The importance of quality sampling for soil mineral nitrogen analysis
NRM
Agriculture
Investing time, money, and effort in taking samples for analysis, of any sort, is one of the first steps towards farming more sustainably. Sustainable production starts with knowing what you have and how to utilise resources without draining what nature provides.
Analysing the soil for nitrogen has proven tangible benefits. But you know what they say: rubbish in equals rubbish out. Using measurement to manage decision making should be the ambition, but those decisions must be based on reliable data.
To ensure quality analytical data is produced, there are important steps that must be followed to ensure that lab analysis adequately represents the supply of soil nitrogen in the field, which can vary spatially and temporally. To avoid pitfalls and get the best out of soil nitrogen analysis, read our expert guidance below.
#1: Planning
Soil mineral nitrogen (SMN) sampling requires a bit more planning than standard soil analysis. This first step seems obvious but deciding where across the rotation and which fields/crops to sample should be the first consideration.
Prioritising the fields where the soil nitrogen supply (SNS) is likely to be unknown, high, or low due to previous management, or where you notice previous crop performance was poorer than expected, is a good start. Or, as an alternative, select barometer fields across the rotation that are annually sampled to gain an understanding and build up a picture of soil mineral nitrogen levels across the rotation. It’s a good way of monitoring supplies from varying soil textures after each crop grown and enables you to apply the soil nitrogen supply across similar fields.
Once you have selected your fields, planning when to sample and booking the courier are the next steps. The advice is always to sample at the beginning of the week (Monday) and arrange courier collection for the next day (Tuesday) so your package gets to lab on day three (Wednesday) for extraction. There are two reasons for this timetable: to minimise sample degradation as your samples may be held over the weekend, and because the lab will not arrange courier collections for a Friday. So, avoid sampling later in the week to reduce the risk of analysis delay and the quality of the results.
Always plan to sample ahead of any nitrogen fertiliser applications, and do not sample within 4 to 6 weeks of any organic manure application. Sampling close to the first application of nitrogen fertiliser is generally a good time. Then you can adjust subsequent nitrogen applications based on the soil mineral nitrogen supply measured before the first was applied.
Once all the planning has been done, it’s time to head to the field.
#2: Taking a representative sample
Pay close attention to where you sample in the field, how many cores you take, and how deep you sample. You should also be careful to avoid cross contamination between sample depths.
Current RB209 guidance provides the following advice on how samples should be taken, including:
- Avoiding unusual areas of the field, such as headlands or where previous in-field manure heaps were sited.
- Sampling the area in question comprehensively by taking a minimum of 15-20 cores in a general representative W pattern.
- Ensuring that the bulk sample is mixed, but not overmixed before a sub-sample is taken.
- Excluding areas within the main sampling area that are different, due to soil texture or previous management.
- Taking more samples in larger fields greater than 10ha, particularly if the soil isn’t uniform.
- Keeping the samples fridge cold as soon as the soil cores have been combined and sub-sampled.
Many researchers have dived deeper to determine how important sample intensity and sampling pattern is for optimal results. Whilst RB209 suggests a minimum of 15-20 cores, a study by Marchant et al (2012) concluded that the ideal number of soil mineral nitrogen cores required relates to the size of the field and the likely soil nitrogen supply. The following table describes the number of cores that should ideally be taken in relation to field size and expected soil nitrogen supply.
Taking more samples can be time consuming, but this study suggests doing so is cost-effective across larger fields because of the potentially greater total yield and therefore financial return. Increasing the number of soil cores is also suggested when the soil nitrogen supply is expected to be high because of the risk of large within-field variability.
The study concluded that, for a 60-hectare field with an expected soil nitrogen supply of 275 kg/ha, the most profitable sampling strategy included taking around 23 cores. This is slightly greater than the maximum core number recommended in RB209, although the guidance advises splitting fields of more than 10ha. A smaller number of soil cores was found to be optimal for fields of 20ha or less, particularly if a soil nitrogen supply of up to 125 kg/ha was expected, compared to the 15-20 cores recommended in RB209. This shows that the accuracy of soil nitrogen measurement doesn’t increase by taking more and more soil cores, particularly on smaller fields.
From a cost-benefit perspective, AHDB’s project “Establishing best practice for estimation of Soil N Supply” concluded that taking 15-20 soil cores from a 10ha field is excessive. For most fields, 10 cores are sufficient. More are only required where fields are variable, greater than 30ha, or where the soil nitrogen supply is expected to be high (more than 150 kg/ha).
As experts in soil analysis, we recommend following the latest sampling approach advice in RB209. This guidance is continually updated and provides guidance based on a broad number of research projects. Click here for the latest guidance.
