How to Test and Amend Garden Soil

Successful gardens and lawns start from the ground up — literally. You need great soil to grow beautiful ornamentals and delicious edibles. While some plants actually prefer poor soil, most won't thrive if their roots can't get the nutrients, water and oxygen they need.

You can buy a few bags of soil from a garden center and dump them in your yard, but for the best results, expect to invest a little time, money and energy, and your plants and grass will flourish. So, how do you get great soil? Start with a plan.

What Is Great Soil?

If you could examine a sample of ordinary soil, about 45% would be weathered rock in the form of sand, silt or clay particles. Air and water, found in the spaces between the particles, account for less than another 50%. The rest is organic matter. Organisms that live in the soil — everything from microorganisms like bacteria and nematodes to creatures like ants, mites and spiders — make up what's called the soil food web. Plant roots are also part of the web. The web helps organic matter decompose, breaks down pollutants in the soil and helps change minerals, fertilizers and nitrogen into forms that roots can absorb.

A healthy soil food web equals great soil, but first you need to know the pH of your soil.

What is Soil pH?

Soil pH refers to acidity and alkalinity. It's measured on a scale of 0 to 14, where 7 is neutral soil, under 7 is acidic and over 7 is alkaline. Some plants need acidic soils (think blueberries and azaleas), and some need alkaline (like daylilies and hostas). Many plants do fine with a neutral to slightly acidic or slightly alkaline soil pH of about 5.5 to 6.5.

Garden centers and nurseries sell test kits you can use to find your soil's pH, or you can ask your local extension service agent to test your soil for you. Follow their instructions and send it a clean, dry sample. Tell them what you plan to grow. They can recommend what kinds and amounts of soil amendments to use and whether you need to adjust your pH.

How Do You Adjust Soil pH?

If your soil is too acidic, adding garden lime, dolomite limestone, poultry manure or wood ashes will make it more alkaline (raise the pH level). Most people use lime, sold at garden centers as a powder or as pellets or granules. Apply it in the fall so it has time to break down and blend with your existing soil. For a new planting area, dig or till the soil 6 to 12 inches deep, spread the lime over the surface, rake it in and water it.

If your soil is too alkaline, add elemental sulfur, available from garden centers, to make it more acidic (lower the pH). Some gardeners use oak leaves, but their acidity decreases as they decompose. Others use pine needles, but researchers say they don't lower the pH significantly. Elemental sulfur is usually inexpensive and won't harm your plants, but it works slowly. For faster results, try iron sulfate, which costs a little more. Some gardeners swear by coffee grounds, but tests have shown that brewed grounds are almost neutral, with a pH of 6.5 to 6.8, and any changes they cause don't last long.

Check Your Soil's Texture and Structure

While you're learning how to test and amend your garden soil, pay attention to the soil's texture — that is, the proportions of sand, clay and silt particles it contains. Light soils contain more sand while heavy soils have more clay. Sandy soils tend to be acidic and don't hold moisture well. Work in organic matter like compost, aged manure or peat to help them retain more moisture and nutrients.

Claylike soils are usually alkaline. They drain slowly and can be sticky and dense, making it tough for roots to grow. Clay soils often contain a lot of nutrients, but plants can't access them if the pH is off. Improve the soil before you plant in the spring by working in 2 to 3 inches of organic matter like compost or aged manure. As the plants grow, mulch or side-dress them with shredded leaves or more compost or aged manure.

Another option to improve clay soils: build raised beds so water can drain more easily. Make the beds narrow enough to reach into, so you don't have to step in and compact the clay.

Mulch clay soils over the winter with straw or another kind of organic matter so hard rains won't pack them down. Mulching also helps prevent erosion and weeds.

After you've adjusted the pH of your soil, you may need to add nutrients. Again, what to add, and how much, depends on what you want to grow. This is why you may want to ask your local extension service or a soil testing lab to test your soil. Most DIY kits indicate only the pH and don't tell you much, if anything, about amendments.

What Nutrients Does Soil Need?

Plants need macronutrients like nitrogen, phosphorus and potassium. Nitrogen helps leaves and stems grow, while phosphorus promotes roots and seeds and potassium helps move nutrients through the plant.

Plants also need micronutrients or trace amounts of elements like boron, zinc and manganese. Follow the recommendations from your extension service test for which ones, if any, and how much to use.

Which is Better: Organic or Inorganic Amendments?

Organic matter like compost, bone meal and untreated grass clippings feed your plants, improve soil quality and even help prevent erosion. Inorganic fertilizers, also called chemical or synthetic fertilizers, feed plants, too, and work faster than organic materials. But they can disrupt the natural balance in your soil and don't improve its quality. They're easy to overuse, which can harm your plants and even contribute to environmental pollution.

Add compost to your soil at any time. If it's fresh or uncomposted, like leaves or a cover crop, wait at least a few weeks to let it break down before working it in.

How Long Does it Take to Improve Soil?

Be patient because organic matter breaks down slowly and can take a long time to significantly improve your soil. Retest periodically to see how the pH and nutrient levels are changing and keep adding organic matter as needed. At the end of the growing season, let fallen leaves remain on the soil and decay, and/or plant cover crops.

Northern gardeners typically plant cover crops of winter wheat after the last harvest or when flowering plants are finished. In the South, crimson clover and oats are popular. In the spring, chop down the cover crops — also known as green manure — and work them into the soil. Improving your soil takes time, but it's worth the wait for a healthy, productive and beautiful garden.