Grape vine nutrient requirements and acquisition

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

Soil is comprised of air, water, mineral particles (a mix of clay, silt, and sand), organic matter (decomposing plant material), and organisms (bacteria, algae, fungi, earthworms, insects, etc.).  Most of the solid soil matter is comprised of mineral particles while fully half of the overall content is pore space (in general a 50-50 mix of air and water).  Organic matter is: (i) critical in holding soil particles together; (ii) a reservoir of nutrients and water; and (ii) a food source for soil organisms.

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 that are resident in the soil matrix is 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.

Soil composition affects the availability of nutrients for soil uptake.  Soil pH is a measure of the acidity (3.5 - 6.5) or alkalinity (7.4 - 9.0) of soil which, through its influence on nutrient solubility and micro-organism activity, affects the number and types of nutrients in the soil. Soil pH between 6 and 7 is considered optimal for vine plant growth as most of the needed nutrients and micro-organisms are available in that range.  The optimal soil type also has a moderate content of low cation exchange capability (CEC) clay (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.).

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.

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.

While the above treats "naturally" occurring nutrients, the viticulturist will fertilize if he/she determines that a nutrient deficiency exists.  The nutrients added to the soil will make their way into the plant in exactly the same manner.


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