Alternatives to Plastic Mulch for Organic Watermelon Production

Alternatives to Plastic Mulch for Organic Watermelon Production

The material below was first presented as a poster:Silvernail, Bomford, and Harvey

Anthony Silvernail, Michael Bomford, and Bryant Harvey. 2006. Alternatives to plastic mulch for organic watermelon production. Kentucky Academy of Science Meeting, Agicultural Sciences Section, November 10, 2006. Morehead, KY.


Abstract

     Black plastic mulch is commonly used in conventional and organic production of watermelon (Citrullus lanatus [Thunb.] Matsum. et Nakai) to conserve soil moisture, prevent weed emergence, promote early ripening, and prevent fruit rots.  Because it is a costly off-farm input with high embodied energy that generates waste instead of building soil organic matter, we set out to evaluate alternatives suitable for organic watermelon production.  In 2006 we grew seeded and seedless watermelon varieties (Sangria and Charleston Jr., respectively) organically, following a winter cover crop of winter rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth).  We compared the standard bed preparation system – cover crop incorporation followed by mulching with black plastic to three alternative systems: 1) cover crop roll-down; 2) cover crop roll-down followed by mulching with hay; and 3) cover crop incorporation followed by mulching with newsprint and hay.  Three weeks after transplanting we measured vine length and counted and identified weeds.  Eight weeks after transplanting we harvested and weighed all fruit and counted and identified weeds.  The black plastic system out-yielded all but the newspaper and hay mulch system.  Early growth and final yields were lowest, and weed counts highest, in the roll-down system without additional mulch.  The newspaper and hay mulch system was the most promising alternative to plastic mulch among the systems tested. 


Introduction  

    Organic weed management tactics should be designed with an awareness of system sustainability, providing sufficient yields for economic sustainability, while maintaining or improving soil quality.1  Black plastic mulch has long been used in conventional vegetable production as an effective means of weed control, which also warms the soil in spring, allowing earlier production of  warm season crops like melons. Organic standards allow the use of plastic mulch, even though its high embodied energy (308 GJ/t),2 off-farm origin, and annual disposal needs run counter to the standards’ emphasis on resource cycling. Many organic growers would prefer to use mulches that can be produced on-farm and incorporated into soil to build soil organic matter. Such alternatives include newspaper, hay, and roll-chopped cover crops. We set out to compare organic watermelon yield and weed pressure in soil mulched with black plastic or these alternatives.

Objectives


Materials and Methods

    The experiment was conducted at the Kentucky State University Research Farm during the summer of 2006 using a randomized complete block design with four mulch treatments and three replicates. Plots were 25 m long and spaced 2.5 m apart. Mulches were: 

     Hairy vetch and winter rye were planted in all plots in September of 2005 at 33 and 66 kg/ha, respectively.  Plastic and Newspaper treatments were prepared with two passes of a Celli Spading Machine to incorporate the cover crop before mulch application. Hay and Rolldown treatments were prepared with a single pass of a Buffalo Rolling Stalk Chopper to kill and flatten the cover crop without incorporating it. Feather meal fertilizer (Nature Safe, 10-2-8) was surface applied at 78 kg N/ha before transplanting. On 15 June, eight week-old seedless (cv. Charleston Jr.) and seeded (cv. Sangria) watermelon varieties were transplanted into each plot at 3 and 1.5 m spacings, respectively.  

        The longest vine from the second plant of each variety was measured three weeks after transplanting to evaluate early growth (Fig. 3). All melons were harvested, counted and weighed eight weeks after transplanting. All weeds in two 0.5 m2 sample areas in each plot were identified and counted after harvest.


Unrolling hay bale over newsprint
Figure 1. Setting up the Newspaper treatment, with a layer of hay mulch over a single layer of newsprint.
Treatments before planting
Figure 2. Unplanted plots after:
  • cover crop incorporation with a Celli Spading Machine (left, Plastic and Newspaper treatments);
  • mulching with hay (right, Newspaper and Hay treatments); and
  • cover crop roll-down with a Buffalo Rolling Stalk Chopper (far right, Hay and Rolldown treatments).
Bryant measuring watermelon vines
Figure 3. Measuring watermelon vine length three weeks after transplanting.

Results

     Early season vine measurements showed more robust growth on Plastic than on the other mulches (Fig. 5). Melon growth in the Newspaper and Hay treatments was similar, with growth slowest in the Rolldown treatment.

     Weed counts were highest in the Rolldown treatment for both watermelon varieties (Figs. 4 & 7). Total weed counts in the Plastic treatment were negligible, and significantly lower than with other mulches. Similar total weed counts were found in the Newspaper and Hay treatments. Dominant broadleaf weeds were smooth pigweed and lambsquarters. Dominant grassy weeds were crabgrass, goosegrass, stinkgrass, and giant foxtail. Broadleaf and grassy weeds showed similar responses to the mulch treatments. 

     The yield response to mulch was similar for the seeded and seedless watermelon varieties. Weed density and yield were inversely related for both varieties (Fig. 7).  Melon counts and weight were much lower in the Rolldown treatment than in the Plastic treatment (Figs. 6 & 7). Yields did not differ significantly between the Newsprint and Hay treatments, or between the Plastic and Newsprint treatments.

Plastic mulch Rolldown with hay mulch
Newsprint with hay mulch Rolldown without additional mulch
Figure 4. The four treatments after harvest. Clockwise from upper left: Plastic, Rolldown with Hay, Rolldown without hay, and Newspaper with hay.

Watermelon vine length
Figure 5. Length (+S.E.) of longest watermelon vine on each plant three weeks after transplanting. Bars labeled with the same letter do not differ significantly  (Tukey's test, alpha=0.05).

Watermelon yield
Figure 6. Watermelon yield, expressed as fruit count (left) and weight (right). Bars of the same color labeled with the same letter do not differ significantly (Tukey's test, alpha=0.05).

Weed-Yield Relationship
Figure 7. Relationship between weed density and watermelon yield, as determined by data collected from four different mulch treatments and two watermelon varieties (n=3).

Conclusion

     The Newspaper mulch treatment slowed early season vine growth relative to plastic mulch, but still offered the most promising alternative to plastic. Watermelon yield was depressed by approximately 33, 50 and 98%, relative to plastic, by the Newspaper, Hay, and Roll-down treatments respectively. All of the alternative treatments offer environmental benefits over plastic: They do not require mulch removal or disposal after harvest; they are more likely to build soil organic matter; and they require relatively little energy input. The yield reductions, relative to plastic, represent trade-offs between yield and environmental impact. This trade-off was least pronounced for the Newspaper treatment, making it the most likely compromise for sustainable growers seeking a balance between economic return and environmental impact.

References 

  1. Roger Fay, Graham Treloar and Usha Iyer-Raniga. 2000. Life-cycle energy analysis of buildings: A case study. Building Research and Information 28: 31-41.
  2. USDA. 1999. National Organic Program Regulations (Standards) and Guidelines. http://www.ams.usda.gov/nop/NOP/NOPhome.html

Acknowledgements

Bryant Harvey completed his work on this project as a student intern with Kentucky State University's Research and Extension Apprenticeship Program. Watermelon seed was donated by US Seedless, LLC. Evans-Allen funds were used for all other salaries, materials, and supplies. We thank Eddie Reed and Ken Andries for help with study setup. The study will be repeated in 2007.

College of Agriculture, Food Science and Sustainable Systems