Organic Mulches Affect Soil and Leaf Nutrient Levels of Young Pecan Trees

Materials And Methods

Container-grown pecan trees (Carya illinoensis ‘Desirable’) were planted in October 1991 on a 100 × 115 m (30 × 35 ft) spacing at the E.V Smith Research Center in central Alabama on a Cahaba fine sandy loam (fine, loamy, siliceous, thermic, Typic Hapludults) soil. The trees were sized (circumference), ranked, and planted in a randomized complete block design with 4 single-tree replications. Trees of similar size were planted within blocks. Each block contained 5 mulch types (hardwood leaves, pine bark nuggets, pine straw, grass clippings, and chipped limbs) each at 3 depths—15, 30, and 45 cm (6, 12, and 18 in.); and a plot of common bermudagrass sod was laid around trees at time of initial mulch application. All 16 treatments were maintained in a 3 × 3m (10 × 10 ft) area centered on the trees.

Mulches were applied initially in February 1992 and replenished annually in 1993 and 1994 during the dormant season to maintain specific depths. Trees were fertilized uniformly with commercialgrade fertilizer based on leaf and soil analyses (O’Barr et al. 1989) taken from all treatments and averaged. Nitrogen (N) was applied underneath each tree at a rate 0.3, 0.6, and 1.3 kg (0.7, 1.4, and 2.9 lb) actual N per 90 m²/year (1,000 ft²/year) from ammonium nitrate in 1993, 1994, and 1995-1996, respectively. This N application was split into 3 applications approximately 30 days between each application, with the first application applied in late February. During this period, no additional phosphorous (P), K, or zinc (Zn) was required based on the averages of soil and foliar analyses and none was applied. Trees received no supplemental irrigation but were otherwise maintained according to standard recommendations for pecans (Worley 1989).

Foliage samples were taken in July 1994 by collecting 50 leaflets from the middle of compound leaves at the midpoint of the current season’s shoots of each tree. Leaflets were rinsed briefly in tap water containing 2% (v/v) detergent to remove surface contaminants, then rinsed again in water and allowed to dry before analysis as described by O’Barr et al. (1989). Samples were analyzed for boron (B), Ca, copper (Cu), Fe, K, Mg, Mn, N, P, and Zn using a dry ash technique (Hue and Evans 1986). Soil samples were collected in July 1994 within the 3 × 3m(10 × 10 ft) treated area surrounding each tree to a depth of 20 cm (8 in.). These samples were analyzed for the same nutrients and for soil pH using established procedures (Mehlich 1953; Hue and Evans 1986). Data were analyzed with the SAS GLM procedure (SAS Institute 1991).

Results And Discussion

Among the organic mulches, neither mulch type nor depth affected soil or foliar nutrient concentrations, with the exception of soil K (P = 0.0269) and foliar Fe (P = 0.0104). The plots mulched with grass clippings had higher soil K concentrations (126 ppm) than did plots mulched with hardwood leaves (53 ppm), pine straw (53 ppm), or pine bark nuggets (43 ppm). The chipped-limb mulch treatment had lower foliar Fe levels (78 ppm) than did plots mulched with leaves (125 ppm), pine bark nuggets (125 ppm), or grass clippings (112 ppm).

When the organic mulches were compared to all treatments, including the bermudagrass sod, similar trends were observed. Plots mulched with grass clippings had higher soil K concentrations than any other treatment (Table 1). Foliar Fe for the pinebark-nugget and hardwood-leaf treatments were higher than for the pine-straw and chipped-limb treatments (Table 1). All treatments were within Fe sufficiency ranges for pecan growth (0.75% to 0.95%) (O’Barr et al. 1989). Leaf Mn for the grass-clipping treatment were higher than those for bermudagrass sod, pine straw, chipped limb, or leaf treatments (Table 1). Again, all treatments were within Mn sufficiency ranges (100 to 800 ppm) (O’Barr et al. 1989).

Foliar K levels were not affected by any of the treatments (P = 0.2697). All treatments were within the K sufficiency ranges for optimal pecan growth (0.75% to 0.95%) suggested by O’Barr et al. (1989). Foliar N concentrations were not affected by any treatment (P = 0.5674). However, it is noteworthy that trees in all treatments were below the minimum recommended N concentrations for pecan trees (range 2.70% to 2.90%) (O’Barr et al. 1989). The nitrogen level for the mulched group was 2.20% and for the sod plots, 2.17%. Soil pH levels were unaffected by the treatments (P = 0.0594).

Increased growth, as measured by trunk crosssectional area (TCSA), for mulched trees observed in this study were reported previously (Foshee et al. 1996). Mulched trees grew approximately 60% more in TCSA than with bermudagrass sod treatments. This study demonstrated that trees mulched with grass-clipping mulch had higher soil K concentrations than all other treatments. In addition, some mulch treatments showed increased foliar Fe and Mn levels. Overall, no other significant nutritional differences or trends were found. It is likely that other effects attributed to mulching—such as moisture conservation, evaporation reduction (Russel 1939), and increased soil organic matter (Ashworth and Harrison 1983)—contributed substantially to observed growth differences.

Information from this study is applicable to establishment of young trees in the urban forest and orchards. Pecans are a particularly suitable tree for evaluating nutritional effects of mulches, due to extensive literature available on foliar nutrient sufficiency ranges.