The roles of soil minerals and cation exchange capability (CEC) in meeting the nutrient requirements of the grapevine

Adequate amounts of the appropriate nutrients are required to support proper growth of the grape vine, fruit development, and fruit maturity and those nutrients are obtained from the soil by the plant.  The table below shows the mineral requirements of the vine plant, the role of each mineral, acceptable ranges of each mineral in the soil, and the impact of mineral deficiency on the vine.

Source: Compiled from LGRGP.org and others

Mineral Sources
Rocks
The earth is made up of varying proportions of the 90 or so naturally occurring elements but, according to Alex Maltman (Vineyards, Rocks, & Soils), four of these -- oxygen at 48%, silicon at 28%, aluminum at 8%, and iron at 6% -- are responsible for 88% of its composition. In most geological materials, these elements combine to form minerals -- "a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice" (opentextbc.ca, Minerals and Rocks).

Lattice structure of the mineral halite. Atoms of sodium alternate
 with atoms of chlorine in all three dimensions
 (Source: opentextbc.ca)

In order to effect the above bond, the sodium atom yielded one of its electrons to the chlorine atom, attaining a positive electrical charge as a result. Atoms which experience a change in the number of electrons are known as ions. An ion with a positive electrical charge, resulting from the loss of an electron, is called a cation. An atom with a negative charge, the result of gaining an electron, is called an anion. The bond that is formed as a result is referred to as a stable compound (Maltman)

In nature, minerals are found in rocks "and the vast majority of rocks are composed of at least a few different minerals." The picture below shows a piece of granite and its constituent minerals.

A close-up view of the rock granite and associated minerals
(Source: opentextbc.ca)

The figure below shows a typical soil profile.

Source: http://www.westone.wa.gov.au

Jackson (Wine Science: Principles and Applications) stipulates that (p. 245) "... the mineral content of soil is primarily derived from the parental rock substrate." The figures below show the weathering of rocks into minerals.

Source: geology.csupomona.edu

Decaying Organic Material
Jamie Goode (Rescuing Minerality) contends that the bulk of soil mineral content comes "from decaying organic material, not decomposed rock and it is microbial activity in the soil that affects the ability of soil to break down organic matter into mineral ions that can be used by the plant." Maltman agrees with Goode: "... in practice, it's the humus that's more important, indeed essential."

According to Schwarcz and Schoeninger (Stable Isotope Analysis in Human Nutrition, Yearbook of Physical Anthropology 34, pp. 293-321), almost 100% of exchangeable nitrogen is found in the atmosphere or dissolved in the world's oceans and is transferred from these environments into the biological system through the processes illustrated in the figure below.  Grape vine plants receive their nitrogen through this terrestrial nitrogen cycle.

Source: http://tolweb.org/notes/?note_id=3920

Cation Exchange
Soil-based nutrients are resident either in the soil solution (water and dissolved minerals in the soil pores) or in the soil matrix (mineral particles and organic matter).  Two problems present themselves, however: (i) the concentration of nutrients in the soil solution is low and (ii) the nutrients resident in the soil matrix are immobile.  Plant roots have developed adaptions to allow growth into the soil matrix and capture of the nutrients needed for metabolic activity (Dr. Paul Schreiner, USDA-ARS) and we will discuss these later.

Most of the mineral nutrients that the vine needs are cations so the soil's cation exchange capacity (CEC) is a major enabler of  it's nutrient acquisition. The positively charged mineral ions bind loosely to the clay and humus colloids in the soil and these minerals are released in exchange for hydrogen ions secreted by the vine roots. (Clay minerals act as harbors for nutrients because the positive ions of the nutrients are trapped by the negative charge of the clay minerals.  The abundance and types of minerals determine whether the clay is classed as low- or high-CEC.). The ion that makes the strongest link with the clay is the hydrogen ion "... and its almost as though the vine knows this! The vine's metabolism can prompt its roots to pump out hydrogen ions into the soil water, which then dislodges the other ions held on the clays, thus making them available to the vine roots" (Maltman). This concept is illustrated in the figure below.

