Phosphorus Management in Louisiana Turfgrass

Ronald Strahan, Koske, Thomas J.  |  6/3/2005 1:21:15 AM

Red coloring from anthocyanin pigments.

This complete fertilizer analysis contains 6% P2O5.

Phosphorus (P) is a major essential nutrient in plants and soils, but it is often eyed as an element of concern. In plants, it is involved in energy-related processes, membranes, enzymes, etc. In most soils, it may be a tenth of 1 percent of all soil elements.

Phosphorous is mobile in the plant but not in calcareous sands or clay soils. Silica sands won’t hold P well. Visual symptoms of P deficiency in turf are initially reduced growth and a darkening of green color. The color enhancement comes as vegetative growth reduces and chlorophyll concentrates in the leaf tissue. Lower leaves show this deeper green first. So far then, lower P is a benefit in better color, less mowing and adequate root growth.

As P deficiency deepens significantly, lower leaves will develop a reddish-purple toward the tips. This color change will progress down the leaf and up the stem. This color comes from accumulations of anthocyanin pigment. This pigment is sparked by a net starch accumulation brought on by less leaf growth and continued photosynthesis.

If P deficiency develops further, photosynthesis is now reduced and several growth-dependent systems, like root growth, all decline.

During establishment and spring root decline recovery, rooting is minimal and low soil P will reduce root growth more readily. This is why root stimulators, starter fertilizers and spring fertility recommendations all feature higher P application. Once you have the root system established, you can then do with lower available P.

The big concern for P is that high levels of available P can spark algae blooms in water ways and decrease water quality. With this threat and its lower visibility of fertilizer response, P fertilization is often questioned and sometimes banned. This need not be the case if best management practices are used for P.

Never dispose of lawn clippings or plant materials in storm drains or on drainage banks where nutrients can wash into surface waters.

Avoid spreading fertilizer on impermeable surfaces like walks and drives. If they receive some granules, sweep or blow them back into the lawn.

Avoid spreading fertilizers, or most chemicals for that matter, very near ponds, bayous or drainage ditches. Leave these areas as buffer strips to catch and process nutrient amendments. Be sure your mower discharges clippings back into the lawn and not into the street or waterway.

Soil test every several years to see if available soil P is very low and pH is adequate. You still deserve good plant growth and color. It's just that most mature and established plants are very content with moderate soil P levels.

We are seeing some modern turf fertilizer blends with higher P because of a greater need for P application to high-sand greens and sports fields. Deficiency of P is more prevalent in sandy, low CEC soils that are well irrigated. Such soils will usually have low pH concerns with the ample use of inorganic fertilizers and the high leaching potential. With these soils, apply at least several light applications of P annually and keep pH close to 6.

Keep soil pH between 6 and 7 for best P availability. Soils with pH  lower cause P to tie up with Fe, Al and Mn; those much higher fix P with Ca and CaCO3.

High P in soils may induce yellowing from iron (Fe) deficiency. It can also counteract some weed control activity from the commonly used arsenical herbicides because of a competitive chemistry. In such cases, more product is required to overcome the competition.

Some P fertilizer is needed to maintain medium soil P levels. We generally replenish with at least ½ pound of P2O5 fertilizer per 1000 sq. ft. per season if clippings are returned and twice that if clipping are bagged. Fertilizer P is listed as % P2O5 by weight, so if you need a pound of P2O5 from a 0-6-0 analysis, you spread 16.7 pounds of the fertilizer. On sands, spread out several light (1/4-1/2 lb.P2O5) applications per season. On heavy soils, one spring application is adequate although apps over 1 pound should be split.

Phosphorus is important for seedling growth and spring reestablishment of our warm-season lawns. Apply l pound of P2O5 per 1000 sq. ft. at establishment or sodding on fertile soils. Less fertile soils can receive twice that as pre-plant incorporate unless a soil test advises more. Always start the spring green-up season with a complete (containing all 3 numbers) fertilizer. This is usually mid to late March or April, depending on your area of Louisiana.

Following our BMPs for P fertilization should yield adequate growth, color and pest resistance without harmful pressures on our water’s quality.

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