1999 Iowa Turfgrass Research Report



Effect of Organic and Mineral Mulches on Soil Properties and

Growth of Fairview Flame® Red Maple Trees

Jeffery K. Iles and Michael S. Dosmann

Abstract. Five mineral mulches (crushed red brick, pea gravel, lava rock, carmel rock, and river rock) and three organic mulches (finely screened pine bark, pine wood chips, and shredded hardwood bark) were evaluated over two years to determine their influence on soil temperature, moisture, and pH, and to quantify their effect on growth of Fairview Flame® red maple (Acer rubrum L.). Mulch treatments (2.3-m2 (25-ft2) plots of eight mulches applied as separate treatments and a non-mulched control) were randomly applied to trees in five blocks. Organic mulches were placed directly on bare ground, while mineral mulches were underlaid with a woven polypropylene fabric. Soil temperatures were highest and soil moisture percentages lowest under the mineral mulches and non-mulched control. Soil pH readings were highest under shredded bark and wood chips, and lowest in the non-mulched control. Despite these differences in root zone environments, there were no significant differences in tree height. Trees growing in river rock, crushed brick, pea gravel, and carmel rock, however, had larger stem calipers than those growing in shredded-bark plots. Crushed brick, pea gravel, and carmel rock treatments also resulted in greater leaf dry mass than shredded-bark. Leaf dry mass also was greater for trees in crushed brick and pea gravel than screened pine. Our results indicate mineral mulches used in this study do not create growth-limiting soil environments.

 

Benefits of using wood and bark by-products as horticultural mulch over the root zones of landscape plants are well-established (Gleason and Iles 1998; Green and Watson 1989; Greenly and Rakow 1995; Skroch et al. 1992; Watson 1988), however, several actual or perceived problems associated with organic mulches such as, unacceptable appearance (Rakow 1992), creation of a temporary soil nitrogen deficiency (Ashworth and Harrison 1983), potential fire hazard (Hickman and Perry 1996), and rapid decomposition (Rakow 1992) have led to increased usage of mineral or rock mulches. But concerns that materials like rock, gravel, and crushed brick may promote potentially injurious high temperatures both above and below the mulch layer, alkalinization of the soil, and mechanical injury to the stems of plants, have caused many landscape and tree-care professionals to reexamine their rationale for using mineral mulches as suitable ground-covering materials around woody and herbaceous plants. This experiment was designed to evaluate and compare the effects of five mineral and three organic mulches on 1) several soil properties, and 2) growth of Fairview Flame® red maple (Acer rubrum L.).

MATERIALS AND METHODS

Ninety bare-root, 1.6- to 2.0-cm (0.6- to 0.8-in-caliper), 1.2- to 1.5-m (4- to 5-ft-tall), branched Fairview Flame® red maple trees were planted in a Nicollet fine sandy loam soil at the Iowa State University Horticulture Research Station, Gilbert, Iowa (USDA hardiness zone 5a; lat. 42°3'N), on April 22, 1996. The experimental design was a randomized complete block with nine treatments, five blocks (replications), with treatments repeated twice in each replication. Trees were spaced 2.0 m (6.5 ft) apart in north-south oriented rows with 3.0 m (10 ft) between rows. Trees were hand watered once on the day of planting to facilitate establishment. Treatments consisted of 2.3-m2 (25-ft2) plots of eight mulches: a 5.0-cm (2-in) layer of 1.9-cm (0.75-in) diameter crushed red brick, 0.9-cm (0.4-in) diameter pea gravel, 1.3-cm (0.5-in) diameter lava rock, and 2.5-cm (1.0-in) diameter carmel rock (chert); a 7.5-cm (3.0-in) layer of 3.8-cm (1.5-in) diameter river rock; a 10.0-cm (4.0-in) layer of 4.0- to 6.0-cm (1.6- to 2.4-in) long finely screened pine bark, 2.0- to 3.0-cm (0.8- to 1.2-in) diameter pine wood chips, and 4.0- to 5.5-cm (1.6- to 2.2-in) long shredded hardwood bark (mostly oak); and a non-mulched control maintained as bare ground. Organic mulches were placed directly on bare ground, while mineral mulches were underlaid with a woven polypropylene fabric (DeWitt Landscape Pro 5). Weeds and other unwanted vegetation within and between treatment plots, and along the east and west borders of the plots (15-cm wide) were controlled with glyphosate (1% v/v). Plots were not fertilized.

