So where are we up to with soils and wines?
We began by exploring the ideas of minerality, and how soils might be having a direct effect on wine quality, even though science seems to indicate that soils impact upon wines rather indirectly. We explored the idea that what is perceived as minerality might actually be a result of ‘reduction’. And then we looked at the way that the root environment affects above-ground growth through hormone signalling.
Now let’s think about the issue of soil biology, and how this could be affecting the vine.
‘The extent, health, and physical and chemical environment of the roots must be a major key to the best ripening and terroir expression.’
John Gladstones, Wine, terroir and climate change (Wakefield Press, 2011).
Vine roots respond to the conditions of the soils they are growing in. First of all, a large permanent framework of roots is established, followed by a network of finer lateral roots, and finally even finer tertiary roots which are vital for uptake of water and nutrients. Nutrient uptake by the roots can be both passive and active. As the vine takes up water, it will usually take up whatever is dissolved in that water. But if it lacks specific nutrients, it can take them up actively, if they are present in the soil. There are some situations where the vine is fooled, though, by mineral ions that look quite similar, such that a deficiency of one can occur when there’s an abundance of another. And, for example, in soils with a lot of limestone, chlorosis can be a problem. This is because in limestone-rich soils, vines find it very difficult to take up iron, which is needed for photosynthesis, as a vital component of the green-coloured chorophyll pigment. As a result, the leaves turn a yellow colour and are diminished in their ability to carry out photosynthesis, the process of transforming light into energy.
A special layer of material called the Casparian strip surrounds the root endodermis, the layer of cells that circle the vascular tissue. This strip contains suberin, a waxy, rubbery material that is impermeable to water. Thus water and solutes entering the roots have to pass through plasmodesmata (pores in the cell walls) and therefore through the cytoplasm of root cells, before they can be transmitted to the rest of the plant. This gives the vine a level of control to what is taken up. The plasmodesmata are significant because they allow direct communication between the cytoplasm of adjacent plant cells, through the otherwise rigid cellulose cell walls.
How do vine roots take nutrients from the soil? One of the key concepts here is cation exchange. Roots are able to exchange hydrogen ions, which they pump out, for the cations attached to the negatively charged soil particles such as clay and humus. Clay often carries a negative charged, whereas humus – decayed organic material – can carry both negative and positive charges, and so can hold both cations and anions. Cation exchange capacity (CEC) refers to the number of positive ions (such as calcium, magnesium, iron and the nitrogen-containing ammonium ion) that the soils can hold. When clay and humus have a negative electrical charge they are able to hold onto positively charged ions. Generally speaking, CEC correlates positively with soil fertility, because it determines how many plant nutrients the soils can hang on to. Soil pH also affects CEC: more acid soils (lower pH) have a lower CEC than more alkaline soils (high pH). One way to increase CEC is to increase the organic content of soils. This has the benefit of both increasing CEC, and thus fertility, and also increasing soil texture. Without organic material or clay, soils find it hard to retain nutrients. For example an excessively sandy or gravelly soil will allow mineral ions to be rapidly leached from the soil by rainfall.
So where do the mineral ions (nutrients) come from in the first place? It is not really from the bedrock, which would be the intuitive assumption on many. Some mineral ions might be bedrock-derived, but these would largely be in the subsoil. Low levels can come from rain, and some can come from the weathering of larger soil particles such as stones and rocks, but the bulk of soil nutrition will come from decaying organic material.
To get a better handle on this I spoke with Tim Carlisle, who has studied soil science, but also has a deep understanding of wine from his current employment as a wine merchant. ‘We need to then look at the microbial activity in soil,’ he states. ‘This affects the speed and ability of soil to break down organic matter into mineral ions that can be used by plants – and also aids the uptake of ions by plants. Because of this, no discussion about soil should exclude them, because whatever the terroir is, the level of microbial activity is an important and always overlooked element.’
Carlisle points out that there are many factors that influence this microbial activity, but primarily water, food and oxygen. ‘Oxygen is more available in a loose uncompacted soil,’ he says. ‘A soil that is overly compacted has little oxygen and so little microbial activity – the same is true of a waterlogged soil – which Is one reason why porous bedrock and/or slopes are important, not just because of vine stress but because the microflora and fauna don’t get drowned.’ The term microflora refers to the bacteria and fungi in the soil. Their existence also governs the level of microfauna, which refers to soil organisms ranging from single-celled protista to small arthropods and insects, through to nematodes and earthworms.
‘The food they need is organic matter. If you visited a conventional agriculture wheat field you’d find that there was very little organic matter in the soil, and as a result very little microbial activity, which is further diminished by crop spraying – hence the vicious cycle of needing to use tons of fertiliser.’
During his studies of soil science, Carlisle looked at the effect of fungicides, herbicides and insecticides on the soil microbes. He did this two different ways. First, he took microbes from the soil, grew them in culture, and then studies the effects of dilute agrochemicals on their growth. ‘What I found in this was that fungicides over herbicides and insecticides kill off not just fungi (which includes yeasts and moulds),’ says Carlisle, ‘but also a high proportion of bacteria, and actinomycetes (I didn’t do anything with algae), but also that herbicides and insecticides also killed off a proportion of all types of microbe, and restricted growth of others.
He also studied the overall microbial activity in the soil. ‘What this showed up was that untreated soil was healthier than anything with any kind of treatment, including one that was sprayed with fertiliser,’ reports Carlisle. ‘If you think about it, minerals are essentially the excretion of microbes. Too much excretion to soil will poison them and so spraying with fertiliser actually caused a check in microbial activity – it continued but at a lesser rate.’ He adds, ‘the thing that was by far the most interesting from a viticulture perspective was that one of the samples was sprayed with copper sulfate, which is permitted in organic viticulture. This sample was the one in which microbial activity was reduced by the most.’
This raises interesting questions. Clearly, soil life is important. And this soil life can be affected by vineyard treatments. It poses a problem for organics and biodynamics: while these approaches aim to encourage soil life, the fact that they need to rely on copper – a traditional but toxic remedy for fungal disease – is at odds with this approach. But then conventional remedies, such as the use of systemic fungicides, are also problematic.