Volume XXIX Number 1 Page 28

By Clint Waltz
University of Georgia

From time to time, concerned citizens try to pressure lawmakers to eliminate phosphorus from lawn fertilizers. They mean well. They're looking out for our water resources.

Unfortunately, they don't understand how phosphorus enters aquatic systems or its role in plants and its behavior in soil systems. They fail to understand, too, that phosphorus is an essential nutrient. It's a "must have" for plants to grow.

Soil phosphorus levels aren't static, either. Low levels of phosphorus have to be applied each year to maintain proper soil nutritional balances.

Besides, applying phosphorus carefully while using "Best Management Practices" can greatly ease the environmental concerns.

Especially here

You have to apply phosphorus in the Southeast. Because it's hotter and wetter here with a longer growing season than in most of the country, Southeastern soils have less phosphorus than in other regions.

And plants can't do without it. It's the second-most essential element, behind nitrogen, for plants' growth. And plant roots readily extract it from the soil.

Phosphorus is in such high demand because plants use it in the metabolic processes of energy transfer. So it has to be added back to the soil for plants to keep growing well.

Too much of a good thing

In excess amounts, though, phosphorus can harm the environment. That's especially true when it runs off into streams, ponds or lakes.

Aquatic plant life must have a balance of nutrients. But high phosphorus levels stimulate excessive growth of aquatic plants and algae.

The problem is that when this excessive plant life dies and decays, the process takes oxygen from the water. And when oxygen levels drop, fish and other aquatic animals die.

Pollution solution

It's important to remember that the way most phosphorus reaches water bodies is in soil erosion. Nonpoint-source pollution of water bodies can be greatly reduced by managing soil erosion.

Of the phosphorus lost to lakes and streams through soil erosion, 75 percent to 90 percent is fixed to soil and organic matter. This fixed phosphorus has been shown to contribute to the growth of algae.

Turf grasses, which need phosphorus to grow well, can help the environment. Turf will greatly slow the flow of water across the soil surface and effectively reduce soil erosion.

A cover of turf will allow water to gradually infiltrate into soil, too. Once soluble phosphorus enters the soil, it's quickly bound to soil solids and organic matter and becomes relatively harmless.

BMP's

Nonpoint-source pollution from phosphorus can best be controlled by using best management practices.

Soil testing is one BMP that can help. But you have to use proper soil-sampling techniques. Your county University of Georgia Extension Service office can help you with this.

In general, it's best to apply phosphorus according to soil test results. There are exceptions. A fertilizer with low rates of phosphorus may help a turf grass that's stressed by cold or wet soil, for instance, or when root-rotting diseases have damaged the roots.

A second BMP is the use of fertilizers with low phosphorus levels. Many modern lawn fertilizers have been engineered to meet the needs of most turf grasses.

It's not uncommon to see products with analyses like 29-3-4 or 27-4-4, in which the content is around 1 part phosphorus for every 8 to 11 parts nitrogen. Zero-analysis phosphorus fertilizers are also available.

A third BMP to keep phosphorus out of water resources is to not apply fertilizer to hard surfaces such as driveways, sidewalks and streets.

Remember, when water-soluble phosphorus contacts soil and organic matter, it quickly becomes immobile in the soil. So just sweeping or power blowing fertilizer that lands on hard surfaces can greatly reduce the amount of phosphorus moving through storm-water systems into reservoirs.

(Clint Waltz is an Extension Service turf scientist with the University of Georgia College of Agricultural and Environmental Sciences.)