This afternoon I was at my favorite park in Bangkok, the King Rama IX Park. This park is near my residence and when I am in Bangkok I frequently go to this park in the early morning or late afternoon for jogging. Today I noticed a lot of algae growing in the ponds —see above the clean water amongst the Victoria amazonica lilies and the algae everywhere else — and was reminded of three things I wanted to share about phosphorus, and phosphorus fertilizer applied to turfgrass in particular. 

1. Phosphorus is essential for food production, is a limited resource, and common sense tells us that we should not apply it to turf if it is not absolutely necessary. Earlier this year I read an interesting article in Foreign Policy entitled Peak Phosphorus. The authors argue that "our dwindling supply of phosphorus, a primary component underlying the growth of global agricultural production, threatens to disrupt food security across the planet during the coming century. This is the gravest natural resource shortage you've never heard of."

I can assure you, if there is adequate phosphorus in the soil, adding more will have no beneficial affect to the turf. How can you tell if phosphorus is necessary? By a routine soil analysis. I use 35 parts per million (ppm) phosphorus on a Mehlich 3 soil test as a conservative threshold level for deciding whether phosphorus fertilizer is required or not. If you are applying fertilizer without doing a soil test, you are probably wasting money and over-applying phosphorus. And if you are applying phosphorus fertilizer to turfgrass growing on soil that has 35 ppm phosphorus or more, you may as well shred money and apply that to the grass. The effect on the turf will be the same.

2. Back to the algae in the water at the Rama IX Park, that algae would not be growing if there was not phosphorus in the water. Phosphorus is usually the limiting factor in algal blooms (eutrophication) of water bodies. Adding phosphorus to turfgrass when that phosphorus is not needed will increase the risk of polluting nearby water bodies and will have no effect on the grass.

3. People ask me how to prevent algae from growing on putting greens. There are a number of techniques to employ, but an essential one is to avoid unnecessary applications of phosphorus. Dr. John Kaminski, Director of the Golf Turf Management Program at Pennsylvania State University, has done research on creeping bentgrass turf during summer conditions to evaluate the effect of different fertilizers on algae. In summary, the results have shown that applying nitrogen only (particularly ammonium sulfate, 21-0-0) prevents algae, while adding phosphorus causes a tremendous increase in algae. From a paper Kaminski published in the International Turfgrass Society Research Journal, there was no algae (0%) in plots fertilized through the summer with ammonium sulfate. A complete fertilizer containing phosphorus (20-20-20) always had algae and in some cases more than 20% of the surface area was covered by algae rather than grass. If you often have an algae problem on your greens, you may want to reevaluate your phosphorus fertilization program.

P.S. At the time I wrote this post in 2010, I was using 35 ppm for P. After the development of the MLSN guidelines, I use them instead, and as of 2016 I'd work with a minimum P level by the Mehlich 3 test of 21 ppm.