Effects of Mulch Type and Fertilizer Placement on Weed Growth and Soil pH and Nutrient Content

in HortTechnology

Four different organic mulches were applied to 1-m2 plots of Margate fine sand soil that were irrigated three times per week. A 8N–0.9P–10K–4Mg controlled-release fertilizer was applied above or below these mulches to determine the effects of fertilizer placement on weed growth and soil pH, nitrate–nitrogen, ammonium–nitrogen, potassium (K), and magnesium (Mg) concentrations. Unfertilized plots were used to determine mulch effects on soil pH and nutrient content. Fertilizer placement generally had no effect on any of these soil fertility parameters nor did it affect weed numbers. Cypress mulch increased soil K concentrations, and pine bark and eucalyptus mulch increased soil Mg over that of unmulched plots when no fertilizer was applied. The presence of any mulch type greatly reduced weed numbers over that of unmulched plots.

Abstract

Four different organic mulches were applied to 1-m2 plots of Margate fine sand soil that were irrigated three times per week. A 8N–0.9P–10K–4Mg controlled-release fertilizer was applied above or below these mulches to determine the effects of fertilizer placement on weed growth and soil pH, nitrate–nitrogen, ammonium–nitrogen, potassium (K), and magnesium (Mg) concentrations. Unfertilized plots were used to determine mulch effects on soil pH and nutrient content. Fertilizer placement generally had no effect on any of these soil fertility parameters nor did it affect weed numbers. Cypress mulch increased soil K concentrations, and pine bark and eucalyptus mulch increased soil Mg over that of unmulched plots when no fertilizer was applied. The presence of any mulch type greatly reduced weed numbers over that of unmulched plots.

The use of organic mulches around the bases of landscape plants has been widely advocated (Black and Ruppert, 1995; Thomas, 2002). Benefits of mulching include soil moisture retention (Ashworth and Harrison, 1983; Greenly and Rakow, 1995; Tukey and Schoff, 1963), reduction of soil temperatures (Ashworth and Harrison, 1983; Greenly and Rakow, 1995; Stinson et al., 1990), weed growth reduction (Ashworth and Harrison, 1983; Billeaud and Zajicek, 1989; Stinson et al., 1990), enhanced plant growth (Brown, 1996; Greenly and Rakow, 1995), improved penetration of irrigation or rainfall (Oliveira and Merwin, 2001), and enhanced nutrient availability (Tukey and Schoff, 1963). Potential problems associated with organic mulches are reduced nitrogen availability (Billeaud and Zajicek, 1989), water repellency (Gartner, 1978), and allelopathy (Duryea et al., 1999; Still et al., 1976).

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One question that is often asked by landscapers is whether fertilizers should be applied above or below the mulch layer. Gilman et al. (1990) concluded from column leaching studies through cypress wood chips that NH4-N and NO3-N readily leached through this commonly used mulch material. However, as organic mulches decay, they may use N in the process (Ashworth and Harrison, 1983). Billeaud and Zajicek (1989) and Sonsteby et al. (2004) found that soil N in mulched plots was lower than in unmulched plots. Brown (1996), as well as Tukey and Schoff (1963), compared pH and soil phosphorus (P), potassium (K), and magnesium (Mg) concentrations in plots mulched with organic mulches or none, but we are unaware of any studies documenting the effects of above versus below mulch applications of fertilizers on soil nutrient concentrations. The objectives of this study were to compare soil pH and nutrient contents as well as weed growth in plots mulched with four different organic mulches with fertilizer applied above or below the mulch.

Materials and methods

A 10 × 10-m area of land at the University of Florida's Fort Lauderdale Research and Education Center (lat. 26°5′5.6″N, long. 80°14’34.6″W) having a 0° slope and a Margate fine sand soil was used for this study. This soil has a cation exchange capacity of 5.6 meq·100 g (U.S. Dept. of Agriculture, 1984). Other chemical properties of the soil in these plots are presented in Table 1. The existing vegetation, consisting of a sparse cover of bahiagrass (Paspalum notatum) and various weed species, was sprayed with glyphosate 1 week before experiment initiation. A factorial experiment with five mulch treatments and three fertilizer treatments was set up using 1 × 1-m plots as the experimental units. There were six replications of each mulch–fertilizer combination arranged in a completely randomized design. Mulch treatments were: 1) no mulch, 2) coarse (2 to 4 inches long × 1 to 2 inches wide × 1/4 to 1/2 inch thick) pine (Pinus elliottii) bark mulch (pH 3.6), 3) finely shredded eucalyptus (Eucalyptus spp.) wood mulch (pH 4.6), 4) finely shredded cypress (Taxodium distichum) wood chip mulch (pH 4.8), and 5) pine (Pinus elliottii) needle mulch (pH 4.4). Two cubic feet of each mulch was spread out uniformly on each plot resulting in an average depth of 2.2 inches. A 8N–0.9P–10K–4Mg landscape fertilizer with controlled-release nitrogen (N), K, and Mg (Nurserymen's Sure Gro Corp., Vero Beach, Fla.) was applied at a rate of 73 g·m−2 using a drop spreader (Lesco, Rocky River, Ohio) according to the following treatments: 1) fertilizer applied before mulching or 2) fertilizer applied after mulching. Six additional plots of each mulch treatment received no fertilizer to determine the effects of the mulches themselves on soil pH and nutrient contents. Irrigation from a single rotary head (Pro Plus model 11003; K-rain, West Palm Beach, Fla.) mounted on a centrally located 2-ft-high riser provided ≈0.5 inch of water every 3 d. Irrigation water was drawn from a pond and had a pH of 7.2 during the first experiment (Expt. 1) and 6.8 during the second experiment (Expt. 2). This water contained 0.5 μg·mL−1 of K and 1.4 μg·mL−1 of Mg.

