Chapter 9
“Not every soil can bear all things.”
-Virgil
Soil and Topography
A wealth of anecdotal knowledge, accrued over centuries, supports the importance of soil both for quality and style of the wine. While climate imposes dominant and inescapable effects on the vine, within broadly similar climates soil is the next most important arbiter of the wine’s quality and personality. The finer the wine, the more this holds true. The loftier the winegrower’s quality aspirations for the wine, the more important planting in a suitable soil becomes. In Roman times, Virgil stated, “Wine loves the hills.” This early and perspicacious observation also applies to the types of soil found in the hills. Hillside soils are generally rockier and better drained than those found on the flats. From the beginnings of modern wine science in the 19th Century, soil was recognized for giving the wine specific qualities. These observations have continued into modern times. Beginning in the early 2000s, sensory science has validated quantitatively soil’s specific effects on aroma. No arguments are more persuasive than when anecdotal knowledge is corroborated and explicated by scientific proof. This linkage between soil and flavor will be explored in greater detail in the sections concerning terroir. A brief overview of the basis for soil and topography’s effects follows.
Soil: physical and structural aspects
Myriad types of soil can successfully produce fine wine—but not all. Suitable sites have certain characteristics in common. The soil should be well drained and at least moderately deep. All early writers on the subject agreed that stony, rocky, or chalky soils result in the best wines. Modern writers tend to attribute this primarily to the superior drainage and moisture regime of such soils. While these are important, there are also other elements at work. Soils with a high proportion of stone or gravel are relatively infertile. This limited fertility creates more balanced vine growth, as excessive vegetative vigor is not consistent with quality wine. Stony soil also absorbs more heat during the day, and then subsequently radiates it at night. This lessens the diurnal temperature fluctuations, giving the vine a longer metabolically active window each day. This is obviously an advantage at the cold limits of viticulture, but it is also helpful even in warmer than ideal conditions. In warm sites, the vines will shut down at mid-day, depriving them of many hours of metabolism. So, the warmth radiated by rocks allows beneficial metabolism to continue during the night, when it would normally be too cool. In the springtime, rocky soils both drain and warm more readily, giving the vines an earlier start to their growing season. As mentioned previously, phenological earliness is generally desirable. Soils with high amounts of stone also resist erosion. Steep slopes in particular benefit from this protection.
The color of the soil surface exerts an influence. Red soils have long been associated with quality wine. Research has proven that light richer in red wavelengths benefits ripening. The red light reflected off reddish soils into the vine likely contributes to this soil’s desirability. Lighter colored soils in general reflect more light up into the vine canopy. A greater quantity of reflected or diffuse light enhances ripening. This effect has been confirmed scientifically by placing reflective material under the vines and monitoring the resulting benefits. Light colored mulches would serve the same ends. That said, practical applications of this knowledge have been limited, no doubt due to the expense and labor-intensiveness of establishing and maintaining mulches in the vine row.
The soil serves a critical function as a reservoir for moisture. This is especially true in a Mediterranean climate, where spring rainfall must sustain the vine through long dry summers. While modern irrigation can offset seasonal drought, soil depth and drainage remain vital. Unseasonably heavy rains will have deleterious effects. Well drained soils are again advantageous, allowing water to move away from the active surface roots more quickly. The chances of the vine taking up water suddenly and splitting berries is thus reduced in deeper well drained soils. Shallow soils are prone to waterlog quickly and to dry quickly—both undesirable.
A perfectly poised chemical balance is far less crucial to the soil than an appropriate structural composition. Nonetheless, if severe nutritional imbalances exist, they will pose problems. These can and should be mitigated. Nitrogen, a primary nutrient, when present in excess leads to overly vigorous vines and lower quality wine. Excess soil nitrogen can be depleted over time using cover crops. Soils deficient in Nitrogen A deficiency of nitrogen in the soil will also weaken the vine. Either animal or green manuring can remediate this, but the deficiency may also be spot treated with fertigation. Potassium, the other primary plant nutrient, likewise needs to be present in moderation. Like nitrogen, an overabundance of potassium is detrimental, and a deficiency of potassium will prevent the vine from producing good wine. Note that both calcium and magnesium are antagonistic to potassium, so their presence will limit or block its uptake. In the case of calcareous soils, the superior structural characteristics offset this blocking of potassium[1].
Soils vary in their capacity to deliver nutrients, based on both structure and composition. Clay soil is the most nutrient rich; sandy soil is the least[2]. Soil organic matter content is important. The origin of organic material in the soil is broken down plant material, which is uniquely well suited for delivering balanced nutrition to the vine. However, exceedingly high organic matter at the soil surface is undesirable because of the heat absorption and color effects[3]. A balanced nutritional supply seems to confer some degree of disease resistance as well. The healthier the vine is in general, the greater its ability to ward off disease.
