Nitrogen Rate Enhances Chestnut Yield and Tree Growth

Published: October 28, 2015

Like all orchard crops, annual applications of nitrogen are needed to optimize chestnut production. Before trees are planted, a soil test is recommended to estimate the nutrients available to the tree. Some nutrients, such as phosphorus and potassium, do not readily move from soil surface into the soil profile with water where they are available to tree roots. Thus, these nutrients must be incorporated into the soil before trees are planted. Soil samples can be submitted to the University of Missouri Soil and Plant Testing Laboratory where they are evaluated for nutrients and pre-plant recommendations are provided.

During the first three growing seasons, nitrogen is important for establishing a strong tree structure before it begins to produce a chestnut crop. Because nitrogen moves into the root zone with rainfall or irrigation, it can be applied to the soil surface. A general recommendation is to apply 0.1 pound of actual nitrogen per tree in a ring around the tree (about six inches from the trunk) about one month after planting in the spring. In the second and third growing season, apply 0.2 and 0.3 pound of actual nitrogen per tree, respectively, just before buds start to produce leaves. A quick way to assess the nitrogen status of chestnut trees is to check leaf color during the growing season and shoot growth in October. If non-bur bearing shoots are yellowish or have less than 20 inches of new growth, the amount of nitrogen applied to trees should be increased by 10% the following spring.

In the fourth growing season, grafted trees will generally begin bearing a small nut crop, which requires additional nitrogen. In the past, recommendations for Chinese chestnut trees have been a “best guess” since a nitrogen response hadn’t been tested in Missouri. Thus, in 2009, Chinese chestnut trees with ‘Peach’ as the scion cultivar grafted onto Auburn Super seedling rootstock were planted in an established sod cover crop at the MU Horticulture and Agroforestry Research Center near New Franklin, Missouri to determine the rate of nitrogen needed to enhance nut production and tree growth. All major nutrients were in the sufficient range as determined by a soil test before planting. The first three growing seasons, the rates of nitrogen as described above were applied to the trees. In 2012 to 2015, chestnut trees were treated with 50, 75, 100, 125, or 150 pounds per acre applied annually as split applications just before budbreak and on June 15. A three foot-wide strip of killed sod was maintained underneath trees during this study.

Shoot growth, trunk circumference, nut numbers, and nut weights were recorded and foliar nitrogen content was determined from tissue analysis for each tree annually. Tree growth, measured by annual increase in trunk circumference, generally increased with higher rates of nitrogen. By 2015, cumulative yields for the lowest to highest rates of nitrogen were 11.3, 11.2, 13.1, 16.1 and 16.5 pounds per tree. Thus, 125 pounds of nitrogen per acre provided a substantial yield response when applied annually to bearing Chinese chestnut trees. This study also revealed that leaf tissue from trees receiving 125 pounds per acre averaged 2.4% nitrogen when foliar sampling was conducted.

For chestnut orchards at varying sites, the best way to evaluate the nutritional status of the trees is by foliar sampling. Between July 15 and 31, obtain a sample of twenty fully-expanded mid-shoot leaves from bur-bearing shoots from an orchard and submit it with the accompanying form at http://soilplantlab.missouri.edu/soil/plantsamples.aspx to the MU Soil and Plant Testing Laboratory. Recommendations are provided based on the nitrogen content of the submitted sample.