Nurturing your soil is the most important step to a flourishing garden. Soil isn't just dirt, soil is alive, ever changing, and no different than any other life cycle. Every season your plants use up valuable nutrients that need to be put back in the soil the following season for new plants to use.
Testing your soil is the best way to know what nutrients need to be replenished in your garden. It'll save you time from guessing what to add, and you won't overfertilize, which can cause pollution. Unless you are looking to make large, quick changes, soil tests can be performed every 3 to 4 years.
HOW TO PREPARE A SOIL TEST
- Although there are lots of home tests available, a lab will give you the most accurate results. Most Cooperative Extensions have, or can direct you to, a soil-testing lab nearby, usually at a university.
- Test at the same time of year each time you test. Metrics like nitrogen and even pH change over the season. Most gardeners test in spring or fall.
- Dig 6" deep for your sample. The majority of plant roots are in the top 6" of soil. If your garden is large or you have several beds in the same area, dig several samples and mix the soil together in a container. Remove any sticks, mulch, or large roots that can skew results.
- Send in your sample with the paperwork that will ask you questions like what you plan to grow. Results generally take about 3 weeks. Soil test results will tell you if you need to improve your soil's components (e.g., phosphorus, organic matter, etc.) based on what you plan to grow and give you some guidance on how to do it.
HOW TO READ A SOIL TEST
Organic Matter: Organic matter is decomposing plant material that not only contributes nutrients to the soil but also improves other attributes like the soil's ability to hold onto water. Compost or aged manure is considered organic material. 5%–6% of your soil should be organic matter.
pH Level: On the pH (potential of hydrogen) scale, 7 is neutral, below 7 is acidic, and above 7 is basic/alkaline. You want soil to be near neutral (in the 6.0–7.2 range) because the soil won't "work" if it isn't balanced, meaning the plants can't take up the nutrients well. If your soil is too alkaline, you can lower the pH using various sulfur-based amendments, unless you have a lot of lime in your soil which inhibits lowering pH. If your soil is too acidic, add lime-based amendments or wood ash.
Nitrate: Nitrogen (N) is one of the three "macronutrients" or major nutrients plants need, promoting green growth (height and leaves). It is also the nutrient we most often need to add to soil because it is so fluid. Nitrate is a "fixed" form of nitrogen (N) making it usable for the plant, Nitrate may or may not be on your soil test because it is constantly in motion and changing forms, which is also why it is the nutrient we most often have to add to our soil. The soil test will tell you if your nitrate level is low, moderate, or high and what you need to do to amend the soil based on the level.
Phosphorus (P): Another of the three macronutrients, phosphorus contributes to root health, and flowering and fruiting of plants. Unlike nitrogen, phosphorus sticks around in the soil. Organic sources of phosphorus are compost, aged manure, and rock phosphate. Your soil test will tell you the level of phosphorus in your soil and what you should do to amend it.
Potassium (K): The final micronutrient contributes to overall plant health, including its ability to use nitrogen and water efficiently in addition to a plant's resilience when handling environmental stress like pests, heat, or drought. Organic sources of potassium include aged manure, compost, kelp, seaweed, wood ash (wood ash also raises pH), and greensand. Again, your soil test will categorize the level of potassium in your soil and tell you how to amend it.
TIP: The above three macronutrients (nitrogen, phosphorus, and potassium) are commonly described as N-P-K and represented on fertilizer packaging in this order but as three numbers separated by dashes. You can also find fertilizers that contain only one of these for more fine-tuned amending.
NOTE: Your soil test may also measure a few of the many other nutrients that plants need like Calcium, (Ca), Sulfur (S), or Magnesium (Mg) and make suggestions for amendment. While these nutrients are needed in lesser amounts, they are still essential and can impact a plant's ability to effectively process macronutrients or lead to other issues.
Texture: Soil textures include sand (largest particles), clay (mid-sized particles), and silt (smallest particles). Where these three types overlap is described as "loam". Likewise, if it overlaps but leans towards one more heavily, it may be called "sandy loam", clay loam", or "silt loam", respectively. Although loam is considered the ideal texture for gardening, you can still have a thriving garden in nearly every soil type. Adding organic material can go a long way to improving your soil texture.
Cation Exchange Capacity (CEC): CEC describes your soil's ability to take up nutrients and hold on to them for plants to use. Remember those magnet science demonstrations in school? Well, CEC is all about negative and positive attraction, too. Many of the nutrients that plants need break down into positive ions, that therefore need negative ions in the soil to hold on to. A soil with good CEC has that. CEC is also related to soil texture but can be improved with organic material and in some cases, a pH adjustment. Your soil test results will tell you the level and how to amend it.
Soil tests are not just for serious scientists! Even new gardeners can easily prepare a soil sample and know what to do when the results come back. And you won't believe the amazing garden that will come when you apply this new information!
Want to learn more about soil? For general soil health information, read The Secret is in the Soil. For more information on fertilizer see our article How do I Know Which Fertilizer to Use?