Soil sampling is easy, isn’t it? Walk into a field, take some soil, send it to the lab, get the results. Job done.
Unfortunately, it’s not quite as simple as that and there are many factors to be aware of when planning a soil sampling and nutrient management strategy. The aim of this article is to try and demystify the soil sampling and analysis process to ensure you get the most out of your sampling, analysis and fertiliser spend.
Sampling should be a simple process, but different approaches, tools, and methods promoted by various suppliers can cause confusion.
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Q. Why sample?
A. The aim of soil sampling is to find and correct any yield limiting factors in the field before they cause yield loss and to ensure the correct amount and type of lime and fertiliser is applied.
Some elements affect yield more often and more seriously than others, e.g. both low and high pH causes yield loss.
Soil indices aren’t the whole story, soil depth and rooting depth are as important to ensure adequate crop nutrition, however indices are a guide and target that can be modified to suit your own situation.
Ultimately, as in most of agriculture, what is appropriate for one farm isn’t necessarily the correct approach for others. One size definitely does not fit all, so its useful to understand the factors involved in soil sampling and analysis.
What are the different sampling strategies
The aim of any sampling plan has to be able to return to the same field, sample again in the same way and get a similar result.
The spatial variation in fields and soils make this difficult as too few samples mean that variations in soil chemistry can’t be mapped properly.
Each element has different variability and causes of variability, different economic costs of sampling, different affect on the crop and therefore a different payback for sampling. In the past, the same soil sampling strategy has been used for each element but this has been superseded by an approach that tailors the number, and therefore cost, of sampling and analysis of each element to their economic and agronomic benefits.
Scotland and the North of England generally have acidic soils while, in many areas of England, alkaline soils dominate in the chalk, limestone regions, so it makes sense to tailor your sampling strategy to your own local conditions.
You wouldn’t take four pH only samples per ha in an area with chalk as its parent material, but it makes sense in our Scottish acidic soils where one of the main yield limiting factors can be soil pH, and lime is a significant cost.
Q. Why do we need to take more than one sample per field?
A. We all know and see every day that soils and crops can vary dramatically over the field, going from a high yielding to a poor area in a few meters. In some cases, this is due to the changes in soil texture, slope and aspect left behind when the glaciers retreated 10,000 years ago.
However, we have changed the soil by farming it over centuries and have created a lot of field and soil chemistry variability due to the amalgamation of small fields into large ones, crop nutrient removal and application and spreading of lime, fertilisers and fym. Grazing animals can move large amounts of fertility. Often areas where they gather often have higher fertility than the rest of the field. The good news is that as we have created the variation, we can fix it by targeted applications.
You can predict the variation of some elements, not others. When we sample a field using high resolution grid pH sampling, we can often see features in the pH map that have been created decades ago, but are still there and affecting crops today.
Field gateways, where lime may have been dumped years ago, still show up as high areas. Other spreading features like strips where the spreader hasn’t been set properly or where the extra last few tonnes of lime left in the spreader have been double spread, all still show up clearly. If you could see this soil chemistry with your eyes then we would all naturally spread lime and fertilisers variably! However, you can’t generally predict where soil pH is likely to changes, so a high resolution grid approach is often the best approach.
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Soil analysis methods
As Scotland and the North of England have similar soil parent material leading to generally acidic soils while Southern England tends to have evolved from basic parent material leading to generally more alkaline soils, the advisory and analysis services in each country recommend different chemical extraction methods to ensure that the results accurately reflect plant available nutrients in each location.
The Scottish method was defined by the Macauley soil science lab which uses pH measured in a buffer solution and a modified Morgans extraction method for other nutrients. In England ADAS defined the best extraction methods for their alkaline soils and use pH measured in water and Olsen phosphate extractions. Why does this matter? If, in Scotland, you use a method designed for alkaline soil then it’s likely that the results won’t reflect the plant available nutrients. Choose the correct analysis type for your location. However, as always, consistency is key, so if you have been using the same extraction method for years, its best to stick with it.
Q. Why use the same sampling strategy for all pH, macro and micro nutrients?
A. In the past we would usually have a single soil sample analysed for pH, macro and micronutrients, as needed. However, this can be wasteful as a more tailored approach that better reflects soil chemistry variation, costs of sampling and economic benefit in the removal of crop limiting factors, has proved to be a better approach.
PH in the field varies quickly and significantly over short distances due to lime spreading history, old field boundaries and soil types. In addition, low and high pH’s cause yield limitations while phosphate and potassium tend to vary less than pH as they are often linked to years of crop offtake and replacement by fertiliser application. It’s common to see areas of fields where the low P and K indices are in the high yield areas and vice versa due to years of spreading a single rate of fertiliser across the field.
However we can use this to our advantage as with the macronutrients, all we need to do is to ensure that there are no low indices that are limiting crop yield. So a popular sampling strategy for macro nutrients is to target a smaller number of routine samples (usually P, K, Mg, Ca and organic matter) to old field boundaries and low and high yielding areas within them, where experience tells us that the variation tends to be.
