For Answers, Put Soil to the Test

Why were my friend’s tomatoes so small? “Early Girl” is not a large tomato, but those from his north Orange County garden were not much bigger than cherry tomatoes.

Another friend in Culver City decided to put in a new vegetable garden, so she and her husband laboriously broke up a concrete patio behind the garage. They found bricklike clay beneath, but what was the black and red stuff on top of the soil? Was it something that might be harmful?

I couldn’t answer these questions, but a test of the soil probably could, so I had them scoop up small bags of soil for me to take to a test facility.

There are four soil laboratories in Southern California: in San Diego, Santa Ana, El Segundo and Santa Paula. To find a lab, look in the Yellow Pages of the telephone book under “Soil Testing.”

While I was at it, I gathered a little soil from my own West Los Angeles garden, from one of the older flower beds where the soil looked great but had been in use for more than 10 years.

The soil had been tested in 1985 by Soil and Plant Laboratory in Santa Ana, and the report then said it was a good sandy clay loam but needed additional organic matter and nitrogen, the main ingredient in fertilizers. I’ve been adding these ever since. Has it made a difference?

Few gardeners get their soil tested, though farmers frequently do, and so do some landscape contractors.

Because so much depends on the soil, from plant growth to disease resistance, it’s a good idea to find out as much as you can about your soil. A printed report will give you a little science to go with your gardening hunches.

Nearby Wallace Laboratories in El Segundo did the testing this time. Its test costs $70, but is quite thorough, using a sophisticated spectrometer to examine 30 elements in the soil, including mercury and lead, and the lab makes recommendations on how to improve the soil.

Before being fed into this $150,000 machine, the soil is sifted through a fine screen and made into a slurry to test different contents and characteristics.

Soil and Plant Laboratory in Santa Ana has a basic test that costs $52. It checks soil pH, the basic fertilizer elements and salt content and also includes recommendations.

The lab will tell you where to get the sample and how much to send along. Garn Wallace wanted me to dig it from 6 to 8 inches down, and from four or five places in the garden so it would be a composite.

Results In

My friend was relieved to find she had no inordinate amount of toxic chemicals in her new vegetable beds, though she and I had noticeable amounts of lead and arsenic in the soil.

The levels were well below those considered safe for leaf and root crops but surprising nonetheless, because we are both careful, mostly organic gardeners.

Wallace wasn’t as surprised, saying that all the older parts of town--from Santa Monica to East Los Angeles--contain some lead and arsenic in the soil. Through the years it has built up from leaded gasoline fumes, house paint, fertilizers, herbicides and soil amendments.

My garden and the Culver City garden had about 8 parts per million of lead, while the newer Orange County garden had only .42. “We flag lead at 30 parts per million for being excessive for edible root, stem and leaf crops,” Wallace said.

Equally surprising, my Orange County friend and I discovered that we had slightly acidic soils, right here in the alkaline West. Dead neutral is 7, but mine had a pH of 6.28, while his was 5.63.

It is important to note that the pH scale is logarithmic, so a 5.6 is 10 times more acidic than a 6.6.

Running from an acid 0 to an alkaline 14, 6.5 is often given as an ideal, though Wallace said that this actually depends on the soil type. A level of 7.5 might be ideal in a clay soil; 5.5 could be ideal in sandy soil.

Too much fertilizer seemed to be the culprit in the Orange County garden. Using complete fertilizers (he uses Miracle-Gro) and fertilizing about four or five times a year has made the soil acidic.

The report also said the soil was too high in fertilizer elements and too salty. He has been overdoing it with fertilizers that contain more than he needs.

Most Western soils are already high in two fertilizer elements, phosphorous and potassium, so we seldom need to add either, yet they come in a lot of fertilizers.

To give you an example of how excess fertilizer can build up, the phosphorous level in my garden was 71 parts per million and in the new Culver City garden was 36 parts per million. Optimum is 4-7, so there is already plenty of phosphorous even in the new ground. But the overfed Orange County garden had an excessive 127.

Ditto with potassium. We had 142 and 92 respectively--with 60 to 120 being optimum--while he had 642.

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.