Saturday, January 19, 2013

Pecan size determined by soil and available water

     Last summer's drought and high temperatures effected agriculture across the Midwest, including pecan groves. In previous posts, I discussed how drought can effect nut size, influence nut shape, retard kernel filling, and prevent proper shuck opening. Today we were evaluating nut samples collected from several locations and soil types. The differences we found  among four Pawnee samples were astounding and I just had to share our observations (photo below).

    Its almost hard to believe that all four samples were produced by trees of the same cultivar. The largest Pawnee nut was produced in Illinois, growing in a deep silt loam soil and irrigated during the mid-summer drought. The combination of great soil and ample water provides ideal conditions for nut growth and kernel fill.
    The second largest nut was produced in Kansas at the Pecan Field. Last summer we were fortunate to have a flood in May that saturated our heavy clay soils. Our Pawnee trees ended up mining that water later in the summer to produce a smaller but well filled nut. However, last summer's heat and drought did cause some kernel defects on our Pawnee nuts. 
    The two samples from Missouri were collected from trees growing at the Horticulture and Agroforestry Research Center in New Franklin. Central Missouri was hard hit by the 2012 drought, decreasing nut size significantly. These samples provide a great example of how site and soil type can effect nut production. The tree growing in the silt loam soil is actually located in a very fertile and moist creek bottom. Trees in this planting have grown exceptionally well with many reaching 12 inches in trunk diameter in just 15 years. Planted at the same time and just 1/4 mile away, the trees growing in the silty clay soil have grown much slower reaching only 8 inches in diameter. Although the silty clay soil is located within the greater Missouri River flood plain, this land is cut off from minor flooding events by a levy.  In the end, the nut size difference between the two Missouri samples can be attributed to a difference in both available water (the silty clay had less) and tree size. If you remember, tree size has a enormous effect on a pecan tree's ability to capture soil water, impacting both leaf and nut size.