Pensacola News Journal

For: Saturday, July 27, 2002

By: Daniel E. Mullins

Extension Horticulture Agent

Santa Rosa County

Fertilizer Leaching Study Completed

Research helps to answer some horticultural questions and when it does, other questions are frequently raised. Such is the case with our waterfront lawn fertilizer leaching study.

Two summers ago, a grant was provided by Master Gardener Dennis Snyder and the Snyder Family Trust for the study. Using these funds, we attempted to learn the effects of different forms and rates of fertilizer when applied to local lawns near surface waters. Two waterfront St. Augustine lawns in the Soundside area east of Gulf Breeze were selected for this work. Plots were laid out approximately 15 feet from the water. There were twenty 5 by 5 foot plots in each lawn allowing for 4 different treatments which were replicated 5 times.

The treatments were as follows:

1. Slow release 18 – 2 – 18 fertilizer applied at one-half the standard rate.

2. Slow release 18 – 2 – 18 fertilizer applied at double the standard rate.

3. Soluble or quick release 16 – 4 - 8 fertilizer applied at one half the standard rate.

4. Soluble or quick release 16 – 4 – 8 fertilizer applied at double the standard rate.

Fertilizer was applied in May and again in July. Both leaf and soil samples were collected prior to fertilizing and at 2 to 3 day intervals for two weeks after each application. Each treatment was rated for color, density and overall quality at these same intervals. Leaf tissue and soil samples were also taken, dried and sent to the University of Florida Soil Testing Laboratory for analysis.

Results showed no differences in visual quality, density or color in either lawn during May. In July scores increased by 50% in both lawns. Quality and color were highest with both rates of the water soluble fertilizer and the high rate of the slow release nitrogen product.

The high rates of nitrogen were not reflected in nitrogen uptake as determined by tissue testing. This may imply that more nitrogen is left in the soil at high nitrogen rates than the grass can readily take up.

Though these tests were not conclusive to determine if nitrogen is leaching into ground or surface water, they did reveal that extremely heavy applications of fertilizer are not necessary. Only minimal differences in turfgrass visual quality occurred in response to these sources and rates.

Based upon this and other University studies, we recommend that homeowners can safely apply no more than one-half pound of actual nitrogen per 1,000 square feet of area. This amount of nitrogen, when using one of the fertilizers in the study, would require only 5½ to 6 pounds per 1,000 square feet, or 35 to 40 pounds for the average sized lawn.

It appears that the best way to prevent fertilizer from reaching ground or surface water is to apply lower rates at more frequent intervals than is customary. In an annual fertilizer regime in Northwest Florida, this would call for 4 to 8 light, separate applications over the entire growing season for best St. Augustine maintenance.

Copies of this 5 page paper are available by contacting the Santa Rosa County Extension office. The title is “Nitrogen Leaching and Turfgrass Response to Fertilizer Rate and Source.”

Question of the Week: There is a black substance on my crape myrtle leaves. What is it and how can I get rid of it?

Answer: This black material is known as sooty mold. It is indirectly caused by aphids which are feeding on the leaves. Please watch this space next week where I will go into more detail and recommend control measures.