Soil moisture was recorded weekly during the growing season (June-August) in 1996 and 1997 with a Theta Probe (meter type HH1, sensor type ML1; Delta-T Devices Ltd., Cambridge, United Kingdom) soil moisture sensor at 6 cm (2.4 in) below the soil surface. Soil temperature also was determined weekly using a portable Barnant 115 thermocouple thermometer (model 600 2810; Barrington, Ill.) at 10 cm (4.0 in) below the soil surface. Both soil moisture and soil temperature readings were taken on the south side of the tree, approximately 0.6 m (2.0 ft) from the trunk.

Stem diameter at 15 cm (6 in) above the soil surface and tree height from soil surface to the highest point in the crown were measured on September 19 and 20, 1997, respectively. Leaves were harvested from each tree on October 4 and 5, 1997, dried at 67°C (153°F) for five days, and weighed. Randomly chosen soil samples (one from each treatment in each replication) taken at the soil surface immediately below the mulch treatment, were retrieved on December 1, 1997 and again on June 17, 1998 to determine pH. All data were subjected to analysis of variance and means separated by least significant difference (P 0.05).

RESULTS AND DISCUSSION

Effects on soil temperature and moisture. In 1997 (data from 1996 are not presented because unseasonably cool, wet conditions caused a lack of statistical significance), highest soil temperatures were recorded in the non-mulched control plots, followed by pea gravel, crushed brick, and carmel rock treatments (Table 1). Plots covered by organic mulch treatments had significantly lower soil temperatures (mean = 23.4°C/74.1°F) than plots treated with mineral mulches (mean = 25.9°C/78.6°F). Loosely packed organic mulches insulate soils by intercepting and absorbing solar radiation instead of conducting heat energy downward (Waggoner et al. 1960; Montague et al. 1998).

Soil moisture content was highest under the three organic mulches and pea gravel, however, the shredded-bark and pea gravel treatments were not different from lava rock or crushed brick (Table 1). Lowest moisture percentages were recorded in the non-mulched control. Soil moisture under mulch is increased through minimizing soil surface evaporation (Himelick and Watson 1990). In our study, organic mulches that meshed together, and fine-textured mineral mulches like pea gravel, presented a greater barrier to evaporation than coarser mulch materials or bare soil.

Effects on soil chemistry. Previous researchers report organic mulches cause no change in soil pH (Greenly and Rakow 1995; Watson and Kupkowski 1991) or reduce pH of the underlying soil (Billeaud and Zajicek 1989; Hild and Morgan 1993; Himelick and Watson 1990). Mulch-induced pH reduction results from the addition or retention of organic matter, with organic acids produced from decomposition of plant-derived materials accumulating or leaching into the soil (Himelick and Watson 1990). At the completion of our study (1997), soil pH was lowest in the non-mulched control plots and highest under shredded-bark and wood chip mulches (Table 1). Elevated pH under these mulches could have resulted from the leaching of basic cations (NH4+) from decomposing organic matter (Tisdale et al. 1993). If so, we would expect the increase in pH from ammonification to be temporary, however, because pH will decrease as ammonia is oxidized to nitrate by nitrifying bacteria in the soil. In 1998, soil pH readings again were highest under wood chip and shredded-bark mulches. Lowest pH measurements were recorded in the lava rock treatment and in the un-mulched control. While some mineral mulches could contribute to undesirably high soil pH, mineral mulches used in this study did not.

