Soil Testing

By Charlie Nardozzi, June 23, 2008

If you've heard it once, you've heard it a hundred times: every garden should have its soil tested. Testing is the easy part, but interpreting the results can be confusing. To help sort out the confusion, this article discusses the reasons for testing, explains the various numbers on the report, considers the advantages and disadvantages of home test kits, and describes a few alternative tests being used by some organic growers.

Why test the soil?

If your garden is growing well, an argument could be made not to bother testing at all. However, if your plants aren't growing as well as you'd like or you're wondering if you're using the right amount of fertilizer, a soil test is the place to start. But a word of caution: a soil test won't solve all your garden problems or tell you everything about your soil. It will give you a periodic snapshot of your soil's mineral health. Tests are most useful when done regularly (every three to four years), at the same time of year (spring is fine, but fall is best because that's when fertility is lowest), and with the same lab (different labs use different tests, and results can vary). This way, you're comparing apples to apples with each set of results.

A sample test

The most accurate tests are conducted by university and private soil laboratories. Contact your cooperative extension service to find a lab in your area. These labs have had years of experience testing the soils in your state and giving specific fertilizer recommendations based on the soil types and crops grown. The basic tests usually cost less than $20 but can be more if you include tests for specific minor nutrients such as zinc or contaminants such as lead.

Results often come as a number and a graph for relative levels of each nutrient (see sample test report below). The graph is more helpful in understanding nutrient levels. This sample describes some of the common results found on soil test forms. I've also included information on deficiency symptoms and some suggested fertilizers high in those nutrients. Symptoms of excessive fertilization tend to manifest themselves as deficiency symptoms of other nutrients. For example, high calcium levels interfere with the uptake of potassium.

1. Type of sample. Laboratories will ask what kind of plants are growing in your test site (vegetables, flowers, lawn, orchard). Each type of planting needs a separate soil test.

2. Soil pH. This is measured on a scale of 1 (acid) to 14 (alkaline). Most garden crops grow well at a pH of 6 to 7, but specific crops such as blueberries and azaleas may need a lower (more acidic) pH. Soils tend to be more acidic in high-rainfall areas (the Southeast and Pacific Northwest), while drier climates (Southwest deserts) can have more alkaline soils. In general lime is used to raise pH, while sulfur is recommended to lower it. If your soil also lacks magnesium, dolomitic limestone (which contains magnesium along with calcium) is recommended. Apply lime and sulfur in summer when the soils are warm and microorganism activity is high.

3. Nitrogen (N). Though nitrogen is one of the key nutrients needed for plant growth, many labs won't test for it because of its high mobility. Nitrogen leaches out of the soil easily, and levels can fluctuate through the growing season. Deficiency usually appears as pale yellow leaves (often the older leaves first) and stunted growth. Since organic matter can hold and then slowly release nitrogen as it breaks down, some labs link the organic matter content to the nitrogen level in the soil. However, organic matter may tie up more nitrogen than it releases (see "Percent organic matter"). Alfalfa meal, cottonseed meal, urea, and fish meal are some fertilizer sources of nitrogen.

4. Phosphorus (P). Phosphorus levels are often reported as either available or reserve. Available phosphorus can be used now, while reserve is tied up due to pH or nutrient imbalance. Often, just raising or lowering the pH to the ideal 6.5 will free up phosphorus for plant use. It isn't used up quickly, and adding too much will build excessive levels that run off, causing pollution. Deficiencies are indicated by purple leaves, brittle roots, skinny stems, and late fruit set and maturity. In early spring, deficiency symptoms on seedlings may disappear with warmer temperatures and increased microbial activity. Superphosphate, rock phosphate, and bonemeal are good sources of phosphorus.

5. Potassium (K). Potassium is vital for stem strength, root growth, and disease resistance. Many soils are naturally high in potassium, and it is readily available to plants. However, sandy and highly weathered soils can be deficient. Signs are irregular yellowing of lower leaves and poor root growth. Muriate of potash, greensand, and wood ash are good sources.