Expt. 1 was established on 14 Nov. 2003 and was ended 6 months later. The entire experiment was repeated on an adjacent block of land beginning on 25 June 2004 and ended 6 months later. Expt. 1 was conducted during the “dry season” (total rainfall = 12.8 inches, rainfall during first 6 weeks = 1.1 inches) in southern Florida, whereas the second was done during the “rainy season” (total rainfall = 33.4 inches, rainfall during the first 6 weeks = 13.7 inches). Rainfall data were obtained from the Florida Automated Weather Network [FAWN (Univ. of Florida, 2006)], which maintains a recording station ≈200 m from the research plots.

Six weeks after initiation of each experiment, soil core samples were taken from each plot for nutrient extractions. Sampling was done by scraping aside any mulch and taking a core of soil 2 cm in diameter × 10 cm deep. The top 2 cm of each core was discarded to prevent possible contamination by fertilizer prills and three cores per plot were pooled for analysis. Soil pH was determined from deionized water extractions, nitrate–nitrogen (NO3-N) and ammonium–nitrogen (NH4-N) were extracted with 2 M potassium chloride (Mulvaney, 1996; Thomas, 1996), and soil K and Mg were extracted with 1 M ammonium acetate (Helmke and Sparks, 1996; Suarez, 1996). NO3-N, NH4-N, and pH were determined by selective ion electrodes (Mulvaney, 1996; Thomas, 1996), whereas K and Mg concentrations were determined using atomic absorption spectroscopy (Helmke and Sparks, 1996; Suarez, 1996).

After 6 months, all weeds growing in each plot were counted and categorized as dicots [primarily large flower pusley (Richardia grandiflora)] or grasses [primarily torpedograss (Panicum repens)]. All data were analyzed using analysis of variance with mean separations by the Waller-Duncan k-ratio method (SAS Institute, Cary, N.C.).

Results and discussion

The numbers of dicot and total weeds per plot were significantly greater for unmulched plots than for plots mulched with any type of mulch in both experiments (Table 2). The number of grass weeds did not differ between mulched and unmulched plots because the predominant grass weed, torpedograss, spreads vegetatively through underground stolons as opposed to seedlings, which can be suppressed by mulches. In general, mulch type had no effect on the numbers of any type of weed. Likewise, fertilizer placement above or below the mulch layer had no effect on numbers of any type of weeds.

Table 1.

Effects of mulch type on unfertilized soil pH and extractable potassium (K) and magnesium (Mg) concentrations.

Table 1.
Table 2.

Effects of mulch type and placement of an 8N–0.9P–10K–4Mg fertilizer above and below the mulch layer on numbers of dicot weeds, grass weeds, and total weeds per 1-m2 (10.8-ft2) plot.

Table 2.

Soil pH was unaffected by mulch treatment or fertilizer (Table 3). Tukey and Schoff (1963) also found that mulching materials had no consistent effect on soil pH. However, other studies have shown that soil pH decreases when organic mulches are used and that this decrease is proportional to the depth of these mulches (Billeaud and Zajicek, 1989). Our soil samples were taken after 6 weeks, compared with 6 months for the latter study, and our shorter equilibration time may have influenced our results. Soil pH was consistently higher across all treatments in Expt. 1 than in Expt. 2. This may have been the result of the slightly higher pH of the irrigation water during Expt. 1 than during Expt. 2.

Table 3.

Effects of mulch type and placement of an 8N–0.9P–10K–4Mg fertilizer above and below the mulch layer on soil pH and extractable potassium (K) and magnesium (Mg) concentrations.

Table 3.