Soil depth & sample mixing
Ideally, on mineral soils, soil cores that are manually sampled should be taken to a total depth of 90cm in 30cm depth layers, or to the topsoil depth if the soil is shallow. A series of gouge augers that get progressively narrower in diameter are recommended to reduce the risk of cross contamination with the soil depth above.
Mechanised sampling to depth, in contrast, typically relies on a 1m long gouge auger of constant diameter. This minimises the risk of cross-contamination between soil depths, as long as the core is taken in one bite.
Thorough mixing of soil samples has been shown to lead to enhanced mineralisation within
the soil sample and therefore should be avoided. In an ideal world, whole bulk samples should be sent to laboratory to avoid the need for sub-sampling in the field. However, bulk samples will often be too large, and subsampling will still be required. The best way to sub-sample and reduce the risk of increased mineralisation is to mix only enough to ensure a homogenous bulk sample and then representatively sample many small portions of soil from that bulk sample.
NRM recommends that a sub-sample of soil is sent to the laboratory for analysis, as opposed to the larger bulk field sample. However, the sub-sample should be gently mixed and smaller portions of soil should be sampled from the larger bulk sample and sent to the lab for analysis.
#3: The importance of keeping your samples cold
To maintain the quality of the sample, it must be placed in a cool box with ice packs as soon as it has been taken in the field. The reason for this is to minimise the risk of mineralisation before the sample has reached the lab, which will distort the amount of soil mineral nitrogen measured. NRM provides insulated boxes and ice packs that keep the soil fridge cold.
In the same AHDB project, studies over a two-year period were conducted to look at the effects of mixing and the effects of the temperature of the sample up to and after 24 hours after sampling on soil mineral nitrogen levels. They also looked at how soil mineral nitrogen levels differed depending on how long the sample took to get to the lab.
Samples were subjected to either an ambient temperature or cooled to 2-4°C, and extracted either within 36 hours, 3, 7 or 14 days in year 1. In year 2, samples were extracted within 6-24 ho
urs, 2, 4 or 7 days. At this time, the study also looked at samples that were thoroughly mixed, compared to no mixing. These samples were kept at 2-4°C and extracted within 36 hours.
In year 1, with one exception, increased with time regardless of the storage temperature. This increase started within 3 days of the sample being taken. However, the rate of increase was much slower for the samples kept at 2-4°C. In year 2, an increase was once again recorded for all samples that were kept between 4 to 7 days before extraction. This was greatest where samples were stored at ambient temperature. Soil mineral
nitrogen also increased in samples extracted between 1 and 4 days after sampling. One sample showed little change, and another showed a small decrease.
Where soil samples were thoroughly mixed and extracted in year 1, significantly higher amounts of soil mineral nitrogen were measured. In year 2, no effect of mixing was seen. The authors suggest that this was down to a colder temperature in year 2 when the samples were taken and mixed compared to year 1.
The study concluded that storing samples at colder temperatures helps to maintain sample quality and minimises sample degradation. However, this doesn’t prevent the need for rapid analysis, and demonstrates how critically important it is to get the samples to the laboratory in the shortest time possible. This is why planning is so important for accurate analysis, which will help you make the right decisions for your farm.
How NRM can help
Planning, soil sampling efficacy, and getting the samples to the lab as soon as possible is the best approach to ensure you can capitalise on the investment of soil mineral nitrogen analysis. Getting these steps right is the key to better results and management of crop nitrogen requirements.
NRM offers two soil nitrogen analysis services: N-Check and N-Check Plus. Whilst N-Check measures soil mineral nitrogen, N-Check Plus includes soil organic matter analysis to provide an estimation of mineralisation. If a spring crop nitrogen value (winter sown crops) is supplied on the sample request form, then a soil nitrogen supply category can be reported alongside the result, ready to be used within RB209.
Click here to contact NRM directly about its N-Check services or speak to your agronomists in good time about soil nitrogen sampling.
References
RB209 Fertiliser Guide
Hoad, SP; Daniel Kindred; Stuart Knight; Pete Berry; Roger Sylvester-Bradley; Damian Hatley; Nathan Morris; Charlotte White (AHDB, 2007-2011), Establishing best practice for estimation of Soil N Supply. Available at: https://ahdb.org.uk/establishing-best-practice-for-estimation-of-soil-n-supply.
Marchant, B.P., Dailey, A.G. and Lark, R.M. (In Press). Cost-effective sampling strategies for soil management. HGCA Report for Project 3189. Stoneleigh: HGCA.
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