A Science Fair Project: How Much Water Does Mulch Save?

by John S. Welsh

[Editor's note: Does the phrase, “science fair project,” make you flinch? It does me. I get calls, not from school children, but from parents, frantically asking for direction regarding science fair projects involving plants. The experiment has either failed or seems too ridiculous to pursue (e.g., what is the effect of coffee on tomato plants?) I am fortunate to have a son who is a gardener, so suggesting to him a plant-type science fair project was well received. Below is John’s (or should I say our) 8th grade science fair project, "Conserving Water With Mulch." It answers the question posed above. John is a “published” gardener. The project was modified for use in the Junior Master Gardener®’s curriculum, Operation W.A.T.E.R. (Texas Cooperative Extension, The Texas A&M University System) and presented here for your edification (and to show a father’s pride) -Douglas F. Welsh]

“Conserving Water with Mulch”

Mulch is commonly recommended for landscape use to reduce water lost due to evapotranspiration (ET). The goal of this experiment was to find out if mulch truly reduces water lost due to ET. Using container-grown Dracaena marginata plants, mulch was added as a treatment compared to control plants. Control plants are the plants with no mulch. ET (water lost) was measured by weighing the plants in grams that convert directly into milliliters. At 6:00 pm daily, for seven days, the weight of each plant (in grams) was taken to get the “day-end weight.” The day-end weight was subtracted from the weight of the plant at “container capacity” to get the “daily ET” (in milliliters). This data was used to compare the ET of mulched plants and those with no mulch. A percentage change in ET was calculated.

Scientific concepts and principles related to my project include:

• Evaporation (E) is when water changes into water vapor.

• Transpiration (T) is when the water in plants changes to water vapor and escapes through plant leaves.

• Evapotranspiration (ET) is the amount of water that transpires through a plant’s leaves combined with the amount that evaporates from the soil in which the plant grows. ET can be used to predict how much water a plant needs per day, week, or year.

• Container capacity of a pot is how much water the soil in the pot can hold after any excess water has drained out.

• Gravimetric method of measurement states that one gram of water equals one milliliter of water. This makes it easy to measure container capacity and water lost through ET.

• Mulch is any loose material placed over the soil to control weeds, keeps soil temperatures more constant, and conserve soil moisture. Mulch is usually a coarse organic matter. Common mulches are grass clippings, leaves, saw dust, straw, and pine bark.

Hypothesis:

Mulch can reduce the amount of water lost through ET by plants.

Materials:

• Scale (diet scale that measures in grams)

• 12 Dracaena marginata plants growing in 4-inch diameter plastic pots

• Shredded pine bark mulch

• Water source

• Light source (indoor or outdoor)

• Graduated cylinder

• 12-inch ruler

Procedure:

Get 12 Dracaena marginata plants that are in four-inch diameter plastic pots

Label six of the plants with numbers M1 through M6 (M standing for mulched); label the remaining six plants C1 through C6 (C standing for “Control” or plants with no mulch)

Place a one-inch layer of bark mulch on the soil of plants M1 through M6

At 6:00am on Day 1 of the experiment, water each plant with enough water to cause water to drain out of the bottom of the pots; let the excess water drain freely from the pots for 15 minutes; this makes the soil in each pot reach “container capacity.”

Weigh all plants to obtain a Start Weight for each plant

Put all 12 plants in a sunny window indoors or in bright light outdoors under a roofed patio. A greenhouse would be perfect to provide light and eliminate rainfall

For one week at 6:00pm daily, weigh all plants to determine a Daily water used, or Daily ET, by each plant. The math concepts and calculations used include:

one gram of water = one milliliter of water

Start Weight (grams) at container capacity – Day End Weight (grams) = Daily ET (milliliters)

Record the Start Weight, Day End Weight, and Daily ET for the plants every day of the experiment

To begin with the same Start Weight for each plant throughout the week, it is important to water each plant daily with the amount equal to the previous day’s Daily ET. This brings the pot back to container capacity every day.

• From the data collected for the week, calculate the Average Daily ET for each plant throughout the week. Using the Average Daily ET per plant throughout the week, calculate the Average Daily ET for ALL the plants in the Control group of plants and then do the same calculation for the Mulched plants.

• To calculate the percent change in Daily ET due to mulch, use the formula:

  [(Average Daily ET for Control plants - Average Daily ET for Mulched plants) / Average Daily ET for Control plants] x 100 = Percent change in Daily ET due to mulch

Observations

The Average Daily ET for the control plants ranged from 14.3 ml to 27.7 ml (Table 1 and Figure 1). The Average Daily ET for mulched plants ranged from 11.7 ml to 19.4 ml. The Average Daily ET for all mulched plants was 15.2 ml, and the Average Daily ET for all plants without mulch was 20.3 ml of water (Table 1).

Table 1. Table presents Average Daily ET (milliliters) of each control plant and mulched plant for a week period. Average Daily ET for all mulched plants was 25 percent less than for control plants.

Plant Number     Control       Mulch
                 (no mulch)    treatment
      1          17.7          13.7
      2          14.3          12.0
      3          16.6          18.9
      4          20.3          15.4
      5          25.1          19.4
      6          27.7          11.7
          
Average Daily ET 20.3          15.2

Conclusion: The hypothesis of this experiment was that mulch can reduce the amount of water lost to ET from plants. This experiment supported the hypothesis. The mulched plants had a 25 percent decrease in Average Daily ET compared to plants without mulch (Table 1). In other words, the plants without mulch used 25 percent more water than those with mulch.

In conclusion, mulch did reduce the amount of water used (ET) in Dracaena marginata plants grown in pots. It appears that the mulch reduced water lost due to evaporation (E) from the soil surface. Transpiration (T) of the plants was likely similar.

It is reasonable to conclude from this experiment that mulch used in a flower or shrub bed, vegetable garden, or outdoor container plants would also reduce water use by similar amounts.