TUESDAY, JUNE 30, 2026. BY STAN GRANT, VITICULTURIST.
Part One of a Three-Part Vineyard Mineral Nutrient Management Lab Analysis Series
Featured image: Borden Ranch Lodi AVA soil. Source: Lodi Winegrape Commission.
Plants acquire nearly all of their mineral nutrients from the soil. For many annual crop plants, the available mineral nutrient supply analyzed in soils closely corresponds with crop yield. With grapevines, close correlations between soil analysis results and yield are much less common. This is largely due to the challenge of characterizing the mineral nutrient supply in the sizeable root zones associated with extensive, low density grapevine root systems.
Figure 1. Analysis of strategically collected vineyard soil samples is an essential element of sustainable mineral nutrient management. Source: Progressive Viticulture, LLC©
Still, with typically more than 60% of vine roots residing in the top 24 inches of vineyard soil, analysis results for carefully collected soil samples provide important insights into the mineral nutrient supply and factors affecting it (Figure 1, above). Such insights are essential elements of sustainable vineyard management, including certification by LODI RULES for Sustainable Winegrowing. Below we will focus on the interpretation of soil analysis results for prudent management of mineral nutrients in vineyards.
Readily Available Mineral Nutrients
The mineral nutrients most readily available to grapevine roots are those that dissolve in water. These are essentially the same mineral nutrients present in the liquid within soils, which is known as the soil solution. To acquire a sample of soluble minerals, a precise quantity of vineyard soil is wetted to a paste consistency. For analysis, the liquid within the paste is sucked from it under vacuum.
Total dissolved solids (TDS) represent the total concentration of all soluble minerals taken from a soil sample. Most agricultural laboratories, however, measure the total concentration of soluble minerals in soils as electrical conductivity (EC). This measurement is possible because, within a soil solution, soluble minerals are a mixture of positively and negatively charged ions (cations and anions, respectively) capable of conducting an electric current.
Combinations of a soluble cation, such as calcium, and a soluble anion, such as sulfate, form salts. In practice, most agricultural laboratories in California orient their panel of soluble mineral tests towards concerns of excess salts. Accordingly, their focus is on soluble sodium, soluble magnesium, soluble calcium, chloride, sulfate, and carbonates.
In reality, scant soil salinity is a greater concern than excess salinity for most vineyards.
Low concentrations of soluble minerals indicate low soil fertility, a risk of mineral nutrient deficiency, and possible benefits of strategic fertilization. Fortunately, many laboratories also analyze soluble potassium, nitrogen as nitrate-nitrogen, and soluble boron, which are often among mineral nutrients in short supply.
Here is appropriate to mention that nitrate-nitrogen is commonly low in vineyard soils. This is so mainly because the slow release of nitrogen from decomposing organic matter and residual fertilizer often cannot keep pace with the comparatively rapid uptake by grapevine roots. As such, high nitrate-nitrogen levels in vineyard soils indicate unusual situations, such as recently applied nitrate fertilizer, high nitrate concentrations in irrigation water, restricted root zone drainage, or inhibited root activity.
Guidelines for determining low soluble mineral nutrient levels, as well as excess levels are presented in Table 1 (above). Some laboratories report their findings for individual soluble mineral ions in units of charge per volume of water (i.e., milliequivalents per liter or meq/L). In Table 1 they are presented in units of ion mass rather than in units of charge (i.e., parts per million or ppm). In this way, they appear similar to the concentrations of other mineral nutrients in soils, as well as in irrigation waters and grapevine tissues.
Reserve and Less Readily Available Mineral Nutrients
There are mineral nutrients other than those in the soil solution that contribute significantly to the total supply available to grapevines. Among these are certain nutrients adsorbed on and near soil particle surfaces, including adsorbed potassium, absorbed magnesium, and absorbed calcium. It is the prevailing negative charges in soils, which are especially present on the surfaces of clay particles and organic matter, that draw these positively charged ions (cations) for adsorption.
Although they are less readily available to plant roots, adsorbed cations are commonly present at concentrations at least ten times greater than those dissolved in the soil solution (Table 2, below). These nutrients, held in reserve on or near soil particle surfaces, replenish the soil solution as supplies in it are depleted.
For analysis, a solution rich in the cation ammonium comingles with and passes through a specific quantity of soil. The applied ammonium saturates adsorption sites on soil particles, displacing absorbed potassium, adsorbed magnesium, adsorbed calcium, and adsorbed sodium. The leached solution of displaced adsorbed cations also contains a much smaller quantity of soluble cations.
As plants require them in relatively large quantities, potassium, magnesium, and calcium are among the so-called macronutrients. In vineyard soils there are also mineral nutrient cations required in comparatively small quantities by plants – the micronutrient cations. These are iron, manganese, copper, and zinc. A chelating compound in a solution applied to a specific amount of soil extracts these nutrients. Chelates are large molecules with a negatively charged opening capable of forming a chemical complex with micronutrient cations.
Phosphorus is mainly present in soils in organic matter and solid compounds that vary in solubility depending on their composition and the prevailing chemical environment. To accommodate the substantial regional variability in soil phosphorus within the United States, several phosphorus extraction methods are available. In California, the Olson bicarbonate method is most common.
