DipWSET Theory: Vineyard Soils

First, let’s define what soil actually is. Soil is the upper layer of the earth and is made up of geological sediment. Sediment is another word for weathered bedrock—aka solid rock. Soil also includes organic remains in the form of humus as well the water and air found in the pores/space between the sediment. That’s…a lot of stuff. But wait. There’s more…

[Information based on DipWSET D1 material]

Soil is like an ogre—it’s got a lot of layers. Those layers were formed over different periods of time, so each will have its own textural and structural characteristics. So, that sediment you see at the very top, where your feet hit the ground, that could have been transported over time by water, wind, ice, gravity, etc. Meanwhile, the harder bedrock underneath—may have been formed in a similar way at some point, but now is buried beneath, solid and stuck in its ways.

For vines, soil is the basis of the root system, the “medium from which vines take up water and nutrients,” as my text explains. Just as importantly, soil houses an ecosystem—you know: bugs, animals, microbes.

Despite what side of the fence you’re on about the influence of soil on wine flavor and aroma (anyone want to use the word minerality, here?), it’s the soil’s physical composition that determines water and nutrient availability to the vine and, therefore, ultimately affects vine growth and berry ripening.

Physical Properties of Soil

The texture of the soil refers to the proportion of the mineral particles of sand, silt, or clay.

• Soils high in clay content are said to be “finely textured,” as clay particles are small. Smaller particles have a larger surface area to volume ratio, which means, they are more effective at holding water and nutrients than larger textured particles.
• Sand particles are relatively large so have a smaller surface area to volume ratio. This means they have limited space to hold water/nutrients and will end up draining that water through. Soil high in sand are said to have a loose texture, so it’s easy for vines to sink those roots quite deep.
• Silt particles are somewhere in the middle of clay and sand, sharing properties of both. 
• I bet you’ve heard the term loam several times before when discussing vineyards. Loam refers to a soil that has moderate proportions each of clay, silt, and sand.
• And where do those large rocks that are basically boulders making up the CDP fit in? Gravel or pebbles (which are actually bigger than gravel) can be found in soil. These guys will help improve drainage as well as lower overall nutrient and water-holding capacity.

Structure refers to how those mineral particles above aggregate, or crumb. How a soil aggregates—the size, shape, and stability of those crumbs—determines water drainage, root growth, and workability of the soil.

• Clay is sticky, so it’s crumbs make it hard for vines to penetrate. Makes sense since clay particles are the guys that like to hold on to water.

• Sand particles, or larger, (gravel, pebbles) are loose structured. In fact, soils high in these larger particle types need clay or the addition of humus (below) to help bind it together.

  

• Hummus is an organic matter found in soil that is formed by the partial decomp of plant material created by microbes and earthworm. Yes, earthworm poop. Earthworm poop is spongey, can absorb water and nutrients, and helps bind soil particles together. So if you’ve got a gravelly, pebbly, or sandy vineyard…get a pet earthworm or two.

So how does this all affect vine health? So glad you asked…

The texture and structure will determine how far vine roots can penetrate. Sandy, stony soils are loose, so vines will be able to dive quite deep and over a large area. So while the soils themselves are poor holders of water and nutrients, vines—if spaced out properly—can look further and make up for that lack of immediate availability close to the surface. On the flip side, clay-based soils are wet and sticky. There’s a lot of water available and not a lot of space to move around—some vines in specific conditions are ok with that.

That being said, the “right” soil type is also dependent on climactic conditions. For example, if you’re planting in an area that gets a lot of rain throughout the year, you may want to make sure you have free-draining soils to ensure good drainage. Let it be known, vines can drown—excess water in the soil, as in our lungs, will displace oxygen, impeding root respiration as well as killing the soil organisms. (Poor Earthworm Jim.) If this was the case, growers can install artificial draining systems. (NOTE: It’s easier to do this before the vineyard is established.) Conversely, if you’re in an area that gets very little rain, or only rain during a certain time of the year, you may want to plant in an area where the soil can hold onto that water until the vine needs it.

Soil, believe it or not, also affects availability of light and heat to the vine. Lighter soils will reflect sunlight back into the canopy. This is great for varieties that may have a particularly bushy canopy, as it can expose areas that would otherwise be shaded to much needed light. Conversely, darker soils absorb heat during the day, hold on to it, and then release it during the night. This will moderate the diurnal range (ie: the difference in daytime and night time temperatures), so can be particularly useful for those areas that experience extremely cold nights—the heat will keep the vine from shutting down in those cold conditions; grapes will continue to ripen—albeit slowly—throughout the evening hours.

If healthy soils makes healthy vines and healthy vines makes healthy grapes, then it stands to reason that poor soil health will effect the vine, the grapes, and ultimately the resulting wine. There are loads of tests that growers can conduct before a vineyard is established and/or on a regular basis on an existing vineyard. Soil health tests can indicate what, if anything, needs to be improved—be it the structure, nutrient level, water availability, etc.

And how can growers adjust these little inconsistencies in the soil. I am so happy you asked because that means you’ll be back for tomorrow’s post. 😉

Thanks again for studying with me!

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