GARDENING\Gypsum can make harsh Valley soil fruitful.

Byline: Joshua Siskin

Perhaps the only nursery item you can purchase confidently is gypsum.

A reader named Jim informs me that he is moving from Woodland Hills to Chatsworth, and I would be remiss without advising him: Beware of the soil. Woodland Hills, for the most part, is in the flat part of the West Valley, where stretches of remarkably good soil may be found. But Chatsworth is something else again, and don't dare start a garden there without gypsum, that great softener of hard ground.

Don't get me wrong. In practically any part of Los Angeles, including Woodland Hills, the soil can be terribly compacted and hard to dig. But as you move into the hills that circle the Valley - into certain neighborhoods of Chatsworth, for instance - the soil gets ever rockier and more impenetrable. Living there, you might wonder if you'll ever be able to garden again.

Jim wonders, in particular, if he'll be able to grow his favorite plant, the bird of paradise bush (Caesalpinia Gilliesii), underneath the skies of Chatsworth. This plant is in no way related to what commonly is referred to as bird of paradise (Strelitzia reginae), yet its own extravagant flowers - enormous yellow blooms spouting bright red stamens - justify its name.

From the standpoint of temperature, Jim has nothing to fear; the cold air that rolls off the Santa Susana Mountains passes over Chatsworth before settling down in Woodland Hills, the coldest part of the Valley. Besides, his bird of paradise bush - a leguminous plant from Argentina - is hardy enough to withstand the coldest weather Los Angeles has to offer.

Jim's soil, however, if it is classic Chatsworth clay, is another matter. Many drought-tolerant plants, including the bird of paradise bush, prefer well-drained, sandy soil. Where drainage is poor, such plants devlop root fungus. This is exactly what happened last summer when a specimen of this plant in a Woodland Hills garden with moderately heavy soil suddenly died. Upon examination of the root ball, it was apparent that root rot had developed as the result of overwatering.

The quickest way to soften hard or heavy soil is with gypsum. Gypsum (calcium sulfate) is one of the best soil amendments, and probably the least expensive; a 40-pound bag costs only a few dollars. Garn Wallace, a local soil scientist, broadcasts a light coating of gypsum over his plants and lawn twice a year.

Hard soil usually is the result of clay particles held tightly together. Gypsum rearranges these particles in such a manner that space is created between them; this space allows water to move through and roots to grow.

An important benefit of gypsum is its reduction of soil pH. Most of the soil in Southern California has an alkaline pH (more than 7.0), whereas plants prefer a slightly acidic pH of 6.5. Recall that the pH scale, like the Richter scale, is logarithmic, meaning that intervals of 1 represent tenfold differences in strength. In other words, a soil pH of 7.5 - that would be typical for Los Angeles - would be approximately 10 times more alkaline than the ideal plant growing pH of 6.5. For this reason, it is recommended that, as part of the soil preparation procedure in our city, 10 pounds of gypsum - dug into the top 6 inches of soil - be applied per 100 square feet of planting area.

The reduction of soil pH makes iron, zinc, manganese and other trace elements more available to plants. Where soil pH remains high, leaves appear yellow with green veins, a sign that these elements are trapped in the soil and thus deficient in the plant. When gypsum is added to the soil and then watered in, sulfuric acid is formed, lowering soil pH and releasing the trace elements for absorption by plant roots.

Gypsum, applied in its pure form, may be dusted lightly over planter beds and lawns where soil compaction is a problem. There is also a product called gypsite, which is gypsum plus nitrogen, that may be broadcast as though it were fertilizer. The beneficial powers of gypsum are demonstrated in its use as a carrier for most chemical fertilizers manufactured for use in Southern California. Chemical fertilizers that are gray in color contain a large percentage of gypsum.

Tip: Do not plant right after a rain, because digging in wet soil will compact it. A good, fluffy soil never should be tread upon, wet or dry, lest it become compacted. To avoid stepping in your vegetable beds, make them no more than three feet wide.