Phosphorus is an essential element for healthy turfgrass plants, but phosphorus can also contribute to algal growth in natural water bodies, so turfgrass managers should try to apply only the amount of phosphorus that is required by the grass. How much phosphorus is enough? Dr. Doug Soldat from the University of Wisconsin posted a very interesting photo on his blog showing a phosphorus deficient turf, but green and healthy bentgrass growing where a leaky spray boom had spilled a phosphorus fertilizer onto the turf.
Phosphorus deficiency symptoms, which appear as purple turfgrass and apparently drought-stricken plants, will occur at Wisconsin when the soil phosphorus is less than 10 ppm on a Mehlich 3 soil test. At California, Drs. Larry Stowell and Wendy Gelernter from PACE Turf have seen deficiency symptoms when the Mehlich 3-extractable phosphorus is at less than 20 ppm. They recommend at least 20 ppm phosphorus on a Mehlich 3 soil test to ensure adequate bentgrass growth.
Dr. Soldat has compared phosphorus to a "light switch," noting that when there is enough phosphorus, the grass grows very well (light switch on), and when there is not enough phosphorus in the soil (light switch off), the grass does not grow well at all. But adding more phosphorus beyond what the grass needs, and to be safe we can say this would be 20 ppm on a Mehlich 3 soil test, will have no effect on the grass, will certainly cost money but provide no benefit in turfgrass quality, and will have the potential to cause pollution of water bodies through leaching or runoff. So we should try to minimize the amount of phosphorus fertilizer applied.
I have done soil nutrient analyses for golf courses throughout Asia, and one thing that is surprising is how high the phosphorus levels tend to be on golf course putting greens in Japan. Creeping bentgrass can grow perfectly well at a soil phosphorus level of 20 ppm (on the Mehlich 3 test), and in one year we can expect to remove about 0.5 g P m-2 though the collection of grass clippings, and we would expect removal of that much phosphorus to cause a decline in soil phosphorus of about 3 ppm. But it is common on golf courses in Japan to have putting green soil phosphorus levels at 200 or 300 ppm, ten or sometimes twenty times more phosphorus than is needed by the grass.
What do I recommend?
- Test your soil every year to ensure you will have healthy turfgrass, will not waste money on unnecessary fertilizer applications, and will minimize the chance of any pollution
- If you have soil phosphorus levels at more than 20 ppm on the Mehlich 3 test, you will not see a turfgrass response from applying more phosphorus, so save your money and don't apply phosphorus in those situations
- Be very careful about the amount of nitrogen fertilizer that you apply, recognizing that nitrogen is the element that will control the growth of the grass and the uptake of other mineral nutrients
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