This is inspired by an observation I made on a field trip last year. We were in the Government Canyon State Natural Area northwest of San Antonio. A guide was explaining all the local plant life. We observed some Spanish moss growing on a tree.
The guide explained the plant (not a real moss) is not a parasite. It gets all of its nutrients from the air. My reaction was a quick rundown of the nutrients a plant receives, and I realized the statement was not correct. One thing that came to mind was phosphorus. A plant definitely does not get any phosphorus from the air. It has to get it from the host plant, usually a southern live oak, which most of us have.
And that got me to thinking about phosphorus and plant life. Phosphorus is essential to all living organisms on this planet. For one, DNA molecules involve phosphorus. Additionally, the association between adenosine triphosphate and and adenosine diphosphate manages energy transfer within living cells. A typical human body will contain about a pound of phosphorus. Nothing can live without it.
Phosphorus is something home owners need to consider when managing their landscape. In the wild there is little need to be concerned about nutrient phosphorus compounds. As a plant grows it takes up phosphorus from the soil in the form of various phosphorus compounds. When a plant dies it decomposes, and the phosphorus is returned to the soil. There are gaseous phosphorus compounds, but they are not involved in plant and animal nutrition. Phosphorus tends not to migrate in nature.
If you compost your tree leaves and your grass and garden clippings and return this material to your landscape, you will not need to add much phosphorus from fertilizer. Moreover, you can cause problems by adding too much phosphate fertilizer. Excess phosphates will be leached from the soil and will end up in the surface water, where it can cause problems. River and pond water will normally not have enough phosphorus to support growth. Addition of phosphates from fertilizer supports the growth of water-borne life such as algae. Rampant growth of algae is detrimental to other life in these bodies of water. When the algae die their decomposition consumes dissolved oxygen, which is needed by fish.
To manage the phosphorus content of your fertilizer, pay attention to the three letters NPK. A designation on a package of fertilizer will give the quantities of nitrogen (N), phosphorus (P), and potassium (K). Often the letters are omitted, and only three numbers are printed. For example 18-4-4 means the fertilizer has 18 pounds of nitrogen per hundred pounds of fertilizer, and so on for phosphorus and potassium. A growing plant needs all three of these macro-nutrients in addition to a number of micro-nutrients. If you are mulching or composting your yard waste, then the nutrient phosphorus is being returned to your landscape, and you should be using a fertilizer with a small second number.