Phosphorous on lawns can be too much of a good thing
Q: I have always read that phosphorous was strongly recommended for good grass and plant growth. Now I find the major lawn fertilizer producers are no longer including phosphorous in their products as a concession to environmental concerns. How should we maintain the proper level of this nutrient if it is no longer included in their products? It is interesting to note that many garden fertilizers still include phosphorus.
A: For lawns, phosphorous is indeed praised for its ability to promote good turf root structure, but it has been pulled from many commercial lawn fertilizers because of its contribution to algae blooms and excessive weed growth in waterways. Several states have banned its inclusion in lawn fertilizers because of this issue. If we all used the right kind of phosphorous fertilizers appropriately and only when necessary, this problem may not exist. But, because many of us apply these fertilizers when they aren't needed and because they are notoriously overapplied, we are stuck with the ramifications of our overuse.
Phosphorous is not very mobile within the soil; in fact, soil particles bind phosphorous readily and are reluctant to let it go. So, intuitively, this may imply that once phosphorous is added to lawns, it's quickly absorbed into the soil and held there until a plant needs it. This isn't always the case, however. When we apply a chemical phosphorous fertilizer to a lawn and we get a heavy rain before it can be absorbed by the soil, a good amount of the fertilizer washes into our waterways (often by traveling on impermeable surfaces and into storm sewers).
Again, this wouldn't be a huge issue if we didn't add so darned much of it. Most of the phosphorous added to our lawns isn't actually necessary and contributes to excessive amounts of fertilizer runoff. You should never add any fertilizer to any plants unless a soil test tells you that you have a nutrient deficiency. If you plan to add fertilizers to your lawn, you need to start by getting a soil test from the Penn State Cooperative Extension every two or three years. The test tells you whether you need to be fertilizing and, if so, exactly what you should be fertilizing with. As with people food, too much plant food is not a good thing.
As for your question about how to provide your lawn with phosphorous if the soil test tells you that you need it, choosing an organic lawn fertilizer is the way to do it. Organic phosphorous sources -- like colloidal rock phosphate, bonemeal and others -- first require the action of soil microbes to break them down and release the phosphorous rather than it being available immediately, and only temporarily, from a chemical source.
Organic granular lawn fertilizers provide phosphorous and other nutrients through plant- and animal-derived ingredients as well as through the inclusion of natural mineral deposits. The nutrients in these fertilizers are available to plants gradually over a longer period of time. Hence, fewer applications are necessary and runoff is limited.
Compost is another good source of phosphorous -- and lots of other plant nutrients. Broadcasting a quarter-inch of finely screened compost over the lawn twice a year provides nearly all of the necessary nutrients for proper turf growth. That being said, it is possible to over-apply organic fertilizers, too (just ask the Chesapeake Bay about the effects of excessive runoff from the overapplication of manures). Please limit all lawn fertilizer applications to times when a soil test deems them necessary.