Nitrate-N in soil extracts was below 1 μg·g−1 for all treatments (data not shown). Neither mulch presence or type nor fertilizer placement had any effect on soil NH4-N concentrations in either experiment with values ranging from 7 to 10 μg·g−1 for all treatments (data not shown). Likewise, neither mulch presence nor type affected soil NH4-N concentrations in unfertilized plots, which were similar to those of the fertilized plots (data not shown). On the other hand, Sonsteby et al. (2004) found that soil NO3-N and NH4-N were significantly decreased when spruce (Picea spp.) bark was applied to an unfertilized morainic loam soil. Their different response may have been the result of a much longer time that the soil was exposed to mulch before sampling (3 years vs. 6 weeks).

Mulch type generally had no effect on soil K and Mg concentrations, but in Expt. 2, soil K concentrations in plots with no mulch were lower than any mulched plots (Table 3). Tukey and Schoff (1963) found that plots mulched with organic materials had significantly higher soil K, but not Mg, concentrations than unmulched plots. They attributed this to K release during the decomposition of the mulch materials. When we compared soil K and Mg concentrations in unfertilized plots, we found that the presence of cypress mulch significantly increased soil K concentrations above that of unmulched plots in Expt. 2 (Table 1). Water-extractable K concentrations in cypress, melaleuca, pine bark, and pine needle mulches of ≈60, 69, 78, and 41 μg·cm−3, respectively, represent significant contributions of this element to soil fertility.

Both pine bark and eucalyptus mulch increased soil Mg concentrations above that of unmulched control plots in Expt. 2 (Table 1). Water-extractable Mg concentrations in cypress, melaleuca, pine bark, and pine needle mulches were ≈3, 7, 10, and 9 μg·cm−3, respectively. These concentrations of Mg are considerably lower than those for K but could still potentially influence soil Mg concentrations. Interactive effects of mulch type and fertilizer placement on soil K and Mg concentrations were not significant in either experiment.

Although fertilizer placed below mulch resulted in higher soil K concentrations than fertilizer placed above mulch in Expt. 1 plots, that trend was not statistically significant in Expt. 2 plots (Table 3). This suggests that some K may have been adsorbed to mulch particles, which retarded the leaching of this element into the soil. Similar effects might also have been expected for Mg, although fertilizer placement had no consistent effects on soil Mg concentrations.

In summary, organic mulch of any type greatly reduced dicot weed numbers with no one type superior to any other. Certain types of mulches such as cypress, pine bark, or eucalyptus mulch by themselves increased soil K or Mg concentrations, but none affected soil N concentrations. Fertilizer placement above or below mulch had little or no effect on soil nutrient concentrations and thus there appears to be no advantage to applying fertilizers under mulches. These conclusions are based on soil sampling 6 weeks after mulching and fertilization and results may differ if sampling is done after longer time intervals after mulch and fertilizer application or under different environmental conditions.

Literature cited

  • AshworthS.HarrisonH.1983Evaluation of mulches for use in the home gardenHortScience18180182

  • BilleaudL.A.ZajicekJ.M.1989Influence of mulches on weed control, soil pH, soil nitrogen content, and growth of Ligustrum japonicumJ. Environ. Hort.7155157

    • Search Google Scholar
    • Export Citation
  • BlackR.J.RuppertK.C.1995Your Florida landscape: A complete guide to planting and maintenanceUniv. Fla. Coop. Ext. Serv. PublSP135

  • BrownS.H.1996Response of hibiscus to organic mulchesProc. Florida State Hort. Soc.1093033

  • DuryeaM.L.EnglishR.J.HermansenL.A.1999A comparison of landscape mulches: Chemical, allelopathic, and decomposition propertiesJ. Arboriculture258896

    • Search Google Scholar
    • Export Citation
  • GartnerJ.B.1978Using bark and wood chips as a mulch for shrubs and evergreensAmer. Nurseryman14795355

  • GilmanE.F.YeagerT.H.WeigleD.1990Nitrogen leaching from cypress wood chipsHortScience2513881390

  • GreenlyK.M.RakowD.A.1995The effect of wood mulch type and depth on weed and tree growth and certain soil parametersJ. Arboriculture21225232

    • Search Google Scholar
    • Export Citation
  • HelmkeP.A.SparksD.L.1996Lithium, sodium, potassium, rubidium, and cesium551574SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • MulvaneyR.L.1996Nitrogen—inorganic forms11231184SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • OliveiraM.T.MerwinI.A.2001Soil physical conditions in a New York orchard after eight years under different groundcover management systemsPlant Soil234233237

    • Search Google Scholar
    • Export Citation
  • SonstebyA.NesA.MageF.2004Effects of bark mulch and N–P–K fertilizer on yield, leaf nutrient status, and soil mineral nitrogen during three years of production. Acta Agriculturae ScandinavicaSection B (Soil and Plant Science)54128134

