Depth Of Soil: The Three Layers To Consider
Soil is one of the main factors that influence wine quality. Not because it directly gives taste and flavors to the grapes but because vines grow better in a certain type of soil.
Indeed, there is no reference or scientific proof that explains the concept of Terroir, which is a very abused word and more a marketing concept than anything else (at least from our humble point of view).
Despite what adherents of the concept of Terroir say, vines will grow and winemakers can make good wine in almost any type of soil (Stephen Skelton MW, 2020).
So let's analyze the composition of the soil and understand why this can be important.
The Three Layers
Topsoil
The topsoil provide the interface between the grower and the plant.
Being the surface that the sun warms, the topsoil is particularly important in promoting the growth of the vine at budburst. The quicker the soil warms up to 10°C, the quicker budburst will occur and the longer will be the growing season.
The color is important too. Specifically, darker soils will warm faster and retain more heat.
Soils that are wet and that dry out slowly will be slower to warm. Cold and dump soils will not favor the formation of the "cytokinin hormones".
Cytokinin Hormones
It affects cell multiplication and the growth and development of both shoots and flowers. Vines low in these hormones will be slower to come into leaf, produce less fruit, and ripen it more slowly.
Source: Viticulture, Stephen Skelton MW, 2020 (pag. 39)
These are the main things to remember when it comes to topsoil:
It's usually friable;
It will immediately absorb any rainfall or irrigation water falling into it;
It contains the majority of the organic matter in the soil;
Contains the soil organisms (yeasts, molds, fungi, bacteria, protozoa, worms...) that are all crucial in healthy soil;
It can be shallow or deep;
Let's focus a bit on the last point in the following table:
Shallow (150 mm or less)
It has limited water-holding and nutrient capacity
Shallow topsoil over chalk may be very satisfactory for vines as vines' roots will be able to penetrate into it (which is both friable and water retentive)
Shallow topsoil over impervious material (clay or rock) isn't so suitable
Deep (up to 1 m)
It may be too fertile leading to excess lush growth at the expense of fruiting
Deep topsoil is less affected by rainfall and or irrigation, fertilizers, and any nitrogen realized by the breakdown of organic matters.
Subsoil
It's here that the vine will hopefully place the majority of its roots.
It's less friable, nevertheless, it must be friable enough for roots to penetrate and be open enough for the root hairs to grow in the early summer;
It contains less organic matters. However, it should provide sufficient flora and fauna to ensure aeration and drainage;
Vines can root as deep as three meters;
Soils rich in stone and gravel are very suitable for vines because they will encourage deeper rooting;
Root hairs
The vine has several main roots that branch in different directions and from those main roots, smaller feeder roots (root hairs) grow. Through the feeder roots, the vine will seek out water and nutrients. These smaller roots can die off once the resources in an area have been exhausted, while the main root will continue to produce smaller roots in other directions continuing the search.
Source: Viticulture, Stephen Skelton MW, 2020 (pag. 5)
Keep in mind that vines with deep root structure wil be less affected by periods of high or low rainfall, and will be generally less vigorous than shallower rooted vines.
Subsolum
Usually few roots will penetrate here. In fact this layer is at least three meters below the surface.
It's important that water should be able to drain away and not cause water logging;
A soil with an impervious layer may be less suitable for vines. It's wise to investigate vineyards' soils before planting the vines. By excavating trial holes, vinegrowers can assess the suitability of the soil for vine growing.
Conclusions
Depth of soil is for sure an important factor to take into account when selecting the site for grape growing.
This article can be a bit boring at first, but trust us when we say that it will set you apart once you understand it. This level of knowledge is required to pass a specific module of the WSET Diploma and it's good if you start familiarizing yourself with it.
We tried as always to keep it as easy and fun as possible but it is what it is. The topic is hard and there is nothing we can do. So, you could deal with this topic with your wine study group, or perhaps you could make an extra effort and think about opening a nice bottle of wine once you have studied the article this evening.