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IN THE GARDEN

For Answers, Put Soil to the Test

August 03, 1997|ROBERT SMAUS | SPECIAL TO THE TIMES

Wallace said that over-fertilizing is very common in Southern California. People think that they are feeding the plants, as they would feed a baby, and the more they feed, the better it will grow. But these things build up quickly and can become toxic to plants, actually inhibiting growth. Often, fertilizing is not the cure but the cause of the problem.

Salts in the Soil

In addition, each fertilizer source brings with it a little mineral salt, so the salinity of the soil increases and begins to affect plant growth. The Orange County garden was very saline. Measured as a value called ECe, two of us had little, .81 and 1.05, but the Orange County friend had an ECe of 4.65.

According to Wallace, anything over 4 affects many plants and probably is the cause of the small tomatoes. At this level, he said, the salinity alone will decrease the size and quantity by about 25%.

Most of this salinity is coming from fertilizers, there being many kinds of salts besides common table salt. Among other things, salts come from muriate of potash, cheap and common in fertilizers (look on the label for potassium derived from potassium nitrate or potassium sulfate instead; they are better sources).

Fortunately, salts and some of the excessive fertilizer can be leached from the soil. A good rainy year would help; so would putting about 6 inches of water on the garden within about a week. That amount will push salts below the level of most roots, removing about 90%, which is what Wallace recommended doing.

This is a one-time job; do it too often and you will also wash all the fertilizer elements out.

Many packages of fertilizer also contain trace elements, such as iron, boron and zinc. My Orange County friend and I had a little more boron than is deemed desirable, but it, too, can be flushed from the soil.

Unfortunately, the lead and arsenic and other heavy metals cannot be leached. The only way to get rid of these is to remove the soil, a good reason to carefully check labels to make sure you aren't adding any more. Incorporating organic matter will bind up the heavy metals so they don't get absorbed by most vegetables, but they will still be there when the organic matter decomposes and disappears, though adding organic mulch to the soil surface helps replenish organic matter.

Stop Fertilizing

Wallace recommended that the Orange County friend back off on the fertilizing and leach the soil to get rid of the excess and the salinity and that the soil be amended to increase aeration and drainage.

He suggested that my friend with the new vegetable garden add gypsum and then leach. This would remove sodium, which was a little high in her plot. The suggestion for my old garden bed was to stay away from fertilizers with trace elements, even organic kinds, and to aim for a more neutral pH.

I actually have to add lime or calcium carbonate to the soil (in very small amounts--2 or 3 pounds per 1,000 square feet), just as an East Coast gardener would.

Though the science behind soils is very complicated, all of his suggestions were very specific and easy to follow. I won't bore you with further details because what my soil needs, or my friends', will be quite different from what yours needs, but most soils need at least some fine tuning.

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