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To Mulch Or
Not To
Mulch:
COMPARING
BROCCOLI
PRODUCTIVITY IN
MULCHED AND
UNMULCHED AREAS
IN NAZARE
PAULISTA
Zena Lee Park
Center for Environmental Research and Conservation,
Columbia University, New York, New York, 10027
U.S.A
[email protected]
INTRODUCTION
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Mulch acts as the skin of the soil, a protective layer covering the surface of the soil. Often the
mulching technique can eliminate fertilizing, thereby economizing the workload of the gardener.
Mulch takes away the sunlight weeds need
The existence of mulch lessens water evaporation and adds essential microorganisms and nutrients to
the soil. Areas that are mulched sustain balanced and healthy soil. The use of mulch is an important
strategy to minimize the risks of soil degradation. Since mulching avoids excess watering, it avoids
the passage of nutrients into subsoil.
The Natural Resources Conservation Service-United States Department of Agriculture claims mulch
does the following: protects the soil from erosion, reduces compaction from the impact of heavy
rains, conserves moisture, reduces the need for frequent waterings, maintains a more even soil
temperature, prevents weed growth, keeps fruit and vegetables clean.
Mulching a garden mimics the natural setting of a forest. Since, in nature, there will always be some
kind of organic litter or some type of protection on the ground. For example, leaves fall and then
cover the soil, thereby cooling and shielding the soil and root system from rain that might cause soil
erosion or hardening. In an unmulched area, sun exposure often kills many of the microorganisms
that make the soil healthy.
The main objective of this study was to determine if there were any differences between mulched and
unmulched areas of broccoli, indicating possible differences in productivity. If so, then whether or
not there was any correlation between type of treatment and factors that show broccoli productivity.
The question was: Does the existence of mulch increase the productivity of an organic garden? The
null hypothesis (Ho) was: The existence of mulch does not effect the productivity of an organic
garden. The alternative hypotheses were: The existence of mulch increases the level of productivity
in an organic garden (Ha). The existence of mulch decreases productivity (Hb).
METHODS
Description of Study Area
The studied area was an organic garden, privately operated by the Center of Light community, in the Atlantic Rainforest of
Nazare Paulista, Sao Paulo State, Brazil. On June 16, 2001, all data was collected. The observed eight broccoli plants
were found at the following location, according to the GPS unit used (Trimble GeoExplorer3): 23  11’56.907” S,
4621’234.408”W, at an altitude of 798.3m. The mulched and the unmulched gardens lay evenly, side by side, in a
combined rectangular area of 880cm x 300cm. The mulched garden included African Red Grass (Melinis minutiflora)
that lay on top of the soil.
Summary of Procedures
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In order to compare broccoli (Brassica oleracea) productivity in mulched versus unmulched treated areas, the following
characteristics were calculated and measured: plant height, plant width, number of stalks on each plant, leaf areas,
number of leaves on each plant, and soil temperature taken at the base of each plant.
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There was only one crop of the same species that had been divided equally between mulched and unmulched treatments,
located in the same area, and planted at the same time. This crop of broccoli was three months old at the time of the
study.
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Eight individual broccoli plants were chosen for observation within this half mulched, half unmulched area of broccoli.
Four rows were chosen randomly, each row had two broccoli plants—one in the mulched area and one in the
unmulched.
RESULTS
Significant differences were found in two out of the six variables looked at: number of broccoli stalks and the
leaf sizes (Table 1). According to the statistical test used (t-test), the two variables gave calculated t values
that were greater than the critical t values. This indicated that the null hypothesis should be rejected. The
average number of broccoli stalks for the mulched area was much greater than the average number of broccoli
stalks for the unmulched area (Figure 1, Table 2). The average leaf size found on each broccoli plant in the
mulched area was higher than the average leaf size found on each broccoli plant in the unmulched area
(Figure 2, Table 3). As for the other variables, leaf abundance (Figure 3, Table 4), plant height (Figure 4,
Table 5), plant width (Figure 5, Table 6), and soil temperature (Figure 6, Table 7), t-tests showed no
significant differences.
