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The Dirt on Dirt


What caused the leaves of the tomato plants to turn purple?

Why did my zucchini die off so early this year?

Should I have watered my garlic more?

THESE ARE THE kinds of questions I am frequently asked when people learn that I grow vegetables for a living. I might suggest that the purpling of the tomato leaves could be due to a shortage of available phosphorous; that the zucchini may have contracted a case of bacterial wilt transmitted by cucumber beetles; and that, yes, it probably would have been a good idea to give the garlic more water during its most active growth phase in May and June, especially if those months were drier then usual. But these are just guesses—there are a number of plausible causes for the symptoms described.

More often than not, growers with questions like these have not had their soil tested recently, if at all.

Often, a simple adjustment in pH—the measure of acidity or alkalinity—will result in healthier and more productive crops.

For about $12, the test will indicate whether the soil has adequate amounts of the major plant nutrients—phosphorous, potassium, calcium, magnesium. (Nitrogen, another important plant nutrient, is not reliably measured in a standard soil test.) If the soil is deficient in a particular nutrient, the lab will give you recommendations on how much of that nutrient to add. The test also will indicate the percentage of organic matter (a very crucial ingredient) and how much of various micronutrients like iron, manganese, zinc and aluminum are present. Perhaps most importantly, you’ll get a reading of your soil’s pH. We have the soils of our vegetable fields tested every three years, usually in the fall. It’s relatively easy to do:

For a given field, we dig down five or six inches with a clean trowel, take a few tablespoons of soil and put them in a plastic bucket. We do this in several different locations, then mix the various samples together. We want to end up with a generous cup full of soil, which is then placed in a small Ziploc plastic bag and labeled with our farm name, field number and date. Next, we fill out a form provided by the lab we use—Agro-One Soils Laboratory (Ithaca). They have forms for both commercial growers and for gardeners. The commercial form asks for the name of the soil being tested, the depth of tillage, crops grown over the past three years (and those expected to be grown in the next three years) and whether animal manure has been applied and cover crops grown recently. The gardeners’ form has fewer, more general questions.

Often, a simple adjustment in pH—the measure of acidity or alkalinity—will result in healthier and more productive crops. For most vegetables, the ideal pH range is 6.2 to 6.8 (a pH of 7 is neutral, less than 7 indicates an acidic condition, more than 7 indicates an alkaline condition). If a soil’s pH is above or below the ideal range, certain nutrients in the soil bind to each other and become less available to plants. If the pH is too low (5.9, for example), much of the phosphorous in the soil will attach to aluminum, leaving an insufficient amount for the vegetables. Adding more phosphorous won’t help much—the soil’s pH must be raised so the phosphorous that is probably already present can be freed up.

Most soils in the Northeast tend to be acidic (pH below 7) and, over time, tend to become more acidic. This means it’s more likely the soil’s pH will need to be raised rather than lowered. This can be done by adding agricultural lime. The general rule in our area is that the application of a half-ton of lime per acre will raise the pH by 0.1 units. So, if the pH of the soil is 5.9 and you want to bring it up to 6.5, you would need to add 6,000 pounds of lime per acre. This sounds like a lot, but lime is cheap—a gardener growing primarily for home use on much less than an acre of land would think in terms of square feet and pounds rather than acres and tons. For every 1,000 square feet of growing area you need about 25 pounds of lime to raise the pH by 0.1. So, if you have a 5,000-square-foot garden and you want to raise the pH from 5.9 to 6.5 (6 increments of 0.1) you would apply 750 pounds of lime (25 x 5 x 6). That’s 15 50-pound bags of lime—still quite a bit of weight to heft around, but well worth it if you want a productive garden. Agricultural lime is sold at most farm or garden supply stores—picking up a small lime spreader while you’re at it will make the application easier.

Whenever we get our soils tested, I’m anxious to see if the organic matter quotient is stable or increasing. When we started farming 27 years ago, most of our fields had 3- to 3.5 percent organic matter content. Now they come in with between 5 percent and 6 percent—a big improvement. Organic matter is not only the “glue” that holds soil together, it retains moisture like a sponge, provides food for soil organisms, furnishes plants with nutrients, helps prevent erosion and reduces soil-borne diseases. A high organic matter component is beneficial in any garden or farming system, especially if the growing plan does not include chemical fertilizers and pesticides.

Organic matter includes the manure of herbivorous animals and all plants and soil organisms, either living or dead and, if dead, at any stage of decomposition. The best way to keep the soil’s organic matter quotient high is to regularly apply compost or other organic material, like mulch. Growing cover crops such as buckwheat, clover, oats or rye, and turning them into the soil when they reach maturity also is a good thing to do.

So, if your tomatoes are stunted or the tips of your cabbages are turning brown, consider collecting a cup of soil and sending it off for testing. A standard soil test won’t tell you everything that’s going on in your garden (far from it—after all, a garden is a complex ecosystem with an almost endless array of interactions occurring among its biological, mineral and chemical parts) but, for a few bucks, a soil test will give you at least some elemental information about its chemical content and what may need remediation. It’s better than amending the soil with something that may not be needed or that might even be counter-productive.

Relatively new from Cornell University is The Cornell Soil Health Test. It costs quite a bit more than the simple test we just discussed, but in addition to the standard chemical and nutrient analyses, this more “holistic” test takes into account the physical and biological aspects of the soil. Stay tuned.

Hudson Valley Restaurant Week is back this April 8-21!