Soil and Petiole Testing

Soil and Petiole Testing
by Hans Walter-Peterson

Fill in the blank in this sentence: "I apply ______ pounds of [nitrogen/potash/boron/magnesium] to my vineyard every year."

Now fill in the blank in this sentence: "I apply that much fertilizer to my fields because ________."

It's easy for most growers to answer the first question. But how do you answer the second one? If your answer isn't something like, "That's what my soil and petiole tests indicated that the vines needed" then your operation is probably less efficient (i.e. wasting money and/or losing production) than it could be. While the nutrient status of your vineyard is only one factor in its overall performance, it is an important aspect of vineyard management, regardless of the grape varieties that you grow. Nutrient deficiencies or excesses can negatively impact vine physiology, impairing functions like photosynthesis, shoot and leaf development, fruit development and ripening, and root function.

Let's look at how both types of tests play a role in developing a fertilization program specific to your operation.

Soil tests
The soil acts as the "reservoir" of nutrients and water that your vines draw from each year. Soil tests look at how much of each of the important elements are present in that reservoir, as well as looking at other factors such as organic matter, pH and cation exchange capacity. Deficiencies that are seen in these tests can then be dealt with by supplementing your soil with different materials like ammonium nitrate, urea, potash, lime, etc.

In most soils in the Lake Erie region, there are adequate amounts of most nutrients in the soil for the vines. Where we run into trouble, though, is the availability of those nutrients to the plant. One of the major factors we have to deal with in western New York vineyards is our highly acidic (i.e. low pH) soil. One of the basic concepts in plant nutrition is that at lower pH, most essential mineral nutrients become less available for uptake by the plant, except iron and manganese which become more available (and why we rarely see deficiencies of these elements here). At these lower pH levels, aluminum is also more available, but aluminum is toxic to the vine's roots, impairing their ability to take up other nutrients such as potassium, magnesium, and even nitrogen. Raising the pH by adding lime to the soil reduces the amount of aluminum available to the plant and increases the availability of other nutrients (see graphs below).

Figure 1. Soil pH influences plant-available aluminum and iron. As soil pH decreases, aluminum and iron availability increase, causing decreased availability of other essential nutrients and restricting root growth. Source: "Vineyard Nutrient Management" by Dr. Terry Bates (http://lenewa.netsync.net/public/Bates/NutrientRec.htm)

Figure 2. Soil pH influence on soil magnesium and calcium availability. It should be noted that a corresponding increase in potassium availability is NOT seen with increasing pH. Source: "Vineyard Nutrient Management" by Dr. Terry Bates

In essence, when we take a soil test, we're trying to find out:

• How much of the various nutrients are present in the soil?
• To what extent are those nutrients actually available to the plant?

Petiole tests
To my mind, the reason to do petiole tests comes down to one thing - the nutrient status of a vine (and therefore its health and productivity) is ultimately determined by what it can pull out of the soil and into its cells, tissues, organs, etc. The only way to determine that uptake is by testing some of those tissues, which is what a petiole test does.

Supplementing the soil does not necessarily mean that we are automatically increasing the amount of a nutrient taken up by the plant. The graphs below are from several years worth of soil and petiole results that have come through here at the Fredonia Vineyard Lab. The graph on the left shows a strong correlation between the soil pH and the amount of available magnesium present. When we look at soil pH and magnesium levels in petioles, however, there is virtually no correlation at all between the two. In other words, we cannot assume that an increase in soil magnesium will result in a corresponding increase in the vine's uptake of magnesium.

Figure 3. Soil pH and magnesium levels in soil (left) and petioles (right).

When only one of these tests is used, we get an incomplete picture of the nutrient status of the vineyard. If only petiole tests are taken, we don't have information about the relative availability of nutrients in the soil, levels of organic matter or soil pH that may help to explain any apparent deficiencies or excesses. If only soil sampling is relied on, we have no knowledge of what the vines are bringing out of the soil and into their tissues, and in the end, that's what will have an impact on your vines' health and production. Using only one of these tests by itself is certainly better than not doing any testing at all. But when they are used together, they can give a clearer picture of the nutritional status of your vineyard, and lead to the development of a sound fertilization program that enhances the efficiency of your operation.