Micronutrient sampling is more expensive still and many use the approach of one micro nutrient sample per field provided that the sample is taken at the same time as pH and routine samples and uses the spare soil from the high resolution pH samples, so it is truly representative of the whole field.
Q. So what’s best?
A. Generally, for arable areas of Scotland and Northern England, a strategy of four pH only samples per hectare on a 50 meter grid, with zoned samples based on old field boundaries and yield zones for macronutrients and OM, with one sample per field for micronutrients, seems to work well and provides the best compromise between sampling cost, accuracy and repeatability while spending the largest share of the cost on the element most likely to be limiting yield. All these samples should be taken at the same time to minimise labour costs and ensure the zone and per field samples have soil from across the field for the best possible sample reliability and repeatability.
In grassland, depending on the type and intensity of management the same principles apply but we suggest dropping the number of pH samples down to one or two per ha while targeting P, K, Mg, Ca and OM to zones or whole fields with micro nutrient samples remaining at one per field. Of course, as discussed above, each farm and rotation is different so its always best to tailor the approach that suits your farm.
Sample timing
Soil chemistry varies over time both within the growing season and over years, so samples should be taken at the same time of year in the same place in the rotation, ideally before the most pH sensitive crops. We prefer to sample when soil is cool and wet to ensure consistency of results as during the summer, plant growth, root exudate, microbiology and nutrient extraction by the crop can affect sample results. Test pH sensitive crops in advance then choose when to apply lime.
Consistency is key. Same sampling locations, same lab, same place in the rotation, same time of the year.
That way comparisons can be made with previous sample results.
It is possible to sample the whole farm at once, say every four of five years, however this strategy makes it difficult to compare results when each field has been sampled at a different time in the rotation.
Lime quality and type
Lime and drainage were traditionally the foundation of a productive farm and are just as important today.
When buying lime, remember, to be sold as agricultural lime, lime must meet the legal standards for neutralising value and fineness of grind.
In addition, the parent material is key as it sets the rate of breakdown of the larger lime particles giving a slow, controlled release over the rotation.
It’s also helpful to ensure you analyse for magnesium and calcium to help choose between calcium and magnesium limes. Magnesium lime is 5% magnesium but also has large amounts of calcium.
Both magnesium and calcium are macro nutrients and are essential for plant and animal growth.
Q. There has to be a better way?
A. Many methods have been tried over the last 30 years to use soil geophysical sensors to map field chemistry. These sensors were designed to use soil conductivity and naturally occurring soil gamma radiation to map soil parent material and soil particle size, at high resolution, relatively cheaply, compared to physical texture sampling and are generally excellent at mapping soil textures.
However, none of them actually detect soil chemistry and are therefore poor at mapping soil chemistry variation in fields, as they rely on there being a correlation between soil texture, parent material and soil chemistry. In countries that have had a shorter agricultural history than the UK, like Canada, Australia and Argentina, soil chemistry is still linked to soil texture and parent material.
However, in the UK and Europe, we have broken the link due to centuries of cropping and application of lime and fertilisers. So, in general ,experience tells us that to get a reliable and repeatable result, its best to spend the budget on more soil chemistry samples processed in a reputable laboratory and sampled in a consistent, planned strategy.
Yield maps from arable and grass crops can be used to estimate nutrient offtakes and make it easy to create variable rate fertiliser application maps which reduces the need for multiple P and K soil samples, once you have checked that there are no low indices in the field.
A refinement of this method is to take targeted grain samples just before harvest in the high and low yield areas. Research has shown that critical grain nutrient concentrations can indicate yield limiting deficiencies, particularly in nitrogen and phosphate.
Soil biology and soil carbon
Soil biology sampling is gaining popularity, ranging from a simple organic matter test to a full DNA analysis of the soil components.
However, it can be difficult to interpret what the results mean in practice and how to improve them, so most of the time, soil organic matter is used as a proxy for soil health. Organic matter and organic carbon are often used interchangeably but carbon is only one component of organic matter. Organic carbon is roughly 58% of organic matter. In Scotland, where we don’t have too many soils with inorganic carbon in them like chalk, coal and limestone, the loss on ignition test is a simple, low cost method of organic matter analysis.
If you do have inorganic carbon in your soil, you need to use a more expensive test like Dumas, which measures organic carbon. Carbon stock measurement for the sale of carbon credits is a difficult sampling problem due to the variation of soil carbon across the field and down the soil profile. To convert from a soil carbon test to a carbon stock you also need bulk density, which is difficult to do well, so sampling for carbon credits is a complicated, specialist subject, best left to companies who are equipped for the job.
Help with sampling costs
The Scottish Government Preparing for Sustainable Farming (PFS) scheme offers funding for carbon audits, soil sampling and analysis, animal health and welfare interventions and finishes at the end of the year.
Another option to keep costs down is to take the samples yourself and soil sampling software is available on your mobile phone to help you take and map samples, report them and make variable application maps if needed. Scottish Government will contribute to your time spent soil sampling under the PFS scheme.
Conclusion
Hopefully, you can now choose the best sampling strategy for your land and business.
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