Garden mulch tests produce some surprises as well as better yields

Few gardeners debate the value of mulching any more. By and large they now recognized that mulching is good for the garden soil, good for the growing plants, and good for the gardener as well.

Indeed, the one abiding problem is where to get adequate quantities of mulching materials. That's why a recent work by Prof. Thomas D. Cordrey of Pennsylvania's Delaware Valley College of Science and Agriculture is so interesting.

Dr. Cordrey compared five mulches, black plastic, straw, leaves, flat sheets of newspaper, and shredded newspaper.

Specifically, newspaper was being evaluated because this is the most freely available and inexpensive mulch available to the average home gardener. Straw, leaves, and shredded newspaper (more accurately, half-inch-wide strips of newspaper) were applied 3 inches thick; flat newspaper, 3 sheets thick.

The crops grown were broccoli and cauliflower in irrigated and nonirrigated beds, replicated three times, with bare-soil beds for comparison.

Past experimentation has shown that mulches improve soil moisture retention by between 6 and 10 percent compared with bare soil; control weeds; reduce soil compaction; and, in the case of organic mulches, ultimately improve soil structure. The net effect of all this is generally to improve yields.

In the Delaware Valley experiment an early excess of water complicated the tests. At planting time (Aug. 5) the soil was dry and all the irrigated beds were given 1.5 inches of water. Two days later the same volume of water fell as rain, initially creating a too-moist situation for the plants under those thick organic mulches that had been irrigated.

The tests found that:

* Soil moisture retention was improved by all mulches, though some drying of surface moisture took place under the flat sheets of newspaper, particularly on windy days. For its part, black plastic was found to shed some 20 percent of the rainwater, which, in a dryer test period, would have resulted in the need for frequent irrigation.

The moist conditions at the start of the test persisted for an extended period under the straw, leaf, and shredded newspaper mulches, indicating good moisture retention. This resulted in inital stunting of the plants, although by harvesttime all plants were generally similar in size.

* Weed control under shredded newspaper and leaves was excellent. Straw also suppressed weed growth from seeds in the soil, but this mulch also contained some weed seeds itself which might present a subsequent weed problem.

The black plastic controlled weeds well but was readily damaged; thus, weeds grew plentifully through the tears even though attempts were made to repair the damage. The sheets of paper gave poor weed control, probably because it was only 3 sheets thick.

* Soil temperatures were an average 6 to 8 degrees cooler than bare soil under the organic mulches. In contrast, the soil under the black plastic was slightly higher than the bare soil controls.

Cauliflower yields were generally higher on the nonirrigated plots, while the irrigated broccoli did marginally better than its nonirrigated counterpart.

The yields under the thick organic mulches might have been considerably higher in a dryer season or on lighter soils. But despite the slow start under adverse conditions, the yields under these mulches were reasonably good. Top broccoli production (12.8 pounds a plot) came from the newspaper-sheet plots, followed by the plots mulched with shredded newspaper (11.6 pounds). Straw came in last at 8.7 pounds.

Sheet newspaper was also the top producer (26.2 pounds) in the cauliflower tests, followed by bare soil (25 pounds) and shredded newspaper (22 pounds). Leaves came in at the bottom, at 13.9 pounds.

It should be noted that whole leaves, rather than the generally recommended shredded leaves, were used in these experiments.

Another summer, another season, and the results might vary. The point of the test showed, however, that freely available newspaper makes a useful garden mulch. This is how Dr. Cordrey sums up the mulches:

Black plastic - expensive and difficult to apply; subject to damage, and, being non-biodegradable, has to be removed at the end of each growing season.

Sheet newspaper - inexpensive, biodegradeable, producing vigorous plants, but difficult to keep in place and only moderate at weed control.

Shredded newspaper - inexpensive, biodegradable, excellent on weed control, with high moisture-carrying capacity.

Straw - readily available, easy to apply, moisture retentive, with moderate weed control, but (at $1.50 a bale) expensive.

Leaves - with good weed control, easy to apply, and inexpensive.