Statistical Analysis: t-test
Type
n df t
Broccoli Stalks
4 6
Leaf Sizes
12 22
Number of Leaves 4 6
Plant Height
4 6
Plant Width
4 6
Soil Temperature 4 6
critical t, P< 0.05 Significance
2.944
2.447 Reject Null Hypothesis
3.141
2.074 Reject Null Hypothesis
1.2175
2.447 Accept Null Hypothesis
1.789
2.447 Accept Null Hypothesis
2.148
2.447 Accept Null Hypothesis
1.842
2.447 Accept Null Hypothesis
Table 1
Broccoli Stalks
Average Number of
Broccoli Stalks
20
15
10
5
0
Mulched
Unmulched
Treatments
Figure 1
Number of Broccoli Stalks
Plant Row
Mulched
Unmulched
A
15
4
B
29
1
C
9
4
D
21
10
Mean
18.5
4.75
Variance
73
14.25
Standard
Deviance
8.544004
3.77491722
Table 2
Average Leaf Area
(cm squared)
Leaf Sizes
200
150
100
50
0
Mulched
Unmulched
Treatments
Figure 2
Leaf Size Results*
2
Plant Row
Mulched (cm2) Unmulched (cm )
A
150
150
A
130
117
A
126
117
B
105
170
B
104
153
B
91
104
C
286
136
C
280
128
C
238
98
D
231
136
D
220
80
D
209
77
Mean
180.833
122.167
Variance
5024.697
837.788
Standard Deviation
70.885
28.945
Table 3
*The three larges leaf areas were taken from each area.
Average Number
of Leaves
Leaf Abundance
50
40
30
20
10
0
Mulched
Unmulched
Treatments
Figure 3
Number of Leaves Results
Plant Row
Mulched
Unmulched
A
55
23
B
58
19
C
29
45
D
46
51
Mean
47
34.5
Variance
170
251.6667
Standard Deviance
13.0384
15.8640
Table 4
Broccoli Plant Height
80
Height
60
40
20
0
Mulched
Unmulched
Treatments
Figure 4
Broccoli Plant Height
Plant Row
Mulched
Unmulched
A
90cm
50cm
B
72cm
44cm
C
67cm
52cm
D
48cm
63cm
Mean
69.25cm
52.25cm
Variance
298.250
62.917
Standard Deviance
17.270
7.932
Table 5
Broccoli Plant Width
Average Width
80
60
40
20
0
Mulched
Unmulched
Treatments
Figure 5
Broccoli Plant Width Results
Plant Row
Mulched
Unmulched
A
58.0cm
40.0cm
B
55.0cm
40.0cm
C
70.0cm
63.0cm
D
59.0cm
44.0cm
Mean
60.5cm
46.75cm
Variance
43.0
120.916667
Standard Deviance
6.557439
10.9962115
Table 6
Soil Temperature
Average Soil
Temperature (C)
20
16
12
8
4
0
Mulched
Unmulched
Treatments
Figure 6
Soil Temperature Results
Plant Row
Mulched
Unmulched
A
18.0C
18.4C
B
18.4C
18.5C
C
18.6C
19.0C
D
18.4C
19.0C
Mean
18.35C
18.725C
Variance
.063333
.1025
Standard Deviation
.2516611
.32015621
Table 7
CONCLUSIONS
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The original hypothesis was that the existence of mulch increases productivity—in this case, of
broccoli. Of the six variables tested, two of them rejected the null hypothesis, recognizing a
significant difference between the mulched and unmulched areas. The two variables that supported
the hypothesis were the number of broccoli stalks and leaf sizes found.
But it is important to remember that even with the two variables—number of broccoli stalks and
average size of the leaves—that rejected the null hypothesis, there was still a five percent chance that
the results were inconclusive.
There were several possible reasons (sources of error) that the four other variables did not support my
hypothesis: the smallness of the sample sizes used, soil quality was not one of the variables,
characteristics of the broccoli stalks found on the plants were not considered, more than one crop in
an organic garden that has been divided equally into mulched and unmulched sections, the type of
mulch used, amount of mulch, the type of mulch used is dependent on the type of crop, and the time
of day and the number of times the temperatures were taken.
Studying the soil and the effects of mulch can help gardeners all over the world maximize their
productivity. The process of mulching protects the soil from weather, efficiently uses the water and
heat, protects the plants and soil from harmful insects, and organic mulching enriches the soil. Such
advantages provide a better, healthier growing environment for a vegetable or fruit.
Researchers and gardeners should be aware that the way to have a productive garden is to have
healthy soil. Since mulch is a protective covering that improves the quality and the condition of the
soil, it can only be beneficial for the crop and increase productivity.