Sunday, 18 August 2013

Prelimary discussion on water deficts in a UK limestone vineyard

The soil we have in Rutland looked so much like the Côte de Nuits that I talked myself into planting Pinot Noir: fissured Bajocian oolite with a gravelly 25 cm covering of clay loam. I’d spent so long in the company of Burgundians that I was entirely convinced that the ceiling for wine quality was set by geology, and that limestone is the plinth upon which great reputations are constructed.

And limestone isn’t just my obsession: the stories of Ridge Monte Bello, Bell Hill and Calera have the rock at their heart. For Paul Draper and Josh Jenson, limestone is intrinsically superior to other soil/subsoil/bedrock combinations.

Limestone is not homogeneous. Chalk, Dolomitic, Bajocian, and carboniferous differ in their physical and chemical properties; their hydrology is diverse, and these variances are carried over into the soil and subsoil to which they give rise. The high magnesium content of Dolomitic limestone can lead to water logging, while calcium carbonate from chalk can flocculate the overlying clays, making them free draining. We need to be circumspect when discussing limestone generically.  

Elsewhere I have written about the relationship between hydrology and grape quality in Burgundy; water deficits between 30%-50% of cropevapotranspiration (CET) positively effect gene expression, encouraging ripening rather than vegetative growth, whilst simultaneously increasing the ratio of skins to juice. Smart claims that reduced vigour impacts canopy density, and the resulting improvement in the light environment benefits grape quality, particularly in cool climates. Conversely, too much stress lowers quality and can lead to a cessation in grape development.  Again one needs to be cautious about extending these principles indiscriminately to all varieties and all environments.

The soil and vine combination we have at Rutland references some of the Côte de Nuits’ best sites. Calciferous clay loam, 25-30cm deep over limestone is the referent for many of the Grands and Premiers Crus, but the clay could be illite, kaolinite or montmorillonite; and the limestone fissured, massive, oolite or marble. Notwithstanding this, our soil/subsoil/base rock sequence could be neatly fitted into the Côte de Nuits fractured geology, I’m just not sure where!

Soil Moisture Results

The Sentek monitor gives us five soil moisture readings at soil depths10cm, 20cm, 30cm, 40cm and 50cm. These readings are aggregated together in the graph below.


Together the two datasets show how the rate of extractability of water varies with soil morphology. Water is extracted easily from the clay loam soil, and its reserves are rapidly exhausted. With a full canopy, CET on an average Southern English summer’s day is about 4.5 mm of water, but can peak at around 7mm on a hot sunny day if access to water is unlimited. When the soil is “full”, water is extracted unevenly: extraction from the clay loam is three times greater than extraction from the limestone, even when we mulched the soil surface with plastic to stop evaporation. Our vines are eight years old, so we might speculate that root density within the clay loam is much greater than that within the rock, and that the mechanical resistance to root penetration and the kinetic movement of water in response to plant suction is different for the limestone. From “full” to “onset of stress”, the extraction of water from the limestone was a constant, at around 1mm per day.  

The period between 14-07 and 24-07 was a period of high water demand: the sun shone continually; daytime temperatures peaked at around 28C; and night time minimums averaged 15C. With the moisture in the upper horizon of clay loam depleted the limestone became the vine’s principle source of water, so despite the fact that soil moisture was midway between the “full” and “onset of stress levels”, water rationing had already begun. In fact, with extraction constant at 1mm per day the reserves were sufficient to cover another fifteen days without rainfall, and 30 days without significant precipitation is rare for both Rutland and Burgundy.

The distinction between water deficits and stress is a fine one. If water is available for a prolonged period at less than 20% CET, vine metabolism and physiology may become compromised. We began to see shortening shoots and internodes on some of the shallower soils of the vineyard towards the end of the dry spell, when CET was equivalent to 7mm per day, but water extraction was only 1mm per day.

On the Côte de Nuits, certain cultural practices are utilised that might exaggerate vine dependency upon the regulated deficits provided by limestone. High vine density yields high numbers of leaves per hectare, so the water demand will rapidly exhaust the moisture content of the upper layers after rainfall. In Rutland, the difference between the soil being “full” and reliance on the limestone aquifer is approximately 30mm, or about 4-5 days of warm, dry July weather. Moreover, in the context of this year’s Burgundian deluge, it is worth pointing out that once the soil is full, it cannot hold any more water, so intense episodes of precipitation will not adversely slow the return to deficit. Poor weather may impact flowering or the health of the crop, but stress will be established as rapidly after 150mm of rainfall in two days as it will after 50mm over the same period. It is conceivable that vines growing on the Côte de Nuits’ stonier Grands Crus were in water deficit within 4 days of July’s 70mm storm.

Ploughing is frequently used in the Côte de Nuits vineyards, and eradicating roots from the upper soil layers forces the vine to source its water from the deeper limestone aquifers. On a particularly shallow soil like “Le Musigny”, ploughing will restrict the roots almost entirely to the limestone strata, which offers the intriguing possibility of a vineyard almost completely buffered from the vicissitudes of seasonal precipitation, collapsing clichéd climatic thinking about calling a bad vintage, or the hapless reductionism of considering the fate of all the Côte de Nuits vineyards together.

The Rutland data also makes the case for vine age. Our vines were planted in 2005, and mechanical resistance to rooting over the intervening years was still limiting extractability in the spring of 2013. The meagre 1mm of water per day proved insufficient for parts of the vineyard, even when the vine shoots were at the 10-12 leaf stage. In all probability, our roots will further explore the limestone over the next decade or so, at which point they should maintain the vine at a beneficial level of water deficit rather than at the current potentially stressful level of extraction. The same is true for the Côte de Nuits, and this is recognised in AC stipulations about vine age.

Climatic differences between Rutland and the Cote de Nuits make comparison complicated. CET rates are not only higher in Burgundy, but phenology is more advanced. Moderate water stress at veraison encourages carbohydrate partitioning to the fruit, and in Burgundy this pivotal growth stage coincides with the warmest part of the year, whereas in Rutland CET rates are declining by the time verasion is reached. Post-veraison, water deficits also accelerate the ripening process, and again this is less likely in Rutland through late September and October. All this points to the delicate balance that exists on the Cote de Nuits between geology, climate and viticultural practice; just having limestone isn’t enough: leaf area, vine age, ploughing and climatic patterns all converge advantageously on the best sites.

I will post again at the end of the season on whether the use of vigorous grasses and water exclusion in Rutland can help overcome the disadvantages of a sluggish phenology.

No comments:

Post a Comment