A recent column in which I discussed the largely work-free ''automatic'' garden of Derek Fell in Bucks County, Pa., drew a larger-than-usual spate of letters.
One letter, for example, notes that the suggestion that black plastic mulch raises early-spring soil temperatures is somewhat misleading. While clear plastic does allow the sun's rays to penetrate right through the plastic and heat the soil (soil sterilization is sometimes accomplished this way), black plastic does not.
Experiments at the University of Connecticut show only a marginal increase in soil temperature under black plastic compared with bare soil - around 1 degree F. at a depth of three inches.
Mr. Fell accepts the validity of this observation; he notes, however, that in previous experiments of his, plants mulched with black plastic grew more rapidly in the early spring than those without it.
Last year, for example, he was picking zucchini by May 31, although the last frost in his area was May 14. He says the zucchini, which he had covered on the night of the frost, had been given an early start indoors. Fell credits the black plastic mulch for the rapid growth of the plants once they were set out in the garden.
Fell says he suspects that, while soil temperatures might not be markedly raised by black plastic, soil temperatures are trapped in place after the sun has set for a good deal longer than in exposed soil, thus helping to speed germination. Also, once the seedlings have sprouted, the stored heat in the black plastic radiates out to keep the plants warmer at night.
Other letters refer to Fell's use of drip irrigation. What happens if one of the emitters clogs up beneath all that plastic? one writer asks.
Fell uses a type of irrigation hose -- a micro-porous tube, to be exact -- that does not require emitters but simply allows the water to seep out along the full length of the hose. Fell's current choice is put out by Irrigro, an improved version of the Viaflow tubing originally produced by Du Pont.
In the past, Fell says, he has been disappointed with this type of drip -- or more accurately trickle-irrigation -- hose. He found that the water seeps out irregularly and the hose is inclined to split where it curves around corners.
The more recently developed hose, he adds, has overcome these problems. He notes, however, that the black plastic mulch does a lot to extend the life of the hosing by protecting it from the degrading effects of the ultraviolet rays of the sun.
Fell finds that this tubing does not plug up. The collection of solid particles in the tube can build up, of course, but does not inhibit the weeping action of the hose.
Once a month the tubes are opened at the end and the solid particles flushed out. The fact that the tube is covered by a light-excluding mulch prevents any growth of algae, which can also cause problems.
Irrigro hosing is available from Irrigro, PO Box 160, Niagara Falls, N.Y. 14304. The catalogs put out by the Burpee and Park seed companies also list it.
Finally, in dealing with the automatic garden I neglected to mention that Derek Fell sprays his every two weeks with a mixture of two naturally occurring insecticides, pyrethium and rotenone, both available under several trade names at garden centers.