When and How to Conduct a Soil Analysis
A good soil analysis is one that allows to improve or maintain soil fertility, to make decisions on what type of amendment or fertilizer to apply or to inform us about the actual availability of nutrients accessible to plants.
Observe by yourself
One should not underestimate the importance of gathering a handful of soil, smelling it, feeling its texture between your fingers, seeing if you can make a ribbon out of it. There is so much useful information to be gained simply by having your hands in the soil, and being able to contextualize the analysis returned from the laboratory.
The right time
The right time to take a soil sample for analysis depends on:
- The type of analysis you want to perform.
- What you want to observe (physical, chemical, biological).
- The pedoclimatic context.
- The type of crop (market gardening, arable farming, tree farming…).
Long-term monitoring is essential in soil analysis because it allows to understand what happens over time.
Autumn is an interesting period to carry out soil analyses:
- The soils are still warm and re-moistening: biological activity is at its peak between stimulation by covers or by returns of organic matter (residues, manure). Biological activity continues during autumn and winter. It slows down significantly when the soil is below 10°C. To be sure to have reliable indicators, it is better to take the sample on soil still somewhat warm.
- The analyses performed allow to anticipate winter inputs and notably to set up base fertilizer strategies.
You must anticipate the delay in receiving results to be able to take the analyses into account for your next decisions.
Take into account the analysis delay
This is an important factor since generally you need the analysis report before planting the crops to make decisions; some analyses are very quick, others, depending on precision and method, can take up to several months. You should inquire with the laboratory or your agricultural advisor.
For example, if you want to perform a Haney test, it is recommended to do the analysis a few weeks before crop establishment. It is a very good analysis to adjust nitrogen inputs, it was partly designed for that.
Another example, it is interesting to perform a chromatography before your installation or land purchase.
Frequency of analysis
Again, this depends on many factors:
- The added value of the crop.
- The amendment history.
- The soil fertility.
- The diversity of soils from a pedological point of view.
- The size and homogeneity of the plot.
- Your curiosity.
To take biological analysis into account, perform soil analyses at least 3 weeks after the last soil work or organic product spreading.
Added value of the crop
It goes without saying that depending on the profitability of the crop, the budget allocated for analyses will not be the same if you grow wheat or berries. The more profitable the crop, the more you can afford to do analyses on a small area.
For arable crops, it is currently recommended in France to do a (standard) soil analysis at least every 5 years, but closer monitoring with different analysis tools provides a more holistic view of the plot.
Some large vegetable crops are so profitable that it can be economically interesting to analyze the soil several times a year.
Amendment history
If you want to chemically rebalance your soils or increase the organic matter content with large inputs, then it is very important to regularly monitor soils with analyses to understand their evolution.
Soil fertility
When everything is fine, crops are in good shape: no disease problems, with a satisfactory production, then it is possible to do a soil analysis only every 5 years. It remains interesting to occasionally monitor the evolution of organic matter and/or biology.
Diversity of soils from a pedological point of view
Balances shift faster on a sandy soil than on a clay soil, so it is necessary to perform analyses more often on this type of soil.
Size and homogeneity of the plot
Homogeneity and plot size play a crucial role in obtaining precise and usable results. A homogeneous plot is an area where soil conditions (texture, pH, organic matter, etc.) are uniform. This allows obtaining soil samples that faithfully reflect the characteristics of the studied land.
To ensure this homogeneity, it is recommended to divide large plots into smaller subplots and ensure each subplot is homogeneous.
If the plot is homogeneous, it is advised to take one sample for a maximum of 10 hectares. If your plot shows variability, please take two or more distinct samples representing the most important zones.
Your curiosity
The more analyses you do, the better you understand your soil.
It is very interesting to cross-reference different types of analyses to better understand your soil: microbiology, total mineral content, organic matter fractions, chromatography…
Sampling depth
Sampling depth for soil analysis is a key factor to consider. In France, the usual standard is to sample from 0 to 20 cm layer, although there is no absolute consensus on this.
Choosing a greater depth risks diluting surface nutrients, which can be detrimental, especially in no-till agricultural practices. In this case, it is often recommended to sample from 0 to 15 cm, as fertility is generally concentrated there. However, to explore deeper layers to measure elements like phosphorus, a second analysis from 15 to 30 cm can be useful, especially if the soil surface horizon tends to dry out.
If the soil is very stratified and presents well-defined horizons, then it is interesting to measure these different horizons separately according to their respective depth. Be sure to indicate the sampling depth on the laboratory information sheet.
A residue to measure nitrates can be sampled very deep. The residue value is calculated from nitrate (NO3-) and ammonium (NH4+) quantities present in the 0-30 cm, 30-60 cm, and 60-90 cm horizons (sampling depth depends on soil depth).
Be supported by experienced field agronomists
To make informed decisions about soil analysis, it is imperative to rely on the expertise of a soil analysis specialist, especially if you are a novice in this field. Errors in sampling protocols are unfortunately common and can have significant consequences.
A poorly taken sample can lead to inconsistent results, which in turn can lead to wrong decisions. The ramifications of such errors can result in significant economic losses, far greater than the initial cost of the soil analysis itself.
Moreover, soil analysis professionals can choose the most suitable type of analysis for the project objectives.
Equipment
A simple spade to take soil samples may suffice, but an auger specially designed for this purpose will probably be a better option. Tubular augers can be found from 20 euros online, which can be an excellent investment if you perform analyses every year. For stony or rocky soils, an Edelman auger will be essential, although it is more expensive. Expect about 150 euros for a quality auger.
Protocol
You must respect the sampling protocol provided by the laboratory or supplier. Some laboratories express their values in kg/ha, so it is essential to respect the protocol so that the values displayed on the analysis report are representative of the sample, as is the case for Kinsey/Albrecht analyses.
The right place
Laboratories have very precise analysis methods. The biggest source of error in analyses is often related to sampling. You must choose a homogeneous and representative place of the plot, perform between 12 and 15 samples.
Keep in mind to take samples at the same place year after year to understand soil evolution. For large plots, it is advised to make GPS points to sample at the same points in following years.
It can be interesting to return to the same places to see how nutrients evolve over time, but sometimes, it may be necessary to take samples from other parts of the field depending on what you want to know.
Another point to consider is the way you sample the soil. You must take into account the placement of nutrients: a fertilization localized at sowing or a full-spread manure application, to obtain reliable and comparable results.
For perennial crops it is more interesting to sample in the inter-row rather than under the row, but there is no absolute rule.
Number of cores
Only 100g of soil will be used to perform the soil analysis over one or several hectares, the more cores taken, the more representative the sample will be of the plot. A minimum of 10 cores is necessary, about twenty is ideal.
