Sally Scalera

Brevard County Horticulture Extension Agent
3695 Lake Drive -- Cocoa FL 32926 -- 321-633-1702

Soil testing and lawn care

April 24, 2010

Now that the cold weather is over, everything is starting to grow again.

It is typical in the spring to fertilize the lawn and landscape in time for the new spring-time flush of growth. What research has shown us though is that we don’t always need to add all of the nutrients. And, the only way to know what you need to add is to have your soil tested.

Most often, when people hear of a soil test, they think of soil pH which is a measure of the soil’s acidity or alkalinity. In the truest sense of the word, a soil test actually refers to the measurement of nutrients available to the plant. The University of Florida has an Extension Soil Testing Laboratory that can also test for P (Phosphorous), K (Potassium), Ca (Calcium) and Mg (Magnesium) in addition to the soil pH and that test costs $7 per sample. The results of that test will help you determine which fertilizer is the best for the plants you are growing.

Recent research on established lawns has shown that after repeated use of a complete fertilizer (one that contains N-P-K) the P (Phosphorous) level is high or very high so that no additional Phosphorous is needed for a few years or so. Phosphorous is not mobile in the soil so if the plant roots are not close enough to the molecule to absorb it, it will remain in the soil for a long time. Many soil tests are also showing K (Potassium) to be in high quantities in the soil so that additional fertilizer may not be needed for this nutrient either. Nitrogen, on the other hand, is very mobile in the soil and if it is not absorbed by plant roots, it can leach out of the soil relatively quickly. Every soil test report will come back with the recommendation to add nitrogen, unless all that was requested was the pH.

You may be wondering what the big deal is if you fertilize when the levels are already high. Here are a few things to consider. Since phosphorous and potassium aren’t very mobile in the soil they can accumulate in the soil. Soils that consist largely of quartz sand and soils that are mainly muck are prone to leach any added phosphate fertilizers.

Another complication from an excess amount of phosphorous is that minor elements can be tied up in the soil, thereby causing deficiencies. The other issue is why pay money for nutrients that are already in high supply if you don’t need to. Save your money and just apply what is needed for the plants to grow well. Remember, the saying “if a little is good then more must be better” is not always true!

When fertilizing the landscape, the most important nutrient to apply correctly so as not to harm the environment is nitrogen. Research has shown that turf grass can only use one pound of nitrogen per 1000 square feet IF at least 30-50% of the Nitrogen is from a water insoluble source OR it is manufactured in a slow-release formulation. Because of its high solubility in water, nitrogen can leach into the Indian River Lagoon, St. John’s River or even the aquifer.

If the fertilizer is from the nitrogen source of ammoniacal nitrogen, urea nitrogen or other water soluble nitrogen then, it is typically a fertilizer made up of quick release nitrogen. The exception to this is if the bag specifically states that it is slow-release, timed-release or Poly Plus. Because high amounts of soluble nitrogen can burn plants or leach, it is very important to apply smaller amounts (1/2 a pound per application) more. This is not only better for the plants but also the environment. High amounts of nitrogen in the foliage of plants (including turf) can result in insect infestations or outbreaks of disease.

For those of you who are interested in finding out what nutrients you need to add to the soil you can give us a call at 633-1702 and we will send you an Extension Soil Testing Laboratory form. The other option is to come by either of our offices and pick up a Soil Testing Kit which includes the form, sample bag(s) and a box.

The form describes how to take a sample and where to send it. It is also very important to fill in the crop code on the form. On page 2 of the form there are seven crop codes for lawn grasses and five different crop codes for Landscape Plants and Vegetable Gardens. If you receive a form through the mail you can simply place your soil in brown lunch bags. Be sure to write your name, address and crop code on the outside of each sample bag in permanent ink. If you pick up the kit, the box will already have the Soil Testing Lab address on it, or you can simply mail it in your own box.