University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article

DOES YOUR LAWN REALLY NEED PHOSPHORUS?


Sid Bosworth, Extension Agronomist, and
Leonard Perry, Extension Horticulturist
University of Vermont
 

If you have a lawn, you should be aware of how much phosphorus you are applying in fertilizers.   Often we apply too much, potentially resulting in pollution to waterways and watersheds.

Phosphorus is one of the main fertilizer elements (often seen as the letter P), the second number of three in the analysis on fertilizers.  So in a 5-3-4 fertilizer analysis, the 3 represents the amount of phosphorus.  The higher the number, the more phosphorus.  Some phosphorus is needed, and good, especially for roots and flowers.

Since phosphorus is in most fertilizers, moves slowly through the soil, and isnt used in great amounts by plants, there is often an excess.  With the potential for water pollution from excess phosphorus, some states and municipalities are promoting (or even legislating) a no P fertilization program for lawns.  Excess phosphorus causes algae to build up in waterways, disrupting the ecological balance.

Recent research has shown that when soils become saturated with water to the point that excess water runs off the site, there is a potential risk that some soluble phosphorus can be pulled out of the soil and into the runoff water.  This risk is much higher when soil test levels are in the high to excessive range for phosphorus.

Research has shown also that a dense stand of high quality turfgrass has essentially no soil erosion.  Further, it helps reduce total water runoff because of better water infiltration.  So if soil test levels become low for phosphorus, such that turfgrass growth and density decline, you could see more phosphorus runoff not less!  This is due to poor ground cover from the poor lawn, so increased soil erosion and runoff.

To make sure phosphorus levels are correct, it is important to take a soil test periodically (every three or four years).  If the test is high in phosphorus, or greater, there really is no need to add more and there could be environmental consequences if you do.  But if phosphorus test levels are low, it is important to provide adequate (not excess) amounts.  This will assure good leaf and shoot growth, which will improve turfgrass density, which will in turn reduce water runoff and pollution.

Other methods, in addition to soil testing for phosphorus and using correct amounts, to reduce the risk of pollution from this fertilizer element include:



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