as possible. The samples should ideally be kept at the same temperature as they were in the ground, which may require refrigeration and transport in insulated containers. Freezing has been used for long storage, but samples must be analysed immediately upon thawing as increased mineralisation is possible. Research in the USA has suggested that air drying at room temperature is a more reliable method for preserving nitrate levels in low mineral N soils.

The standard procedure for analysis of available soil nitrogen is well documented, and consists of extraction with KCl, filtration of the extract, analysis by colorimetry, and conversion of nitrate and ammonium ppm to kg/ha based on bulk density of the soil. At each of these stages there is the potential for variation, but in particular bulk density could vary by +/-20%. Consultation revealed strong support for the re-introduction of ring testing or an accreditation scheme for SMN testing. It is widely acknowledged that mineralisation of organic matter can make a significant contribution to the SNS, and it is likely that this accounts for most of the variation in optimum applied nitrogen dose that cannot be explained by SMN status. Current guidelines suggest that net mineralisation should be small in mineral soils of low or average organic matter content, but research has not always supported this. Various methods, including incubation, modelling and chemical analysis, have been explored as a means of determining Potentially Available Nitrogen, but no single approach has universal support.

SMN testing is not recommended on peat soils (due to high net mineralisation), established grassland or in the first year after grassland is ploughed out, or within 3 months of organic manure applications. In these situations, knowledge of previous nitrogen fertiliser use, or the available nitrogen content of manures or other nitrogen-rich waste, may be a more useful guide. Previously, sampling on sandy or shallow soils has been considered less valuable than on nitrogen retentive medium or heavy soils, but recent milder and drier winters have questioned this. In Scotland, where light soils are more prevalent, and rainfall is higher, SMN testing is considered less reliable as a guide to optimum nitrogen fertiliser doses in spring, but is used to quantify soil reserves remaining post harvest to meet autumn needs.

The accuracy level for SMN tests was generally assumed to be within 10-20% (or 5-20 kg/ha) of the total, on 70-80% of occasions. Predictions of mineralisable nitrogen, or optimum nitrogen fertiliser doses based on SMN results, were felt likely to be much less accurate. There were differing views on how best to use SMN information, in particular the importance of results obtained for individual fields compared to overall trends year on year, or in like for like soil/crop situations.

Research suggests that a single measurement of SMN in late winter or early spring is a good indicator of the likely nitrogen capture by an unfertilised crop over the growing season, with effective recovery of 100%. However, there have been opposing conclusions as to the efficiency with which SMN will be recovered compared to fertiliser nitrogen. In practice this is likely to depend on where the nitrogen is located in the soil profile, the effective rooting depth of the crop, and available moisture at that depth. Maximising the uptake of nitrogen present at depth is important, as this can provide a useful buffer during periods of summer drought, and if not taken up could be most at risk from loss by leaching.

Although the review revealed some widely differing views as to when and how best to determine soil mineral nitrogen, and how to interpret the information gained, it was concluded that:

• SMN results are a reasonable guide the amount of available nitrogen present in the soil at the time of testing, but differences of less than 10-20% (5-20 kg N/ha) should be ignored.
• For most mineral soils, testing once in late winter or spring provides a satisfactory guide to the likely soil nitrogen supply during the growing season, in the absence of applied fertiliser nitrogen.
• For soils with a high indigenous organic matter content, where significant quantities of nitrogen may be mineralised, testing in the autumn might give a better guide to the rolling soil supply.
• SMN testing has a valuable role in quantifying potential nitrogen losses, and in avoiding or identifying significant over-application of fertilisers. However it is only an approximate guide to optimum doses of applied nitrogen, and is likely to be more than 30 kg/ha out in 1 in 3 situations.
• The efficiency with which SMN is utilised relative to applied fertiliser nitrogen when both are present is crucial. A lack of certainty about this undermines the value of SMN measurements.
• Assuming that current fertiliser use is adjusted for crop and soil type, SMN testing is unlikely to give an economic benefit where it varies by no more than 30 kg N/ha in the majority of years, or where reserves are unlikely to exceed 100 kg N/ha.

In order to increase confidence in the reliability and interpretation of SMN test results, the following actions are recommended:

1. The introduction of a unified set of guidelines or best practice code for SMN testing, to include what and when to sample, what to analyse, and how to interpret the information.
2. The re-introduction of ring-testing, or implementation of an accreditation scheme, for SMN analysis, to eliminate laboratory procedural differences as a cause of variation.
3. The inclusion of a statement on all test results indicating the likely accuracy of the information, and their limitations as a guide to optimum doses of applied nitrogen fertiliser.
4. Careful matching of sampling depth and timing in relation to the information sought, the crop and establishment date, seasonal rainfall pattern, soil type and organic matter content.
5. Full account should be taken of the amount of nitrogen already in the crop at the time of SMN testing. The tendency towards milder winters and earlier drilling of wheat underline this need.
6. Further research is needed to better understand the interaction between, and relative recoveries of, fertiliser nitrogen and soil nitrogen present at different depths, within a single season.
7. There would be a benefit from further research to improve the ability to predict accurately release of nitrogen from soil organic matter, under field conditions and in a wide range of situations.