Illustration of the cation exchange between
vine roots and surrounding soil particles
(Source: bio1903.nicerweb.com)

Roots have developed a number of physical and chemical adaptations to allow them access to an immobile nutrient set resident in the soil matrix (Dr. Schreiner).  The first adaptation is the root size and structure.  The vine plant deploys an always-growing, three-part root structure to meet its needs for anchoring, water- and nutrient-acquisition, nutrient storage during plant dormancy, and hormone production.  As it relates to nutrition, the plant deploys quick-growing, short-lived roots close to the surface to aid in moisture collection and primary roots for nutrient uptake (The woody roots (anchoring and transport) take up limited amounts of nutrients due to the presence of a waxy coating designed to keep ions in.).  According to UCDavis, about 60% of a vine plant's root structure is located within two feet of the surface but individual roots can grow as deep as 20 feet depending on soil permeability, water table levels, and rootstock variety.

Source: bccs.bristol.ac.uk

The second adaptation is the formation of symbiotic relationships with arbuscular mycorrhizal fungi (AMF), a non-specific fungi which extends its apparatus beyond the plant's zone of influence in order to retrieve minerals such as Phosphorous and Zinc and, in return, utilizes plant-derived carbon for its growth and reproduction.  Both the plant and fungi benefit from this relationship.

A third adaptation is the ability to secrete protons, organic acids, and enzymes and release these into the surrounding soil in order to increase the solubility -- and absorbability -- of certain ions.

Nutrient Transport
In order to effect nutrient transfer, the roots of the plant has to be in direct contact with the soil matrix and the nutrients have to be delivered to the root surface.  Nutrients reach the root surface in a combination of three ways: interception, bulk flow, and diffusion.

Source: baileybio.com

Only a small fraction of the plant's nutrient needs are met by interception.  In this case, as the root grows into new areas, it displaces nutrients resident in the soil matrix.  Once on the root surface, the nutrients transit through the root's plasma membrane using available ion-selective channels.  The transfer is effected as the ions flow from areas of high concentration to areas of low concentration.

Bulk flow is the movement of nutrients towards the root as a result of transpiration water uptake. Water enters a vineyard through precipitation or irrigation and that water either runs off, flows to levels beyond which it can be accessed by the vine plant, or remains in the rooting zone where it is available for the plant's use.  The plant uses water as an internal distribution vehicle (in addition to other functions) and facilitates this by expelling water through pores (stomata) in the leaves.  As water is transpired from the leaves, replacement water is drawn in at the roots.  This replacement water moves undiluted nutrients to the root surface but also carries dissolved nutrients into the roots as a part of its transit. Nitrogen is the nutrient most frequently acquired by the roots in this manner.

Source: talktalk.co.uk

Diffusion is the mechanism whereby nutrients move toward the roots as a result of agitation caused by the concentration gradient that develops near the root surface as a result of nutrient uptake. Phosphorous and Calcium are the nutrients most susceptible to this type of capture.

In addition to the above mechanisms, as mentioned previously, the plant can utilize AMF to reach beyond its depletion zone in order to bring Phosphorous to the root interface.

©Wine -- Mise en abyme

Minerality in wine? Fuggedaboudit

Or so says Alex Maltman of the Institute of Geography and Earth Sciences, Aberystwyth University, Wales.

In a previous post, I outlined a battle revolving around the role of soils in wine quality/typicity. Alex Maltman (Role of Vineyard Geology in Wine Typicity, Journal of Wine Research 19 (1), 2008) noted that it had become de riguer, when describing a vineyard, to specify its geology and this, coupled with the geological indications common in tasting notes, has served to infer "... a direct link between the vineyard substrate and the resulting wine." According to Maltman:

Such perceptions bolster a valuable tactic for the wine trade, as, being one of the few aspects of wine production that cannot be translocated or easily replicated elsewhere, a vineyard's geology is something that can be invoked to promote a wine's typicity, to give it a marketable uniqueness.

