- Walnut trees grow most vigorously in soils that are fertile, well aerated, deep and moist. Ideal soils should be well draining, but retain sufficient moisture for plant health. Soil texture is vital to walnut growth as well, and is determined by the percentage of silt, clay and sand a given soil contains. Sandy loam, loam, silty clay loam and silt loam soils are the best suited for walnut trees. Soil should be free of bedrock, gravel or significant amounts of clay to a depth of at least 4 feet.
- The best soils for walnut cultivation will have a pH between 6.5 and 7.5, to a depth of at least 6 inches. Because walnut trees are heavy feeders, soils should be composed of at least 3.5 percent organic matter by volume, with a healthy amount of micro and macronutrients. Walnut trees require soils with 0.25 to 0.30 percent available nitrogen, 60 to 80 lbs. of phosphorous per acre, and 225 to 275 pounds of potassium per acre. Calcium levels should be at least 2,000 lbs. per acre for best growth, and magnesium should be present in excess of 250 pounds per acre. A soil test is the best method to determine exact nutrient levels, and should always be obtained before planting walnut trees.
Soils to Avoid
- Avoid planting walnut trees in soils that flood frequently or remain wet for long periods of time. Small amounts of flooding are acceptable for walnut culture, as long as the site is well drained and wet periods do not exceed a few days per year. Soils on south- and west-facing slopes should be avoided, as they are more likely to be subject to drought conditions. Examine potential planting sites for soil with mottled coloring, or gray-blue clay layers predominating in the first 3 to 4 feet. These often indicate excessive moisture retention and poor drainage.
- Although the soil suitable for the growth of walnut trees may seem like an ideal site for many other plants, most varieties of walnut are extremely allelopathic, and are thus intolerant of companion plantings. Walnut trees exude a chemical called hydrojuglone, which oxidizes into juglone on contact with soil or air. Juglone is an extremely toxic growth inhibitor for most plants, and it is produced by all parts of the walnut tree. Juglone interferes with basic physical processes necessary to plant life, such as cellular respiration, resulting in leaf yellowing, wilting and death.