Effects on tree growth. Temperature, moisture, and chemical differences in root-zone environments brought about by the various mulch treatments did not translate into differences in tree height, however, trees growing in pea gravel, crushed brick, carmel, and river rock had larger stem calipers than those growing in shredded-bark plots (Table 2). Stem calipers of trees in the three organic mulch treatments, lava rock, and in the non-mulched control were not different.

Crushed brick, pea gravel, and carmel rock treatments resulted in greater leaf dry mass than shredded-bark plots. Leaf dry mass also was greater for trees in crushed brick and pea gravel than for trees mulched with screened pine. Dry mass of trees in the three organic mulch treatments, river rock, lava rock, and in the control were not different.

Differences in tree growth are most likely linked to temperature differences in the soil environment. Although we did not measure soil temperatures in April and May, based on summer readings it is logical to assume soil temperatures under organic mulches would be cooler and possibly more growth limiting (at least for the shredded-bark and screened pine mulches) than warmer, growth-enhancing temperatures under the mineral mulches (particularly pea gravel, crushed brick, and carmel rock). Holloway (1992) reported similar results in Alaska, where five woody plant species grew best in stone mulch treatments. Elevated pH also might have contributed to poorer growth for trees in the shredded-bark treatments.

CONCLUSION

Our results indicate mineral mulches used in this study do not create growth-limiting soil environments. In fact, the capacity of crushed brick and pea gravel to conduct heat to soils below, particularly in early spring, may be responsible for the observed advantage in leaf dry mass for trees growing in these materials over those growing in soils kept relatively cool by insulating organic mulches such as shredded-bark and screened pine. Mineral mulches used in this study also proved to be relatively inert, causing equal or smaller increases in pH than shredded-bark or wood chips.

These results, however, should not be interpreted as an indictment of organic mulches. Because soils at the ISU Horticulture Research Station are fertile and well-drained, the organic matter and nutrient contributions made by organic mulches may be of less consequence than if the study had been conducted on poor soils. Moreover, had conditions been drier and warmer during the years of the study (1996-97), or if the experiment had been conducted in a warmer climate, organic mulches may have outperformed many of the mineral mulches. Finally, because stem caliper and leaf dry mass measurements of trees growing in wood chips and any of the mineral mulches were not statistically different, blanket statements and generalizations regarding the performance of woody plants mulched with organic or mineral (rock) materials are unwise.

The nursery and landscape industry is fortunate to have a wide variety of mulch materials to choose from, and each has its place in the landscape. But in the final analysis, cost and maintenance considerations dictate which mulch materials will be used.

Acknowledgments. The authors wish to acknowledge and thank the International Society of Arboriculture Research Trust and the Iowa Nursery & Landscape Association Research Corporation for funding this research.

LITERATURE CITED

Ashworth, S. and H. Harrison. 1983. Evaluation of mulches for use in the home garden. HortScience 18(2):180-182.

Billeaud, L.A. and J.M. Zajicek. 1989. Influence of mulches on weed control, soil pH, soil nitrogen content, and growth of Ligustrum japonicum. J. Environ. Hort. 7(4):155-157.

Gleason, M.L. and J.K. Iles. 1998. Mulch matters. Amer. Nurseryman 187(4):24-31.

Green, T.L. and G.W. Watson. 1989. Effects of turfgrass and mulch on the establishment and growth of bare-root sugar maples. J. Arboric. 15(11):268-272.

Greenly, K.M. and D. A. Rakow. 1995. The effect of wood mulch type and depth on weed and tree growth and certain soil parameters. J. Arboric. 21(5):225-232.

Hickman, G.W. and E. Perry. 1996. Using ammonium sulfate fertilizer as an organic mulch fire retardant. J. Arboric. 22(6):279-280.

Hild, A.L. and D.L. Morgan. 1993. Mulch effects on crown growth of five southwestern shrub species. J. Environ. Hort. 11(1):41-43.

Himelick, E.B. and G.W. Watson. 1990. Reduction of oak chlorosis with wood chip mulch treatments. J. Arboric. 16(10):275-278.

Holloway, P.S. 1992. Aspen wood chip and stone mulches for landscape plantings in interior Alaska. J. Environ. Hort. 10(1):23-27.

