Soil sampling vineyards and guidelines for interpreting the soil test results

Determine soil nutrient levels in your vineyard and save money by applying only the nutrients you need.

Spring is a good time to test the soil nutrient levels in your vineyard. For established grapes, you are monitoring changes in nutrients, pH and organic matter over multiple years. Organic matter content and pH impact nutrient availability in the soil. Extractable nutrients tend to be lower in fall after harvest. Soil pH tends to be higher in spring than fall. To be able to more easily compare soil nutrient changes in the vineyard from year-to-year, it is advisable to make soil tests at the same time of year. For established vineyards, testing every two to three years is adequate. Michigan State University offers soil testing services at the Soil and Plant Nutrient Laboratory. There are also a number of private labs that can test soil.

To sample soils properly, you need a few tools: a soil probe, trowel or spade; a clean plastic pail; sample bags or boxes; a map of your vineyard; and a marker. Separate the vineyard into smaller sections with relatively similar soil texture, slope, organic matter and cropping history. Mark the areas you are testing on the map so that you can retest the same locations next time for comparison.

Sample the vineyard in a zigzag fashion, five acres or less per sample. If your vineyard is very uniform, then no more than 10 acres per sample. Avoid sampling from the ends of the rows and the edges of fields. Collect soil to a depth of 8 inches from 15 to 20 locations, mix them thoroughly in the plastic pail and take out a pint of soil for testing. Testing forms and soil boxes for the MSU soil lab are available at your Michigan State University Extension county office or by contacting the lab.

Cation exchange capacity (CEC) is a measure of the fertility or nutrient holding capacity of the soil. The CEC is calculated by adding together the amount of soil values of potassium (K), calcium (Ca), magnesium (Mg) and hydrogen (H) held on the soil particles. The greater the clay and organic matter content of the soil, the higher the CEC. Nutrient levels and pH tend to be more stable in soils with higher CEC. In soils with a CEC less than 6me/100g K, Ca and Mg may change more rapidly.

Loamy sands and sands usually have a CEC less than 8. The CEC of sandy loams frequently falls between 8 and 12. Loams, clay loams and clays usually have a CEC greater than 12. As the soil pH changes, the CEC value will also vary somewhat. The higher the CEC, the greater the capacity of the soil to hold nutrients.

Soil pH for grapes. pH is a scale used to measure acidity or alkalinity. The pH scale is from 0 to 14. A value of 7 is neutral, less than 7 acidic and greater than 7 alkaline. A soil pH in the range 5.5 to 6.5 is considered optimum for grapes and generally has better nutrient balance for plant growth than soils that are more acidic or alkaline. Vines will grow from pH 4.0 to 8.5, but a pH below 5.5 and above 8 will depress yields and create vine problems. Soil pH affects the availability of nutrients and microbial activity in the soil. The availability of many micronutrients (Mn, Cu, Zn and B, for example) decreases as soil pH increases. Soil pH often drifts down over time with the use of fertilizers and sulfur.

Adjusting soil pH

Soil organic matter. Organic matter (OM) improves soil structure, moisture retention and fertility. Two to 3 percent is considered ideal for grapes. Nitrogen is released from organic matter at approximately 20 pounds N per acre per year for each 1 percent of organic matter present. So at the optimum 2 to 3 percent OM for grapes, there is 40 to 60 pounds N per acre per year released from the soil. Grapes grown on high organic soils tend to be less winter hardy. With the indeterminate growth habit of grapes, excessive N promotes vegetative growth late into fall, and shoots don’t have time to acclimate for winter.

Recommended application amounts for potassium and phosphorus will be listed on the soil test results. Nitrogen recommendations will also be listed. Usually, half the nitrogen should be applied at bud break and the other half at bloom. Nitrogen applications should be completed by veraison, allowing vines to take up and use the nitrogen during the growing season. Nitrogen applied during the mid to late summer will encourage vegetative growth into the fall season, when vines should be hardening off and preparing for dormancy.

Test

Desirable soil test ranges for grapes

pH

5.5-6.5

Organic matter

2-3 percent

Phosphorus (P)

40-50 ppm

Potassium (K)

250-300 ppm

Magnesium (Mg)

200-250 ppm

Boron (B)

1.5-2.0 ppm

Zinc (Zn)

8-10 ppm

Source: Midwest Grape Production Guide

References