How to Read Your Soil Test

Although ordering and interpreting a soil test is far from glamorous, it will give you key facts about your soil that will help make your garden a success.

by Grace Hensley
PHOTO: Hemera Technologies/

Whether you’ve been farming for years or have just started digging up your backyard, understanding your soil is the foundation for healthy vegetables and robust crops. Experienced farmers can “read” their weeds to determine whether the soil is acidic (they may see more plantain) or alkaline (there’s more bellflower), poor-draining (an abundance of buttercups) or too dry (thistles are a problem), or even nutritionally depleted (forage grasses in the Sorghum and Digitaria families grow in abundance). But unless you’re Superman with X-ray vision, you can’t determine the exact chemical content of your soil without sending it out to a lab.

Where to Take Your Soil Sample?

You can find a reputable soil-testing laboratory through any land-grant university’s extension program. Before you choose one, determine which analytical tests you want to run, and get an example soil test result to ensure that you’re comfortable interpreting the results. A test report that includes the optimal range is as important as the actual test result. Soil composition, pH and macronutrient tests give you important information, but they don’t tell the whole story. Plant health is also dependent on micronutrients and trace elements. If you’re in an urban area, choose a laboratory that also tests for soluble salts and heavy-metal contaminants.

The testing agency will have detailed instructions for preparing and shipping your samples. Send them samples from each of your growing zones and include separate soil samples from both problem areas and productive ones. Plan on repeating the tests at the same time every couple of years to watch the health of your soils improve.

Interpreting your soil test results can be as dry as … well, dirt, if you don’t know what you’re looking at. However, depending on the type of test you order, a soil test can give you numerous insights about your soil including its composition, pH levels and nutrient density, which all translate into healthier, better-tasting food. Here’s how you can put the information from your soil test to use.

Improve Soil Structure

Start by looking at your soil composition, and determine the texture and amount of organic matter in your samples. Loam, a balanced mixture of sand, silt and clay particle sizes, is ideal for food production, but even sandy and clay soils can be productive. A soil organic matter (SOM) test measures the amount of decomposed carbon (aka organic matter) and typically should be around 2.5 percent. Organic matter improves the soil structure of clay soils, as well as aids water retention in sandy soils.

Balance Soil pH

Most crops prefer a neutral to slightly acidic soil pH between 6 to 7.5, but pasture land can tolerate a wider range from 5.5 to 8. However, when the soil is 100 times more acidic (pH of 5) or alkaline (pH of 9), the nutrients essentially can’t stick to the soil and are washed away or evaporate. Small pH changes can be made with the addition of organic matter, like compost or cover crops. For larger full-point changes, amendments, such as agricultural or dolomitic limestone for acidic soils or elemental sulfur for alkaline soils, can be added to bring your land into balance. Because heavy clay soils will need considerably more amending than sandy soils, follow the recommended amounts listed in your report. Seeing the results of your amendments on soil pH could take up to a year or more.

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Pack in Nutrients

Plant health is dependent on many different elements. Nitrogen, sulfur, phosphorus and boron are building blocks that affect leaf and root growth and cell-wall formation. Potassium, magnesium, calcium, manganese, zinc, copper and molybdenum all affect photosynthesis and plant metabolism. Your soil-test report will identify any significant macronutrient (N, P, K) deficiencies or excesses and their optimal ranges. Micronutrient deficiencies, such as in boron, copper and iron, are typically resolved by using a balanced fertilizer or adding compost. Manganese and zinc deficiencies are typically resolved by balancing the pH.

Add-On Tests

The laboratory you choose might not include all tests in a standard report, so you might decide to order additional tests. One in particular that might be worth ordering is a test for soluble salts or contaminants. Fields exposed to high levels of fertilizers or run-off from winter deicing chemicals can contain excessive amounts of salts, which cause plants to dehydrate. Make sure you have adequate drainage, reduce the amount of inorganic fertilizers and don’t apply fresh manure.

The most common modern contaminant found in soil is lead, and if a test reveals lead levels over 300 ppm, avoid growing leafy vegetables and root crops. If it exceeds 400 ppm, avoid growing all vegetable crops by using raised beds and new soil. You only need to test for arsenic if you are near coal-burning or mining operations.

How to Make Changes

Your soil test report should list the optimal range for all of the nutrients and chemicals discussed. Typically, the best way to improve your soil health is to apply a balanced organic fertilizer, but sometimes, you might have an excess of one element, making better sense to selectively apply amendments like blood-meal for nitrogen, colloidal or rock-phosphate for phosphorus, and greensand or kelp for potassium and micronutrients. The Oregon State University Extension Service has an organic fertilizer calculator to convert inorganic agricultural recommendations from your report into practical amounts for backyard and hobby farming.

Organic matter will improve your soil structure and fertility, but making changes to your soil will take time. With regular soil testing and amendments, you’ll literally begin to see your garden grow.

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