Do you have questions about soil testing? You are not alone. There are lots of questions on soil tests coming in and an explanation of how it all works may be of some help.

Soil tests provide a portion of the information that you need for planting, but not everything. There are different types of soil tests. The UF/IFAS Lake County Master Gardeners provide a simple pH test at our office for free. Soil samples sent off to the University of Florida for analysis will provide more details, which cost between $3 and $7 plus shipping. What type of test do you need?

Soil pH is a measure of the acidity or alkalinity. The range of numbers is from 1 to 14, with 1 being the most acidic. A pH of 7 is considered neutral, lower numbers acidic and higher numbers basic. The numbers represent the logarithmic progression of the number of hydrogen ions in the soil solution (the water around the soil particles). The difference between each number is 10 times the number of ions different, and not just a simple difference of one number. There are a lot of hydrogen ions and this is an easier way to express it. A pH of 1 is equivalent in acidity to battery acid. A pH of 13 is equivalent to lye. Most plants prefer a pH around 6.5 to 7, but more acid loving plants like azalea and blueberry prefer a pH of 4 to 5.5. Although many native Florida soils are slightly acidic, many of the soils in developments are more basic. 

If the soil pH is too high or too low, it will directly affect plant growth by root injury. It can indirectly affect plant growth by changing nutrient bioavailability. Nutrients are most available to plant roots around 5.8 to 6.3. If pH gets higher, many micronutrients, like iron, are not available to the plant even if they are present in the soil. At lower pH, many of the micronutrients become so available that you get problems with toxicity. The pH will also affect the nutrient holding capacity of soil or media and the microbial associations with roots, so getting it right is important. 

Adjusting soil pH is not easy. If the soil is too acidic, you can add lime. The soil type will determine how much is needed. If the soil has buffering capacity, it will need much more than soil without a buffering capacity. The soil test done by the UF lab will give you a recommendation for the lime requirement because it measures the buffering capacity. If the pH in your soil is too high, sulfur can be added, but it is not successful in the long term. The finer the particle size of sulfur added, the more quickly it will lower the pH, and reapplication will be necessary. Our experience has been that trying to lower the pH of our high pH soils with sulfur does not last long and it is best to choose plants that grow in higher pH soils. The factsheet at http://edis.ifas.ufl.edu/ss480 provides more details.

Soil tests may also give you nutrient amounts. The UF soil analysis will give you the amounts of phosphorus, potassium, calcium and magnesium. Special tests for commercial producers include tests for organic matter, micronutrients copper, manganese and zinc. Nitrogen, the most common nutrient in fertilizer and most needed by the plant, is not provided. Nitrogen is constantly changing forms in soil, from available to unavailable, and the tests cannot measure available nitrogen easily so they leave that number out. However, they do make recommendations for the amount of nitrogen to apply based on research data for each crop.

Phosphorus is naturally high in Florida soils and a major cause of water pollution when applied without precautions. We always try to reduce the amount of phosphorus applied in fertilizers to the landscape, but some vegetable gardens may need fertilizer with phosphorus for good fruit production. Potassium is a very important nutrient for plant growth that is usually limited in our soils and not a pollution risk. Calcium and magnesium will naturally be high in soils with high pH, but are important to plant growth and may be supplied by the lime source in low pH soils.

To take a soil sample, we recommend a bucket, a trowel or shovel and a small paper bag. Take samples from the top down to about 6 inches. Take several samples from your yard, mix them together in the bucket, then take out about a pint of soil to send for analysis in the paper bag. The soil sample needs to be air dried for analysis, so paper bags are best. Wet samples in plastic bags are not acceptable. If you think there are different types of soils in your yard, consider sending in separate samples from each area. Details on how to take and where to send can be found at http://soilslab.ifas.ufl.edu/ESTL%20Tests.asp, but you can also stop by our office to get forms, directions and materials. The results will be emailed to you and a copy sent to the Extension office if you want to consult with us.

Visit the Discovery Gardens and plant clinic from 9 a.m. to 4 p.m. weekdays at the Extension Center, 1951 Woodlea Road in Tavares. Go to lake.ifas.ufl.edu for details and class registration.

Juanita Popenoe is the director of the UF/IFAS Lake County Extension Center and Environmental Horticulture Production Agent IV. Email: jpopenoe@ufl.edu