By: Orest Protch
In a previous article (The Grapevine Magazine Nov/Dec 2021 issue) I touched on how water chemistry is rocket science. And now I am suggesting that soil testing is in a similar category. So, what does this mean to a vineyard?
Every part of a vine can be sent to commercial or university labs for in-depth analysis. You can get reports covering everything from the robustness of the microbes that thrive on the root hairs to the gas expired by the leaves. To get the gas you simply put a branch into an airtight plastic gas sample bag.
Very few labs can do all types of analysis well. It just does not work that way. So, shop around and meet with various lab account managers. Consider them your long-term partners. At my previous employment before retiring as the senior laboratory technologist for an oil company, I used a combination of over ten different commercial and university labs, each for specific tests even though most could do them all. I continuously re-evaluated their accuracy, precision, and repeatability in doing my required analysis by sending both duplicate samples and spiked sampled.
Also, consider joining with other vineyards to request similar test analysis since bulk samples mean reduced costs. It takes a commercial lab time to set up to run specific tests and the more samples of a similar type that they can push through, the less their overhead cost per sample. Vineyards are all in a friendly competition for customers and this works since no two wines are the same. So, take advantage of bulk sample shipments to save money.
Soil testing is simply one of many available analytical tools to try and ensure a quality grape every season that allows you to make the award-winning wine for which you are striving. This article is going to shed light on several factors that understanding soil chemistry may seem to be shrouded in darkness.
Vineyards are one of the few places where crop rotation does not occur. In normal agricultural practices, this would deplete the harvest potential of a plant within just a few years without massive additions of fertilizers.
On an incredibly positive note, vineyards are one of the few areas where the salmonella bacteria are not a permanent resident in the soil and therefore transferred to the growing crops and fruits. At any given time, one can culture the bacteria on most produce and vegetables found in grocery stores. But be very selective where you buy any soil enhancers. Most vineyards in my area burn their fall time clippings to prevent cross-contamination. The valley fills with smoke every fall.
Grape vines rely on their root systems burrowing deeper into the soil as they mature to reach new sources of essential nutrients, but this does reach a maximum depth in a relatively brief time. Unless vineyards add copious quantities of fertilizers, which for in the field is a marketing no-no, the vines mostly rely on the water they receive from rainfall and irrigation to get what they need to sustain grape growth.
When it rains or when you use irrigation, the water does not move only downwards strictly by gravity. There are hydraulic and other forces working on a single drop of water, and the bottom line is that water moves in all directions in the ground.
Below the ground, water movement is not limited to the warm months. If the vineyards are in areas that experience cold winters, the topsoil still gets groundwater infiltration even in freezing temperatures.
Factors that affect groundwater movement:
• Capillarity action.
• Coefficient of permeability.
• Gravity.
• Ground permeability.
• Ground topography.
• Hydraulic gradient forces. (static head)
• Molecular attraction between soil and water.
• Rock porosity.
• Water surface tension.
Relax, we are not going to be delving into calculus and quantum physics to describe what is going on under the ground. There is no test at the end of this article.
The smaller the soil particles, the greater the surface area per unit mass of soil. The large surface area of clay and its mineral composition make it the storage depot of soil nutrients. Soils with more clay have more nutrients than sandy soils. Clay particles have about 1,000 times as much external surface area as the particles in an equal weight of sand. (Figure 2)
Figure 1: Soil classification system. Grains of soil have vastly more surface area than their volume suggests.
The effect of particle size on surface area as demonstration with a deck of cards: Stacked together, the deck has only twenty-five square inches of surface area. When separated as individual cards, the deck has a surface area of approximately 1,000 square inches. The spread-out cards represent the pores in soil granules.
The soil is dried, run through a sieve shaker that separates the grains according to diameter using stacked sieves with different sized mesh screens, and then viewed through a microscope.
Sieve shakers separate soil grains according to their diameters and piled from largest mesh screen on top to the smallest mesh size on bottom. The author recommends that all vineyards have a shaker and stereo microscope with camera attachment.
Dried soil sieve analysis can indicate soil degradation over the years if samples taken from the same location.
Figure 2: Soil sieve analysis can indicate soil degradation over the years. The author did hundreds of sieve analysis tests and created this chart.
The soil grain porosity and permeability of the soil controls the rate of movement of groundwater through it. And how chemical elements such as iron and phosphorous move with the water.
To illustrate this, imagine a hundred people standing on a soccer field spaced evenly apart on the field. There are two other lines of people at both ends of the field with one side constantly throwing balls to those that are on the field. Now, you divide all one hundred people into two teams, one team is throwing and catching yellow balls and the other team is throwing purple balls. Both teams want to get their balls from one end of the field to the other.
However, one team has more balls than the other and the other team can catch and throw them slower than the other team. Eventually both teams will get their balls to the other end of the soccer field, but one team will get their balls there faster. Now think of the balls as chemical molecules.
The people on the field are like the soil or rock granules making up the ground below us. As the different chemicals move with the groundwater, they released by the soil particles making up the ground below us. These are chemical bonding sites inside all the pores and crevices. For those that are familiar with laboratory equipment such as gas or liquid chromatographs, this is also how these instruments work and measure the chemicals in samples run through them.
Blocking the movement of water and chemical compounds are immovable gas voids and water bubbles caught in the matrix of gas/air. This is a problem with well compacted vineyard soils after multiple years of use.
Figure #3: Chemicals in soil caught and released by chemical bonding sites of the soil grains. The different chemicals move through the ground at different speeds, but they all do move with the groundwater flow. Different chemicals simply move through the ground faster than others and are locked up forever in soil grain pores and stop moving and are not available to the root system.
Figure 4: Unmovable soil gas/air pockets may act as barriers to the movement of water and nutrients. This is a problem with well compacted vineyard soils after many years of continued use.
Not only should vineyards be taking close-up photos of leaves and grapes throughout the growing season on specific plants, but microscope apps for laptops are an indispensable tool for them also. Digging holes in one spot along a vine row through the years can show changes in soil compaction and organic material.
Another tool that the author used frequently for determining an estimate of soil organic matter was a simple small hand torch. The sample was dried, weighed, burnt with the torch flame, and then reweighed again. It was a surprisingly accurate method since a muffle oven was not available.
The soil, leaves and grapes are sent by the vineyard for XRD analysis (X-ray Defraction), and this analysis usually comes with bonus electron scanning microscope analysis, depending on the lab used. It gives a comprehensive break down of the constituents.
Soil analysis can be as simple or complicated as you want. It all depends on what value you place on analysis results that may help you produce award winning wines year after year. There are a multitude of labs both university-based and commercially available to you. Make the lab account managers your partners for the long haul.
Tests should not be single once-a-year snapshots but conducted consistently 3-4 times a year to get a baseline as to what your vineyard has in its arsenal. Analysis costs money. Do not treat it like throwing darts at a dartboard. It is more than that. Treat them like life saving medical checkups. The next article in the series will be on in-house lab testing nuances and best practices.