Montague, T., R. Kjelgren, and L. Rupp. 1998. Surface energy balance affects gas exchange of three shrub species. J. Arboric. 24(5):254-262.

Rakow, D.A. 1992. Mulching: Benefits backed by survey. Arbor Age 12(9):22-29.

Skroch, W.A., M.A. Powell, T.E. Bilderback, and P.H. Henry. 1992. Mulches: Durability, aesthetic value, weed control, and temperature. J. Environ. Hort. 10(1):43-45.

Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil Fertility and Fertilizers. MacMillan Publishing Co., New York, NY. 634 pp.

Waggoner, P.E., P.M. Miller, and H.C. DeRoo. 1960. Plastic mulching - principles and benefits. Bull. No. 634, Conn. Agric. Exp. Stn., New Haven.

Watson, G.W. 1988. Organic mulch and grass competition influence tree root development. J. Arboric. 14(8):200-203.

Watson, G.W. and G. Kupkowski. 1991. Effects of a deep layer of mulch on the soil environment and tree root growth. J. Arboric. 17(9):242-245.

 

Table 1. Effect of eight mulch treatments and a non-mulched control on soil temperature, percentage (%) soil moisture, and soil pH.

 

Temperaturez

Moisturey

pHx

Treatment

(°C)

(%)

1997

1998

Control

29.3w av

19u d

6.03t d

5.86s d

Pea gravel

27.6 b

31 ab

6.44 b

6.14 bc

Crushed brick

26.2 c

30 bc

6.29 bc

6.04 cd

Carmel

26.2 c

29 c

6.29 bc

6.06 cd

River rock

25.2 d

29 c

6.47 b

6.33 b

Lava rock

24.5 d

30 bc

6.21 cd

5.82 d

Shredded-bark

23.6 e

31 ab

6.82 a

6.81 a

Wood chip

23.3 e

32 a

6.81 a

6.37 b

Screened pine

23.2 e

32 a

6.13 cd

6.14 bc

zSoil temperature measured at 10 cm (4 in) depth, between 2:00 and 4:00 p.m., CST.

ySoil moisture measured at 6 cm (2.4 in) depth, between 2:00 and 4:00 p.m., CST.

xSoil samples for pH measurements collected at 0- to 10-cm (0- to 4-in) depth.

wData shown are means of 12 dates x 5 replications (n=60) in 1997.

vMean separation within columns by LSD, P 0.05.

uData shown are means of 12 dates x 5 replications (n=60) in 1997.

tData shown are means of 5 observations. Soil samples collected on December 1, 1997 for pH determination.

sData shown are means of 5 observations. Soil samples collected on June 17, 1998 for pH determination.

 

Table 2. Effect of eight mulch treatments and a non-mulched control on stem caliper, height, and leaf dry mass of Acer rubrum Fairview Flame®.

 

Heightz

Stem calipery

Leaf dry massx

Treatment

(cm)

(cm)

(g)

Lava rock

222w av

4.1u ab

441t abc

Wood chip

222 a

4.1 ab

423 abc

Pea gravel

220 a

4.2 a

467 a

Crushed brick

219 a

4.2 a

478 a

Control

219 a

4.1 ab

419 abc

Carmel

218 a

4.2 a

463 ab

River rock

214 a

4.2 a

449 abc

Screened pine

214 a

4.0 ab

398 bc

Shredded-bark

210 a

3.9 b

383 c

zHeight measured from ground level to highest shoot apex on September 20, 1997.

yStem caliper measured 15 cm (6 in) above ground level on September 19, 1997.

xLeaves harvested October 4 and 5, 1997.

wData shown are means of 10 observations.

vMean separation within columns by LSD, P 0.05.

uData shown are means of 10 observations. Each observation is the average of measurements taken at (1) the widest point on the stem, and (2) rotated 90° clockwise from the first measurement.

tData shown are means of 10 observations.




Iowa State University ISU Horticulture:Publications:1999 Turfgrass Report College of Agriculture