Interpreting Test Results
So, you’ve got your soil test results back from the lab for your lawn, garden, or landscape. What do the numbers mean, and what do you need to look for? A basic soil test will provide information in several categories
- soil organic matter
- cation exchange capacity
- phosphorus levels
- potassium levels
Soil organic matter provides insight into how productive a soil will be. The higher the organic matter content of the soil, the more productive the soil. Organic matter helps soils retain water for plants and helps create soil structure which allows roots to move more easily through the soil.
- Most of the dark soils in Illinois typically have between 2% and 4% organic matter.
- Lighter soils usually have 2 to less than 1%. Low organic matter soils may benefit from the addition of organic matter, such as finished compost.
pH shows how acidic or alkaline your soil is. The pH scale runs from 0 to 14, with 7 being neutral, above 7 being alkaline and below 7 being acidic.
- Most plants like soils in the range of 6.3 to 7.0.
- Having soil with a high or low pH may limit the ability of plants to take in some nutrients, or may allow the plants to take in so much, it becomes toxic.
If soil pH needs changed, it’s usually because the soil is too acidic. Application of limestone will raise soil pH, while sulfur will decrease pH.
Cation Exchange Capacity, or CEC, indicates how well the soil can hold plant nutrients. The amount of organic matter, which has very high CEC, and the amount and type of clay minerals, which also has high CEC, affects the overall CEC in a soil.
- Soil with a CEC of 15 to 20 will be in good shape to hold nutrients.
- Soils with a CEC of 6 or lower will need to be managed close for nutrient deficiencies such as potassium.
Phosphorus is one of three nutrients needed in large amounts by plants. The other two are nitrogen and potassium. Phosphorus is vital for energy transfer within a plant.
- A soil phosphorus level (shown typically with the symbol of P1) of 20 to 40 pounds per acre (10 to 20 ppm) is good for gardens, and landscapes.
- Too much phosphorus in the soil may allow some to dissolve and may eventually move into ponds and streams causing excessive plant growth there.
Potassium is important for moving water and nutrients around in plants and with enzyme activity. It also helps stalks and stems. A potassium test of 250 (125 ppm) or higher shows no additional potassium is needed.
Most soil labs will provide recommendations on what to apply and how much, based on soil test results. Taking soil tests every three to six years will let you examine longer term trends. Is organic matter levels decreasing? How quickly does pH change and in what direction? Examining these trends will help determine how often to soil sample and whether current management is working.