Effects of double-digging in a backyard garden.

In this experiment, the results were a surprise

Increasing yields in home gardens is the goal of many new approaches to bed preparation, layout and planting. Sustainability and minimizing impact on the environment has also led to new techniques in gardening. One low-tech method of improving yields that is gaining popularity in biointensive gardens worldwide is the double-dug method. In an effort to quantify the benefits of this method on corn production in our own backyard garden, we planted corn in two newly dug, adjacent beds of the same size. One bed was double dug to provide an anticipated improvement in yield. The other bed was single dug to provide a control.

Methods

An area of lawn with a slope less than one inch in five feet was chosen for the garden plot in the fall of 1999. At that time the soil was broken up and turned to a depth of about one foot by shovel. Chicken manure that had lain in a covered chicken coop for an unknown number of seasons was sprinkled evenly on the plot and the plot was allowed to rest for the winter of 1999-2000.

In early April of 2000, a bed of five feet by 10 feet was double dug using the method of Jeavons and Cox in The Sustainable Vegetable Garden, Ten Speed Press, 1999. The top one foot of soil is removed, the next foot down is turned by shovel and loosened by pitchfork as needed, and the top one foot of broken up soil is returned to the top. This is done to minimize the disruption of microorganisms in the soil that are sensitive to depth.

An adjacent bed of the same size was single dug south of the first bed. The soil below one foot in this bed was not disturbed. Both beds were in partial shade with a full-size tree to the east of both beds.

On April 19, 2000, both beds were planted with 20 Golden Bantam Sweet Corn seeds from Bountiful Garden Seeds (5798 Ridgewood Rd., Willits, CA 95490) in a hexagonal pattern about one foot apart to maximize density. Both beds were watered with equal amounts of water initially to initiate germination. Thereafter, rains were the only source of water for both beds until mid-July. Because of a very dry July, both beds were watered by hand equally at that point. No fertilizer was added during the growing season. The circumference of the overall garden was planted with Border Patrol[TM] flowers produced by Clyde Robin Seed Co.

Corn ears were harvested by hand all at once when there was no evidence of further development in the ears. The harvesting for both beds occurred on July 29, 2000 for a total of 101 days from planting. No ears were left on the plants no matter how small. Unshucked ears were weighed on a kitchen scale that measures in one-quarter ounce increments with a maximum of 16 ounces. The ears were weighed in groups to minimize additive error as many of the ears weighed less than an ounce.

Results

As can be seen from the table, the total weight of ears of corn produced by the single dug bed was about 28% more than the weight of ears of corn produced by the double dug bed. In general, the ears were small and showed evidence of poor pollination no matter which bed they came from. Some had no kernels of corn on the ears.

The corn plants varied in height in both beds with the minimum 24" and the maximum 69", both occurring in the single dug bed. The height of the corn plants showed the same trend as the weight of the ears. The average height of the corn plants in the single dug bed was 27% higher than in the double dug bed.

Germination was over 95% in both beds. Attrition of plants was 25% in the double dug plants and 15% in the single dug bed.

Discussion

Although our family garden experiment yielded the opposite results of what we expected for our corn plants, it led to a more intense scrutiny of the condition that produced the substantial differences in corn production between the two beds. Since germination and attrition did not account for the differences in production per plant, we looked closely at three other possible causes.

Differences in water supply can lead to a difference in production in most plants and corn is no exception. In our situation, the majority of the water was supplied by rain. As this was an unblinded experiment, the bias of the experimenters may have entered in at watering times.

Besides the waterings being timed and being at the same time of day, two things argue against this bias being a significant cause of the difference in production:

1) Our expected results were for the double dug bed to be the more productive and our bias in better care for the plants would have been on the double dug bed.

2) The appearance of the corn plants at the end of the season showed a clear preponderance of brown leaves on the single dug bed plants compared to the double dug plants. This was not quantified, but the brown leaves in the single dug bed would lead to an expectation of less water available and therefore stunted growth in the single dug bed plants. This, however, was not the case and thus a difference in moisture is not the likely explanation of the difference in corn production.

The second possible cause of the difference in production that should be considered is amount of sunlight available to the plants. Corn is a sunloving plant. Our plants had a mature 30-foot high deciduous tree three feet to the east of both beds, providing partial shade until noon every day. A 50-foot tree about 40 feet to the west also provided partial shade for the last four hours of the day. As the sun is high overhead in midday and the beds are separated by about two feet in a north/south direction, there was no shading of the corn plants in one bed by shade from plants in the other bed.

Although the light levels were not strictly quantified, I do not think the possible differences in sunlight exposure is the cause of our difference in production.

The third possible cause of the difference in production is the soil condition in each bed. We know the beds were prepared differently, but why would a single dug bed produce more than a double dug bed?

One of our clues came at the time of harvesting the stalks, after the ears were harvested. Seeing the shallowness of the corn plants' root system, no matter which bed was being harvested, it appears that corn gets most of its nutrition from the top few inches of soil. It may be that the nutrients in the single dug bed, including the added chicken manure, were maintained near the root system of the plants, while the nutrients in the double dug bed were able to leach away to the lower level because of our aeration of that lower level.

The soil in the double dug bed may have drained water more quickly, resulting in less water in the upper level between rains early in the season resulting in stunted plants. The problem of tree roots as a source of water consumption needs to be considered as well.

To the east of and adjacent to both experimental beds, more double dug beds were created, exposing the tree roots from the tree that was east of the experimental beds. Tree roots were present in both of these double dug beds in a similar density and pattern, implying that there was no difference in the root patterns of the experimental double dug bed compared to the single dug bed. However, it must be kept in mind that many of the roots of the tree in the double dug bed were chopped in the preparation of the soil whereas the single dug bed had only the first layer of roots disturbed. We would expect that this would lead to more water consumption from the tree roots in the single dug bed. This effect would be opposite to the effect of stunting observed, so this is not likely the source of our difference in production.

Conclusions

Although we think there are plenty of highly productive double dug beds in people's back yards, our experience has shown that there are many factors that enter into the harvest results and until it is studied further, double digging is not a guarantee of higher productivity, and in fact may be counter productive in certain circumstances. By the way, we enjoyed eating our ears of corn from both beds, small and rubbery as they were.

Table

                            Double dug         Single dug

Date of planting            April 19, 2000     April 19,2000
No. of seeds                20                 20
Sq. ft. of beds             15                 15
No. of shoots counted
   on 5/10/00               20                 19
No. of plants counted
   on 7/5/00                15                 17
Avg. ht. of plants
   on 7/5/00                44.7"              56.8"
No. ears harvested
   7/29/00                  18                 19
Avg. wt. of unshucked
   ears harvested           1.19 oz.           1.45 oz.
Total wt. of unshucked
   ears harvested           21.5 oz.           27.5 oz.

The authors wish to acknowledge the assistance of students from the Agrimissions Club of MidAmerica Nazarene University for their assistance in the garden bed preparation.

RICHARD PRUIKSMA AND MARGARET HOLCOMB DEXTER, KANSAS