What Is Happening Below 15 cm? The Importance of Subsoil Testing in Vineyards.

When a vineyard is soil tested, the focus is often on the top 10–15 centimetres of soil. It is a logical place to start. This is where organic matter, fertiliser inputs, biological activity and much of the fine root activity are concentrated.

But for an established vineyard, looking only at the surface can provide an incomplete picture of the soil environment the vine is actually growing in.

Grapevines are capable of developing extensive root systems that explore considerably deeper than the conventional soil sampling zone. Research has demonstrated that grapevine roots can occupy soil layers well below the surface, with fine roots responsible for much of the water and nutrient uptake occurring throughout the soil profile. A recent study of ten-year-old vines found substantial root density through the 40–60 cm layer, with roots extending to approximately 1 metre in some areas. (PubMed Central (PMC))

This makes understanding the subsoil particularly important when trying to explain vineyard variability, poor vine performance or unexpected tissue analysis results.

The soil profile is not uniform.

One of the fundamental problems with relying exclusively on a surface soil sample is the assumption that the soil profile is chemically and physically consistent with depth.

It rarely is.

As soil depth increases, pH, organic matter, nutrient availability, cation exchange capacity, salinity, sodicity, texture, structure and water-holding characteristics can all change. A surface soil may appear nutritionally adequate while the underlying B horizon contains a very different chemical environment.

The Australian Wine Research Institute recommends separating surface and subsurface samples because the physical and chemical properties of these layers can be substantially different. Its vineyard sampling guidance suggests sampling at approximately 5–15 cm, 25–35 cm and 55–65 cm where soil horizons are not obvious. (AWRI)

This is particularly relevant in vineyards because vines are perennial. Unlike an annual crop, grapevines are repeatedly interacting with the same soil profile for decades.

Nutrients can look very different with depth.

Nutrient concentrations are not necessarily uniform throughout the soil profile.

Phosphorus, for example, is generally relatively immobile and can accumulate in surface soil following repeated fertiliser applications. Potassium can also be concentrated in particular soil layers, while nitrate is considerably more mobile and can move deeper into the profile following rainfall or irrigation.

This means a surface sample can sometimes suggest that a nutrient is adequate when the deeper soil available to the roots tells a different story.

The reverse can also occur. A nutrient may be relatively low in the surface layer but more available deeper in the profile.

This is one reason soil testing should not be interpreted as simply producing a number that determines how much fertiliser to apply. The location of that nutrient within the soil profile matters.

Nitrogen is a particularly good example.

Nitrogen illustrates why depth matters.

Nitrate is highly mobile in soil and can move below the primary surface root zone following significant rainfall or irrigation. A surface soil test taken several months later may therefore provide little indication of how much nitrate has moved deeper into the profile.

For a vineyard, this can have significant implications.

A grower may observe low vine nitrogen status and respond by increasing nitrogen fertiliser. However, if the underlying issue is that nitrogen has moved into a deeper root-accessible layer, or that roots are unable to access it because of compaction, waterlogging or another soil constraint, simply applying more nitrogen to the surface may not solve the problem.

This is where combining soil testing with petiole or tissue analysis becomes particularly powerful. Soil analysis tells us what is present in the soil, while tissue analysis provides evidence of what the vine has actually acquired. The University of Minnesota Extension similarly recommends using both soil and tissue testing because they answer different questions about vineyard nutrition. (University of Minnesota Extension)

Subsoil pH can be just as important as surface pH.

Soil pH is another area where depth can make a substantial difference.

A vineyard may have a relatively favourable surface pH while the subsoil becomes increasingly acidic or alkaline with depth. This can influence nutrient availability, root development and the effectiveness of fertiliser applications.

Subsoil acidity can restrict root development and alter the availability of nutrients such as phosphorus, calcium, magnesium and molybdenum. Conversely, alkaline or calcareous subsoils can influence the availability of iron, zinc, manganese and phosphorus.

Importantly, these problems cannot always be corrected simply by applying a fertiliser.

If the underlying issue is a chemical or physical constraint within the root zone, adding more of a particular nutrient may have little effect.

This is one of the reasons the AWRI recommends investigating the soil profile when assessing vineyard soil health. Their guidance recommends soil pits to approximately 1.2 metres where appropriate, with separate sampling of the topsoil and subsoil horizons. Subsoil sampling is considered particularly important in duplex soils where changes in structure and drainage can create significant constraints to vine growth. (AWRI)

Salinity and sodicity can hide below the surface.

Subsoil testing is also extremely valuable for identifying salinity and sodicity.