In a follow-up article (Minerality in Wine: A geological perspective, Journal of Wine Research, 2013), Maltman has honed in on minerality, a thoroughly modern (according to him) invention which had received no mention in the works of the "masters" (Peynaud 1987, and Vine 1997, for example) or the science-based tasting schemes (Jackson 2009 and Noble et al., Aroma Wheel 1987, for example). Maltman's 2008 and 2013 articles are built around a similar core argument but in the 2013 article, in addition to focusing on minerality, he advances explanations as to what tasters could be confusing with minerality. The latter aspect of his research will be dealt with in a later post.

There is, in general, a lack of understanding of the differences between minerals in foodstuff (of which wine is a part) and geological minerals (Maltman).

While it is true that the vine plant needs a variety of mineral nutrients, and that weathered bedrock is the source of much of these nutrients, the path from geological mineral to nutrient mineral is a protracted, rocky, and time-variant road.



To illustrate the latter point, Maltman uses the example of feldspar, the most commen geological mineral resident in modern vineyards. Feldspar is a family of minerals containing various combinations of minerals that are "ionically and covalently bonded into a crystalline lattice that gives a grain of feldspar strength and rigidity." This feldspar particle is bound together with a number of other mineral grains to form the aggregate we call rock or, in its fractured form, stone. The minerals contained in feldspar are not directly accessible by the vine plant. To be accessible by the plant the minerals have to be ionic and in solution. The process of transformation from the geological mineral feldspar to vine-accessible mineral nutrients is illustrated below.

Source: geology.csupomona.edu

Source: letslearngeology.com

The gulf between the vineyard chemical profile and that of the wine is further widened as a result of vine activity. The amount of nutrient ions absorbed by the vine roots is not directly related to the amount of nutrient ions in the soil. Rather, it is dependent on the transportation proteins and cell wall hydrophobic deposits among other factors. Further, once in the xylem, differential amounts of ions are directed to the various components of the vine architecture. Even within the berries there is a differential allocation of ions between, skin, seeds, and juice (Maltman 2013).

The disconnect grows even wider once cellar activities commence (shown below) and, as a result, the "proportion of mineral nutrients in finished wine bears only a complex, indirect, and distant relationship with geological minerals in the vineyard" (Maltman 2013).

After establishing a less-than-tenuous relationship between vineyard minerals and the mineral-nutrient signature of the finished wine, Maltman goes on to argue that minerality, as such, cannot be tasted in a wine. First, he argues, the concentration of inorganic material, in general, and mineral elements, in particular, in wine is miniscule -- between 0.15 and 0.4% for inorganics and very low levels for the mineral elements. Of the minerals, potassium has the largest concentration at 577 ppm (.06%) while calcium ranges between 30 and 200 ppm and magnesium registers at .005 ppm. Of the 50 organic elements identified in wine, 25 are trace elements (1 - 100 ppm) while 20 are ultra-trace (parts/trillion).

Compounding their relative scarcity in wine, these mineral elements have no flavor, a situation that also holds for geological minerals which are both solid and insoluble. Flavor-detecting organs in the mouth can only deal with solutions thus only sodium chloride, of all the geological minerals, registers a flavor in the mouth (Maltman).

So mineral nutrients resident in wine occurs in small amounts and are lacking in flavor. Vineyard geological minerals are flavor-free and would not register a taste in the human mouth. Aroma, the other component of flavor, requires volatilization in order to register on the organs on the olfactory bulb but neither rocks nor minerals possess this capability. Minerality as a taste descriptor, then, requires a leap of faith on the part of the taster.

How about texture, you ask? The geological minerals that existed at the beginning of the journey have been transformed beyond recognition in that they now exist as ions, rather than compounds, and in extremely small quantities. Any minerality imbuing capability would have to survive the journey detailed above and still have enough "surviving energy" in order to "infect" the completed wine in such a way that it exhibits "minerality" in a uniform manner.

Does Maltman's analysis consign minerality to the scrap heap of taste descriptors? Let me know your thoughts.

©Wine -- Mise en abyme