Grumpians, it has come to my attention that many weekend gardeners are wasting their precious time and money following stupid advice that has no basis in science. Therefore, every Friday from now until I feel like stopping, I’m going to debunk some popular myth for the benefit of you, my loyal readers.
Question — My lawn service says I need to apply a high-phosphorus fertilizer this fall to encourage a vigorous root system. Is this true?
Answer — Only if you’re employed by a lawn service and make a living getting as many people as possible to do things that aren’t necessary.
Every hort and agronomy student remembers learning that the three most important plant nutrients are nitrogen (N), phosphorus (P), and potassium (K). This is the order in which percentages of each are listed on the fertilizer bag. For example, 16-4-8 means the fertilizer contains 16% nitrogen, 4% phosphorus, and 8% potassium.
Nitrogen and potassium are water-soluble and readily leach from the soil following rain or irrigation. Thus, they must be replenished periodically for proper growth of lawn grasses and other plants. Phosphorus, on the other hand, stays put. It binds with other minerals in the soil. The phosphorus you put down last year is almost certainly there this year.
The only way to really know whether your soil soil needs additional phosphorus is to have your soil tested in a lab. Ask your local Cooperative Extension service for a soil test kit (it won’t be free, but it’s not expensive). The vast majority of soils in this country have all the phosphorus they need. Excessive levels of phosphorus can make other important nutrients, such as iron and zinc, unavailable to plants.
Most of the phosphorus sold in this country is mined in Florida. Not coincidentally, most fertilizers formulated for Florida gardens contain little or no phosphorus.
High-phosphorus levels in “bloom-booster” fertilizers are a waste too. Regular garden soil doesn’t need it and you won’t get any more flowers. Oh, it may make some sense to use a high-P “bloom-booster” fertilizer on plants grown in sterile potting soil once, but after that they have all the phosphorus they need.
What have we learned here? That most likely, your lawn and garden already has all the phosphorus it needs. The best thing you can do for your plants is work lots of organic matter into the soil. And use a composting mower if you have one.