    • Search Google Scholar
    • Export Citation
  • StillS.T.DirrM.A.GartnerJ.B.1976Phytotoxic effects of several bark extracts on mung bean and cucumber growthJ. Amer. Soc. Hort. Sci.1013437

    • Search Google Scholar
    • Export Citation
  • StinsonJ.M.BrinenG.H.McConnellD.B.BlackR.J.1990Evaluation of landscape mulchesProc. Florida State Hort. Soc.103372377

  • SuarezD.L.1996Beryllium, magnesium, calcium, strontium, and barium575602SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • ThomasG.W.1996Soil pH and soil acidity475490SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • ThomasM.2002Best management practices for protection of water resources in FloridaFlorida Dept. Environ. ProtectionTallahassee

  • TukeyR.B.SchoffE.L.1963Influence of different mulching materials upon the soil environmentProc. Amer. Soc. Hort. Sci.826976

  • Univ. of Florida2006Florida automated weather network6 Nov. 2006<http://fawn.ifas.ufl.edu/scripts/reportrequest.asp>

    • Export Citation
  • U.S. Dept. of Agriculture1984Soil survey of Broward County, Florida, Eastern partSoil Conserv. Serv., U.S. States Dept. AgrWashington, D.C

    • Export Citation

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Contributor Notes

This research was supported by the Florida Agricultural Experiment Station.

I thank Susan Thor, Anita Durden, and William Latham for their assistance in this study.

E-mail: tkbr@ufl.edu

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Article References

  • AshworthS.HarrisonH.1983Evaluation of mulches for use in the home gardenHortScience18180182

  • BilleaudL.A.ZajicekJ.M.1989Influence of mulches on weed control, soil pH, soil nitrogen content, and growth of Ligustrum japonicumJ. Environ. Hort.7155157

    • Search Google Scholar
    • Export Citation
  • BlackR.J.RuppertK.C.1995Your Florida landscape: A complete guide to planting and maintenanceUniv. Fla. Coop. Ext. Serv. PublSP135

  • BrownS.H.1996Response of hibiscus to organic mulchesProc. Florida State Hort. Soc.1093033

  • DuryeaM.L.EnglishR.J.HermansenL.A.1999A comparison of landscape mulches: Chemical, allelopathic, and decomposition propertiesJ. Arboriculture258896

    • Search Google Scholar
    • Export Citation
  • GartnerJ.B.1978Using bark and wood chips as a mulch for shrubs and evergreensAmer. Nurseryman14795355

  • GilmanE.F.YeagerT.H.WeigleD.1990Nitrogen leaching from cypress wood chipsHortScience2513881390

  • GreenlyK.M.RakowD.A.1995The effect of wood mulch type and depth on weed and tree growth and certain soil parametersJ. Arboriculture21225232

    • Search Google Scholar
    • Export Citation
  • HelmkeP.A.SparksD.L.1996Lithium, sodium, potassium, rubidium, and cesium551574SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • MulvaneyR.L.1996Nitrogen—inorganic forms11231184SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • OliveiraM.T.MerwinI.A.2001Soil physical conditions in a New York orchard after eight years under different groundcover management systemsPlant Soil234233237

    • Search Google Scholar
    • Export Citation
  • SonstebyA.NesA.MageF.2004Effects of bark mulch and N–P–K fertilizer on yield, leaf nutrient status, and soil mineral nitrogen during three years of production. Acta Agriculturae ScandinavicaSection B (Soil and Plant Science)54128134

    • Search Google Scholar
    • Export Citation
  • StillS.T.DirrM.A.GartnerJ.B.1976Phytotoxic effects of several bark extracts on mung bean and cucumber growthJ. Amer. Soc. Hort. Sci.1013437

    • Search Google Scholar
    • Export Citation
  • StinsonJ.M.BrinenG.H.McConnellD.B.BlackR.J.1990Evaluation of landscape mulchesProc. Florida State Hort. Soc.103372377

  • SuarezD.L.1996Beryllium, magnesium, calcium, strontium, and barium575602SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • ThomasG.W.1996Soil pH and soil acidity475490SparksD.L.Methods of soil analysis. Part 3. Chemical methodsSoil Sci. Soc. AmerMadison, Wis

    • Search Google Scholar
    • Export Citation
  • ThomasM.2002Best management practices for protection of water resources in FloridaFlorida Dept. Environ. ProtectionTallahassee

  • TukeyR.B.SchoffE.L.1963Influence of different mulching materials upon the soil environmentProc. Amer. Soc. Hort. Sci.826976

  • Univ. of Florida2006Florida automated weather network6 Nov. 2006<http://fawn.ifas.ufl.edu/scripts/reportrequest.asp>

    • Export Citation
  • U.S. Dept. of Agriculture1984Soil survey of Broward County, Florida, Eastern partSoil Conserv. Serv., U.S. States Dept. AgrWashington, D.C

    • Export Citation

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