Topography and aspect
Topography and aspect are both significant. Topography is loosely connected to soil type, as rocky well drained soils are often found on the lower slopes of hills and the sides of valleys. The best sites are those which have the smallest daily temperature variation. These normally possess some of the following characteristics. They will be slopes with good air drainage above the average fog line. They will be found on either isolated hills or projecting hills in a range. Exposure to the sun will be at least partly easterly. In inland situations, the best sites will be close to rivers or lakes. Topography exerts its effects by modifying the mesoclimate of the vineyard. Altitude has a more primary effect, and almost all high-quality wine production takes place below 500 meters of elevation. Some of altitude’s deleterious effects are due to a lowering of air pressure, limiting the amount of CO2 available. Over time, increasing levels of CO2 in our Anthropocene atmosphere may offset this negative impact of altitude. An easterly aspect, which gives the vines greater amounts of morning sunlight, is usually best. In the northern hemisphere, southerly aspects are desirable in regions at the cold limit, as they increase insulation and warming effects. The converse is of course true in the southern hemisphere.
Planting and layout decisions
Decisions made regarding vineyard establishment will influence wine quality for decades. Thus, they should be undertaken with great care and consideration. Properly executed plantings will produce quality wine for 30 years or more. Ill-considered decisions mean that wine quality will suffer, and, in extreme cases, the viability of the enterprise itself may be compromised. It is beyond this book’s scope to elaborate the details involved in making these decisions, but a general discussion of the concepts will illustrate how these decisions impact quality. Understanding the principles of good vineyard layout will also allow the winemaker to analyze existing vineyards for their quality potential before purchasing the fruit.
With trellis design and the related vine spacing decisions, two main goals co-exist, spacing and circulation. Spacing between the rows must be such that one row does not cast excessive shade on the other. The lower leaves and fruit zone must receive sufficient sunlight. Enough air should be able to circulate, precluding excess humidity and disease pressure. A vertically trained trellis accomplishes this best in most cases. Even if the design is optimum, some leaf plucking, and other routine annual handwork will still be required. Most situations respond best to north-south row orientation, but arguments can be made for east-west as well. For example, in very warm regions, east-west will offer shade through much of the day and protect the fruit from sunburn. The exigencies of shape of the parcel, aspect to the sun, and the slope will have strong influence on these decisions. If rain is common during the final ripening period, a row orientation that follows the prevailing winds will allow the vineyard to dry out more quickly, lessening disease pressure.
Experimental evidence has confirmed that vine density exerts a positive effect on wine. On the best sites, quality improves as density increases. In most of the finest European appellations, a high-density meter-by-meter spacing is the standard. With lower quality appellations within the same regions, less density is the rule; both vine and row spacing are wider. This pattern of higher density in high quality areas of Europe has fostered a belief that this is true everywhere, but it is not universally the case. In some situations, less dense plantings will yield the best wine. Historically, vine spacing has been wider in Mediterranean climates because each vine needs a greater volume of soil to draw upon for water. Modern irrigation techniques, if water is available, can offset this. Closer spacing and higher density also offer economic advantages. As a vineyard ages and average productivity of each vine declines, tighter spacing can serve to stabilize crop quantity. The loss of any given vine to injury or disease has less effect. In the first few crop years when clusters per vine must be limited, the greater vine counts of tight spacing yield an earlier influx of both crop and income.
The head height and the fruit zone are related to the spacing. Tightly spaced vines require a low head height. A typical meter by meter spaced vineyard will have a head height of 16–18”. A wider 2m x 2m spacing should be headed higher, at 22–27”. Low head height makes the required routine handwork of fine wine farming quite cumbersome. From a purely practical viewpoint, moderate spacing and head height are more efficient. If the highest quality is the prime concern, and high economic returns support it, tighter spacing becomes the path to follow. The temperature advantages of low head height are more critical in cooler conditions. The heat radiating from the soil at night enhances the ripening of the fruit. In areas where spring frost is common, a higher head height will offer some protection. Tighter spacing and lower head heights should be reserved for the best sites within the region or vineyard.
Lyre-type trellising systems, like those developed by Carbonneau in Bordeaux, can provide many of the advantages of tight spacing[4]. If the shoot is healthy and the leaf count is in balance with its fruit load, the chances of achieving perfect ripeness are maximized. The extremely high shoot count per vine in this system means particular care must be taken to thin the shoots and clusters appropriately. It is worth emphasizing that whatever the vine training system, the unit to focus on in wine farming is the individual shoot.
Soil preparation and amendment prior to planting are almost always advisable. Perfectly balanced and suitable soils are few and far between. Correcting basic compositional aspects such as organic content and soil pH can only be accomplished efficiently during the pre-planting preparation when they can be mixed deeply into the soil. Adding compost at this time is always a good idea. During ripping is the only time when additions can be made deeply into the soil. Once the vines and trellis are in place, amendments can only be applied to the surface of the soil, or in its upper profile—both of which are notoriously ineffective. If undesirable layering or compaction exists pre-plantation deep ripping is indicated.
Rarely does a vineyard block have uniform soil structure and chemistry across its entirety. The surface of the soil can only tell you so much. To understand the environment that the roots will be living in, you must dig soil pits and analyze structure, composition, and chemistry of both topsoil and subsoil. This detailed revelation of what is hidden beneath the surface of the soil will enable you to match vine materials, particularly rootstock, more appropriately to the site. Digging multiple pits is the best way to clearly understand the soil that underlies each block.
