August 31, 2009

Plant Health Starts With the Soil

By Jane Morse, University of Florida/IFAS Extension Agent, Pinellas County Extension

Did you know there are more than 70,000 different kinds of soil in the United States?

With all these different kinds of soil you can imagine the variations of their soil properties. How much water or nutrients they hold, their pH and drainage can all be wildly different. Unfortunately for most Florida gardeners our soils are mainly sand. Sand does not hold on to much of anything, so water, nutrients and pollutants flow right through and into our ground water.

Soils are also characterized by their alkalinity (high pH) or acidity (low pH). Most plants prefer soils in the 5.5 to 6.5 pH range, which is slightly acidic. This is also the pH range where most plant nutrients are easily available to plants. When soil pH is either above or below this optimum range plants may start to show nutritional deficiencies (hunger signs) or toxicity symptoms.

Our Florida coastal soils tend to be very alkaline, whereas soils that formed under pine flatwoods can be very acidic. Sea shells, marl and limestone are very high in calcium and are the main reason for our coastal soils being so alkaline. Some calcium-rich building materials such as concrete and stucco can also raise the soil pH. Plants grown in alkaline soils are commonly lacking these plant nutrients: iron, manganese, zinc and boron.

To determine your soil pH you can send a sample to a trustworthy lab such as the University of Florida Extension Soil Testing Laboratory ( Once you know the pH of your soil you can choose the plants best suited to your soil, or understand why some plants are not growing well. Strongly alkaline soils are generally a greater problem in landscapes and proper plant selection is very important. Acid-loving plants such as azalea, ixora, gardenia and blueberry will never do well in an alkaline soil. There is no way to permanently lower the pH of soils formed from high calcium materials, so proper plant selection is critical for plant health.

The University of Florida Extension Soil Testing Laboratory (ESTL) can also provide a fertility analysis. This test tells you how much phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) are in your soil. This report also includes pH, lime and fertilizer recommendations for selected lawn and landscape plants based on your soil sample.

To avoid damage to your landscape plants, always have your soil tested for pH and/or lime requirement before adding lime or sulfur to the soil. If you want to grow plants that are not suited to your soil pH, consider growing them in pots with another soil amended to provide the proper pH.

How to Collect a Soil Sample
1. Identify the area to be sampled. Turf areas, vegetable gardens and ornamental beds should all be sampled separately. Also, any problem areas (such as depressions, rocky areas, etc.) should be sampled separately to avoid contaminating samples from good areas.

2. Using a shovel (or soil probe), remove soil from 10 to 15 locations within the sampling area. Soil should be removed from the top 6 inches. Walk in a zigzag pattern, stopping occasionally to remove soil for the sample.

3. After taking each sub-sample, remove any plant material or mulch and deposit the soil into the plastic bucket. Mix the soil in the bucket to ensure it is well blended.

4. Spread the soil out on a newspaper or paper grocery bag and allow it to dry thoroughly.

5. Once dry, pack approximately 1 pint of soil (fill to the dotted line) into a soil sample bag (available free from your county Extension office). Alternatively, you may pack soil into a zip-top plastic bag.
Soil samples being submitted to the Extension Soil Testing Lab (ESTL) should be accompanied by a completed Landscape & Vegetable Garden Soil Test Information Form ( ) or you can pick up the form at the Extension Office, 12520 Ulmerton Road, Largo. The ESTL offers two soil tests for the home lawn, landscape and vegetable garden. Test A ($3) includes soil pH and lime requirement analyses; Test B ($7) adds analysis of phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Include your check or money order for the test you’ve chosen, along with the form and your soil sample.

If using another soil testing lab, you should contact it first to obtain instructions on how to submit soil samples. Be aware that private laboratories may not use soil tests that are calibrated for our region.

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