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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.)