The correct matching of rootstock to soil is vital. The rootstock mediates the vines’ interactions with the soil, so in a very real sense the winegrower is farming rootstock as much or even more than the scion material[5]. Rootstocks play a substantial role in the vine’s ability to take up moisture and nutrients. They must be resistant to soil-borne pests and disease; compatible with the soil’s structure and chemistry; well matched to the proposed vineyard design and to the scion wood’s growth characteristics. Given these sometimes-competing needs, it should come as no surprise that there is no universally ideal rootstock. All ramifications of the choice must be considered for each specific site. Some rootstocks will shorten the ripening cycle compared to others, and this is invariably desirable when making fine wine is the goal.
Varietal and clonal choice are of course critical. The former is driven largely by what is deemed historically and commercially successful in each region, and many regions in Europe mandate this by regulation. It would not be wise to plant Riesling in Napa or Cabernet in the Mosel for obvious reasons. While there are more than a thousand distinct wine grape varieties worldwide, only a dozen or so each of red and white varieties dominate. Choosing to plant a rare or obscure variety can be successful from the wine quality point of view, but these unknown wines are more difficult to sell. Clonal choice within the variety on the other hand allows the grower a wide latitude. The caveat that other attributes of the planting decisions were correct must be kept in mind[6].
When selecting scion and rootstock, the health of the plant material is paramount. Viral and other diseases introduced at plantation will both shorten the productive life of the vineyard and ultimately lower wine quality. I cannot in good conscience recommend field selection of bud wood as a viable choice,[7] as this presents too great a danger of spreading disease. Choosing which nursery will supply your plants or materials should be undertaken with great care. Some commercial nurseries are less impeccable than they should be in ensuring absence of disease in their material.
As with virtually all agricultural decisions these days, climate change must be a factor when choosing both rootstock and scion. Availability of irrigation water to offset heat events must be evaluated. The relative drought tolerance of the rootstock is important, as is the varietals’ ability to make fine wine within the projected temperature ranges in the upcoming decades. Simply because a certain rootstock and varietal have succeeded historically in a region does not mean that it will flourish in the warmer decades to come. The likelihood that it will not is high.
[1] Calcareous soils are held in near mystical reverence by some winemakers. In traditional European winegrowing, these soils are only appreciated in cooler northerly and continental environments and not highly valued in Mediterranean climates. Even where calcareous soils are appreciated, they tend to favor only certain varieties, notably Pinot Noir, Cabernet Franc, Chardonnay, and Sauvignon Blanc. In general, only the very highest quality wines will reflect differences between soil chemical compositions.
[2] Clay is an extremely poor soil both in terms of drainage and structure, and the nutritional advantage of a high clay content is offset by its poor structure. Sand offers the best drainage, but pure sand, because of its nutritional poverty, is nearly impossible for winegrowing. Most soils are mixtures, and the important fact is that some clay content provides good nutrition for the vine.
[3] William Albrecht’s theoretical work and Neal Kinsey’s practical applications of the same are the best source of information on organic materials’ effects on agriculture. Much of this work was done on high yield grain crops with enormous nutritional needs. While this is not the case for wine grapes, Albrecht and Kinsey’s work illuminates these complex relationships and is well worth perusing.
[4] Many claim that such trellis systems are only appropriate to high yield/modest quality scenarios, but I have personally farmed both Pinot Noir and Chardonnay to the highest quality using these trellises.
[5] Rootstock’s influence on flavor and quality was made clear to me at a tasting hosted by Saintsbury winery in the 90s. Their estate vineyard had three rootstocks, each of which had been paired with three clones of Pinot Noir. The nine possible wines were made accordingly. When these wines were tasted, the rootstock had a greater effect on quality than the clone did. This may not be the case in all situations, but I believe that the emphasis on clonal importance is a bit misplaced. Raymond Bernard, who led the clonal research in Dijon, was asked to identify the three most important influences on quality and replied, “Location, location and location.” He stated that if the site was good, then clone contributed to quality, but if the site was not good, then clone was not capable of improving it substantially. I think the same can be said of rootstock: If the rootstock choice is poor, then the clone cannot succeed.
[6] In New World winegrowing, planting monoclonal blocks is common practice. In Europe, mixed clonal plantings are the norm. The practical argument for mixing clones within the blocks is that the slight variations in the timing of flowering and set between the clones will protect the crop from the vicissitudes of less-than-ideal weather during bloom. Inclement weather, should it arrive during peak of bloom, will only affect one of the several clones planted in a vineyard. The slight variations between the clones in ripeness at harvest can also lead to a more complex wine. A monoclonal vineyard will have only one level of ripeness, which may lead to dullness.
[7] It is possible to select in-house clones from vineyards by cooperating with either academic or commercial nurseries capable of heat treatment and testing for disease-free status.
Nice intro to viticulture. The complexities of grape growing are hard to communicate to someone who is not a farmer.