Do You Know What's In Your Dirt? - William Bryant Logan (Previously published in Organic Gardening, Volume 38 Issue 2, February 1991)
Should you test your soil yourself or give the task to a state or private laboratory? If you choose to do it yourself, you should be aware of a few facts.
"Every soil test kit is a compromise," says Robert Miller, Ph.D., analytical lab director at the University of California at Davis. "For the person who wants to learn what's going on in the soil, it will perform well. But you can't buy a kit and expect indisputable accuracy."
Kits are an aid to the gardener's art, not a scientific substitute for hard work and common sense. Little racks of bottles and tubes--part chemistry set and part magic--mix soil collected from your garden with the kit's reagents, and present the results in colors ranging from red to pale blue to deep purple. Using a series of coded color charts, you can determined the pH and the nitrate, phosphate and potassium levels of the soil.
These changes are interesting and fun to observe, but to make the tests useful, follow a few simple precautions. Don't collect samples from the surface of the soil. Fertilizers not yet used by your crops, stray chemicals (organic or not)--even the sun's rays--can affect the results. For a consistent reading, dig down at least 3 to 5 inches. That's the part of the soil that the roots of your plants are using.
While you're down at that depth, take more than one sample. It's best to have four or five from any particular garden area. Also, if you have an azalea bed in one corner and a vegetable patch in another, you'll want to test those beds separately, since their needs are different. Be particularly cautious about samples taken from around the house foundation, since gardens close to the house are likely to have their own special problems, because of residues from the building itself.
To prepare your basic mix for testing, combine four or five separate samples thoroughly and let them dry overnight. In the morning, crush the lumps out and sieve the soil to remove foreign matter and stones. Now, you're ready to conduct a test.
Since the analyses rely upon color comparisons between solutions and standard charts, it is important to work in a well-lighted place, preferably under natural light. The work area should be free of contaminants. "A lot of people make the mistake of doing the tests in the garden shed," says soil chemist Liz Anger of the LaMotte Chemical Company. "A little stray fertilizer can change your results dramatically."
Kit makers' instructions generally are specific about the exact mixings, stirrings, shakings and filterings needed to extract the soil nutrients into the solutions they provide. When the color change occurs, you match the observed color to the chart color. This is not as easy as it sounds. "You and I could look at the same test together at the same time, and not come up with the same answer," explains Mike McPhail of LaRamie Soils Service.
The eye is more faulty than the chemistry. Distinguishing accurately between two similar color values on a chart can be very difficult. Readings of pH show the most dramatic differences and are easiest to discern. Even then, however, most solution colors fall between chart values, so there's some guesswork involved. The nutrient tests frequently will require fine distinctions, say, between three different intensities of blue. At best, these results should be considered approximate. (With pH, for instance, one major company suggests its readings could be as much as three-quarters of a point off.)
Can you use such rough results? Generally, the answer is yes--especially when you consider that even professional lab tests often show wide divergences testing the same soil. If you prepared and tested your soil carefully, your results should serve as a guide--not a prescription--to solving soil problems.
The pH scale is the most forgiving. The neutral point is pH 7; above it, the soil is alkaline, below it, acid. Most plants will grow well if the pH is between 6.5 and 7.8. As low as 5.5 and as high as 8.3 still are tolerable. Aside from its technical definition, pH is a way of expressing what different plants like.
When settlers moved west, they found three kinds of soil: the acid soils in coniferous forests, the neutral soils of broadleaf forests, and the more alkaline soils of the grasslands and drier West. They didn't know about pH, but their experience taught them that a farm on broadleaf land generally did best.
Today, we have a more precise idea of why this is so. An acid soil with a low pH releases too much aluminum, which is toxic to many crop and garden plants. (A few stunted pines can eke out an existence at pH 3.) Acidity ties up phosphorus and causes other essential nutrients, such as potassium, calcium and magnesium, to be leached from the soil. It also discourages the microorganisms that convert organic matter into a usable form of nitrogen.
Likewise, a very alkaline soil can tie up phosphorus, iron, zinc and other needed elements. If your testing indicates a very low or high pH, you have a problem.
A "low" reading on any major nutrient--nitrates, phosphates and potassium--means that your plants probably are suffering. The "high" reading is less definite. It may mean a desirable level of nutrient, or a level much too high. The test isn't sensitive enough to distinguish. You needn't be alarmed. "If you have too much phosphorus or potassium, it's like putting money in a bank account," says John Mortvedt, Ph.D., senior scientist with the Tennessee Valley Authority. "You're building soil fertility."
The nitrate test is another matter. Unlike the other two, it measures a nutrient whose levels fluctuate according to season, rainfall, uptake by your plants, and rate of organic matter decay. Dr. Miller recalls conducting a lab test for a woman in Innis, Mont., whose home test had indicated a high nitrogen level. She was puzzled that all the flowers in the soil died. A finer-resolution lab test revealed an astounding 1,300 ppm of nitrates, enough to kill any plant.
This may be an uncommon situation, but it's important to realize that nitrate levels are distinctly seasonal. Will Brinton of Wood's End Laboratory notes that there should be little available nitrate left in the soil at the end of your growing year. And, of course you want to take your nitrate reading in the spring, so you have a rough idea of how much is available for your young plants.
You have your results. Now, what do you do with them? Let's say the tests indicate that you should raise or lower the pH, or supply a deficient nutrient.
A soil, as Hans Jenny, dean of American soil scientists, puts it, is "a body in nature." When you change one factor, you may unwittingly affect another. In Eastern Virginia, for example, the soil is naturally low in the nutrient magnesium. If you have an acid pH reading and correct it with a calcium-based lime, you may balance the pH, but the calcium will drive out even more magnesium, aggravating the deficiency. In a calcium-deficient, acidic soil, adding wood ash will raise the pH, but won't increase the calcium. And if you add dolomitic limestone--a magnesium-based limestone--you will exacerbate the conditions.
Likewise, you must know how much of a nutrient your soil can hold. An accurate idea of your soil's texture (the size of its mineral particles) and structure (the arrangement of particles) is needed. Both indicate the amount of water and air in your soil, which are critical for fertility. With practice, you can learn to measure them by hand-but in the meantime, what to do?
This is when a laboratory test becomes very handy. Some state-run labs will provide all the detail you need, including precise parts-per-million readings for available and reserve nutrients; some, however, give only recommendations (usually non-organic). Check whether your state's service provides numbers or levels for important nutrients, such as calcium and magnesium, as well as for the NPK and pH. A good lab test also should include both a texture measure and a number for cation exchange capacity, the scientific name for an estimate of how much electrical charge is available in the soil for holding onto nutrients. A sandy soil may have a CEC of less than 4; a clay loam may have one as high as 17. How much of a nutrient you can add without waste depends on how much the soil can hold. While texture is geological and cannot be improved, structure can be managed through the addition of green manure and compost.
If your state lab doesn't provide the right details, you may want to try a professional test by an independent laboratory or a firm specializing in organics. The Necessary Trading Company, Ohio Earth Food, LaRamie Soils Service and A&L Laboratories all deliver thorough test results and organic recommendations.
Confused? Don't worry. Get the best test you can, keep an eye on your plants, and remember a rule of thumb you can always fall back on. "One of the best things you can do, regardless of your test results," says Jeff Phillips, extension research agronomist at Purdue University, "is to add organic matter to the soil." He adds that he means compost, not fresh manure.
Now, you could have the compost tested, but that's another story.