6. Calcium (Ca). Calcium is important for cell-wall integrity and root and leaf growth. If you're liming your soil regularly to keep the pH above 6, calcium deficiency would be unlikely. However, on alkaline soils (pH above 7) add gypsum (calcium sulfate) instead of lime. Low levels of calcium show as deformed new leaves and branches, and weak stems and roots.

7. Magnesium (Mg). Magnesium is essential for chlorophyll and green leaf development. Pale leaves with green veins are a sign of deficiency. Adding dolomitic lime to raise the pH often corrects deficiency symptoms; on alkaline soils, add Epsom salts (magnesium sulfate).

8. Cation exchange capacity (CEC). CEC measures the ability of soil particles to hold and release specific nutrients. In general, sandy soils tend to have a lower CEC than most clay soils. Adding well-rotted compost raises the CEC. High CEC usually means a more fertile soil. If your soil has a low CEC, add small amounts of fertilizer throughout the growing season to prevent runoff and waste. Most labs report CEC levels in milliequivalents per 100 grams of soil (meq/100g). A rating of 5 is considered low, while 25 is high.

9. Percent organic matter. Organic matter is essential for nitrogen absorption and release, and as a food for microorganisms that help make other nutrients available. A level of 3 to 5 percent organic matter is considered ideal. But it's the quality, not the amount, that can make the difference. Soils high in undecomposed organic matter, such as wood chips or sawdust, can tie up nitrogen and create a deficiency. The best-quality organic matter to apply, especially right before planting, is well-rotted compost.

10. Percent base saturation. Some experts consider the relationship between four key elements (calcium, potassium, magnesium, and sodium) an indication of soil health. The ideal ratio is approximately 60 to 80 percent calcium, 10 to 15 percent magnesium, 5 to 7 percent potassium, and less than 3 percent sodium. Adding these figures gives a number called the base saturation. In general, the higher the number, within the given ratios, the more fertile the soil. Labs that test for base saturation believe that the optimum levels of specific nutrients aren't as important as the relationship among these nutrients.

11. Recommendations. Most labs give recommendations for adding specific nutrients to bring them to their optimum levels. Recommendations are often given in pounds of that element per 1,000 square feet of garden.

Calculate the amount of fertilizer to apply to reach that recommended amount. For example, our sample test recommends applying 3.5 pounds of phosphorus. If you're using bonemeal (12 percent phosphorus, as indicated on the bag), calculate how much actual phosphorus is in the bag by multiplying the percentage of P in the fertilizer (0.12) by the total weight of the bag (say, 20 pounds) to get 2.4 pounds. To add the recommended 3.5 pounds of phosphorus, you'd apply about 1-1/2 bags (30 pounds) of bonemeal per 1,000 square feet.

If you're adding bulk organic fertilizers such as manure, you can reduce the amount of other fertilizers by a fourth to a third by applying 15 bushels of well-rotted cow or horse manure or 7 to 8 bushels of poultry, sheep, or goat manure.

Do-it-yourself soil tests

A laboratory soil test has definite advantages, but it takes time to get the results. For a quick look at your soil, many home tests are available. These rely on color charts to match the nutrient levels in a soil solution. Unless you buy an expensive test kit, the specific nutrient tests aren't, in general, as accurate as in a professional soil test. Their usefulness is limited to a basic guide of the pH and nutrients that are immediately available, and they're best for gardeners who are knowledgeable and inquisitive about soil chemistry.

Alternative soil tests

Some private labs are responding to the need of organic gardeners and farmers to go beyond testing the mineral content of the soil to offer tests for biological content (fungi and bacteria). These labs, such as Soil Food Web Laboratory in Oregon, believe that the soil's biological life is just as important as the mineral content in determining soil and plant health.

It's believed that the more diverse the microorganism population in the soil, the better the soil structure, the more nutrients available to plants, and the less disease on the plants. Some labs, such as Woods End Laboratory in Maine, have a home test kit (Solvita) that tests biological and textural components in the soil, with a special emphasis on organic matter management. These tests, though more expensive than traditional university tests, give gardeners and farmers valuable information about their soil's microbial life. Since these tests are relatively new, cropping and fertilizer recommendations based on them don't have a long history of experience.

Charlie Nardozzi is a senior horticulturist at National Gardening.

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