Salts can accumulate at depth as water moves through the soil and subsequently evaporates or is taken up by the vine. Sodium can also accumulate within particular soil horizons, potentially degrading soil structure and reducing infiltration.

This is particularly important in irrigated vineyards and in regions where groundwater contains elevated levels of salts.

A surface soil sample may therefore underestimate the severity of a deeper salinity problem.

The result can be a vineyard where vines appear to have adequate water and fertiliser, yet continue to show poor vigour, inconsistent growth or nutrient deficiencies. In some cases, the underlying problem may not be a lack of nutrients at all, but a root environment that is restricting nutrient and water uptake.

Physical soil constraints are part of the nutrient story

Nutrient analysis should never be considered independently of soil structure.

Roots require oxygen, water and physical space to function effectively. A compacted layer, dense clay horizon, hardpan, calcrete or seasonally waterlogged layer can restrict root penetration and alter where roots are able to explore.

Research into grapevine root distribution has demonstrated that higher soil density and compaction can limit root development at depth. Conversely, where surface competition occurs from cover crops, vines may develop greater root exploration deeper within the profile. (PubMed Central (PMC))

This creates an important distinction between nutrient availability and nutrient accessibility.

A soil test may show that a nutrient is present, but if the roots cannot effectively explore that soil layer, the nutrient may have limited value to the vine.

Sampling the subsoil.

Subsoil sampling needs to be done carefully because contamination between soil layers can significantly distort results.

The Australian Government’s Agriculture Victoria guidance recommends sampling at least two separate subsoil depths where subsoil assessment is required and stresses the importance of preventing material from one soil layer contaminating another. (Agriculture Victoria)

For an established vineyard, a practical approach is to sample according to the actual soil profile rather than blindly applying the same depth across every vineyard.

For example, samples might be collected from 0–15 cm, 15–30 cm, 30–60 cm and 60–90 cm where the soil profile and rooting depth justify it. In other situations, sampling according to identifiable soil horizons may provide more useful information.

The sampling location is also important. A sample taken directly under the vine, in the mid-row or between the two may represent very different environments because irrigation, fertiliser placement, cover crops and root distribution are not uniform across the vineyard.

What should be tested?

A useful subsoil analysis should go beyond simply measuring N, P and K.

Depending on the vineyard and its history, analysis can include pH, electrical conductivity, exchangeable cations, cation exchange capacity, sodium, calcium, magnesium, potassium, phosphorus, sulphur and a range of micronutrients. Where problems are suspected, additional analysis for chloride, boron, carbonate, sodicity or other site-specific constraints may be appropriate.

Physical assessment should also accompany chemical analysis. Texture, structure, compaction, effective soil depth, drainage and evidence of waterlogging can be just as important as the nutrient concentrations themselves.

Soil testing and tissue testing work together

Perhaps the most important point is that subsoil analysis should not replace plant tissue analysis.

It should complement it.

A soil test tells us what the soil contains and provides information about the chemical environment surrounding the roots. Tissue or petiole analysis tells us what the vine has actually taken up.

When the two are considered together, they can provide considerably more information.

For example, a low tissue concentration of potassium combined with adequate surface-soil potassium may prompt further investigation of subsoil potassium, soil pH, calcium and magnesium relationships, salinity, root distribution or soil moisture. Similarly, low boron in tissue does not automatically mean that boron should simply be applied. Understanding where boron is located within the profile and how the vine is accessing it provides additional context.

This approach moves vineyard nutrition away from simply asking, “What fertiliser does the vine need?” and towards the more important question: “Why isn’t the vine accessing what is already present?”

Looking deeper can change the management decision.

The greatest value of subsoil testing is not the additional numbers on a laboratory report. It is the ability to understand the vineyard as a three-dimensional growing environment.

A surface sample might suggest adequate fertility, yet a deeper sample could reveal high sodium, elevated salinity, low calcium, an acidic horizon, a severe nutrient imbalance or a major change in soil texture.

That information can fundamentally change the management response.

Instead of automatically increasing fertiliser rates, the solution may involve improving drainage, addressing sodicity, changing irrigation management, modifying cover crop competition, correcting pH or investigating a physical root restriction.

For premium wine production, where vine balance and consistency are critical, understanding what is happening below the conventional sampling depth can therefore be extremely valuable.

The soil beneath a vineyard is not simply a reservoir of nutrients. It is the environment in which the vine’s root system must function for decades.

If we only analyse the first 10 or 15 centimetres, we may be making nutritional decisions based on only a fraction of that environment.

Good vineyard nutrition starts at the surface, but it shouldn’t necessarily stop there.