In addition to the Olson bicarbonate method, many California agricultural laboratories offer the Bray 1 extraction. The Bray 1 method is somewhat more effective than the Olson method for extracting phosphorus from acid soils. Nevertheless, the concentrations of phosphorus extracted by the Olson and Bray methods are highly correlated in acid soils, which greatly diminishes the need for the Bray test. As with the other methods of extracting less readily available and reserve mineral nutrients, phosphorus extraction involves a precisely measured mass of soil and volume of extractant solution, which are mixed and filtered to collect the liquid containing the extracted nutrient for analysis.
Cation Ratios
Evaluating the percentage ratios of the cations potassium, magnesium, calcium, and sodium, both in solution and adsorbed on or near soil particles, can be insightful. In the soil solution, cation ratios suggest the likelihood of competition between macronutrient cations for uptake by grapevine roots, as well as the risk of sodium toxicity in foliar grapevine tissues (Table 3, below). In practice, a special soluble cation ratio, the sodium adsorption ratio or SAR, is normally the most reliable indicator of sodium toxicity risk (Table 1).
Ratios of adsorbed cations can indicate the risk of mineral nutrient deficiencies, especially in acid soils, but usually they are more indicative of soil structure stability. A high ratio of adsorbed calcium promotes stable soil aggregation and favorable porosity, while high ratios of either adsorbed magnesium or adsorbed sodium induce soil aggregate degradation, particle dispersal, and poor soil permeability to air, water, and elongating roots (Table 3).
Base saturation is an alternative name for the ratios of adsorbed cations. To further complicate matters, the exchangeable sodium percentage (ESP) is another name for the ratio of adsorbed sodium (i.e., the sodium base saturation).
Every laboratory analysis result, even with refined laboratory techniques and state of the art instrumentation, is associated with a minute amount of error. In calculations involving multiple results, such as cation ratios, the relative errors associated with each result are additive. For this reason, be mindful that cation ratios are slightly less accurate than the reported concentrations for the individual cations.
Figure 2. In Egbert clay in the Sacramento River Delta, the available supply of calcium and cation exchange capacity are related. Source: Progressive Viticulture, LLC©
Agricultural laboratories routinely perform analyses of soil characteristics affecting the mineral nutrient supply. The capacity of a soil to adsorb cations (i.e., it’s negative charge), known as the cation exchange capacity or CEC, is one such factor (Figure 2, above; Table 4, below). While the cation exchange capacity can be determined through specific laboratory procedures, it is common for laboratories to estimate it using the sum of the concentrations of adsorbed cations. It appears in laboratory reports in units of charge per quantity of soil (meq/100 g). Cation exchange capacity resides mainly in organic matter and clay particles, and many laboratories perform analyses that measure the former and infer the latter.
In addition to being a mostly negatively charged medium for holding mineral nutrient cations in reserve, organic matter is also a source of mineral nutrients, especially nitrogen, phosphorus, and sulfur. The actual quantities of nutrients available from soil organic matter, however, are difficult to determine due to considerable variation, even between short distances within vineyards, in organic matter quantity and composition, as well as in the factors that affect the rates of decomposition, nutrient release, and nutrient transformation (i.e., mineralization). Somewhat more consistent are relationships between organic matter concentration and soil characteristics affecting water (Figure 3, below).
Figure 3. The capacity to absorb water and the organic matter concentration are correlated in Egbert clay. Source: Progressive Viticulture, LLC©
The saturation percentage (SP), shown in Figure 3, is the amount of water required to wet a soil to a paste consistency, as described in the first section of this article. For mineral soils, the saturation percentage provides an estimate of texture, which is a function of the relative quantities of mineral particle sizes classified as sand, silt, and clay. As such, it provides a rough estimate of the relative contribution of clay particles to cation exchange capacity in mineral soils (Table 5, below). 
Soil pH is an indicator of the general chemical environment within a soil. On the pH scale a value of 7 is neutral. As pH values decrease and increase away from pH 7, soils become increasingly acid and alkaline, respectively. At the same time, they become less favorable environments for mineral nutrient availability (Figure 4, below). This is so largely due to certain reactions in acid and alkaline soils that render mineral nutrients unavailable, as well as diminished microbial activities fostering nutrient availability. Grapevines are most impacted when acid soils have a pH lower than about 5.5 and alkaline soils have a pH greater than about 7.5 (Table 4).
Figure 4. In El Dorado County, the percentage of adsorbed calcium increases as pH approaches neutral in Holland coarse sandy loam. Source: Progressive Viticulture, LLC©
Conclusions
Making best use of on-site resources, including mineral nutrients available in grapevine root zones, is among the principal goals of sustainable vineyard management. Carefully collecting soil samples and evaluating their analysis results using the guidelines presented in this article or from another trusted source is the first step in achieving this goal.
This process makes clear the availability of individual mineral nutrients, as well as soil characteristics impacting them. At the same time, soil analysis results suggest management actions to increase the supply of available mineral nutrients. These actions may include adding organic matter, altering soil pH, or simply applying specific mineral nutrients as fertilizers.
Evaluation of soil analysis results is also critical for another pillar of sustainable vineyard management – making efficient use of applied resources. Clearly, applying a soil amendment to make less available mineral nutrients more available or applying fertilizers to directly increase mineral nutrients is efficient only when there is an identified need. Further, soil analysis results may suggest efficient fertilizer application rates.
The author dedicates this article to his friend and mentor Gary Patterson.
This article was originally published in the Mid Valley Agricultural Services March 2009 newsletter and was updated for this blog post.
Further Reading, including primary guideline sources
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