Phosphorus is included in most plant foods and fertilizers because it helps develop plant roots and improve bloom production and size. Plant foods with high phosphorus content can help when soil has plenty of the other major nutrients but lacks phosphorus, or when a plant has especially high phosphorus needs.
Plants need high amounts of phosphorus because it is essential to building their genetic material and helps plants store and transport nutrients quickly. When nutrition transport happens rapidly, the plant is free to produce ample blooms, push roots further into the ground faster, and develop quickly overall. This is one of the reasons that high-phosphorus plant foods and fertilizers are often labeled "bloom-boosters."
On a plant food or fertilizer label, there are three numbers separated by dashes. Phosphorus is the middle number. Therefore, if the middle number is higher than the other two numbers, then it is a high-phosphorus plant food. High-phosphorus plant foods come in liquid or granular form. The form does not affect how well the plant food is absorbed, so long as it is applied correctly. Granular fertilizers and bone meal are best mixed into the planting medium before planting, while liquid fertilizers should be applied after the plant is established. Guano and rock phosphate are other organic phosphorus sources, but guano sometimes has a higher nitrogen content.
Annual flowers and annual fruit, seed, and root crops benefit most from high phosphorus plant foods, since they have only a year to absorb, transport and use nutrients before the end of their lives. Perennial flowering plants and crops that are valued for their blooms or fruits benefit from an application of high-phosphorus plant foods, but take care when re-applying. Phosphorus does not leach through the soil the way that nitrogen does, so test the soil before reapplying.
Repeat applications of phosphorus, whether organic or chemical, can harm your plants. Too much phosphorus in the soil locks up zinc and iron, which plants need to survive. It can take several years for phosphorus to stabilize in the soil so that it no longer inhibits zinc and iron. Excess phosphorus on the surface of soil can also wash down into nearby clear water, where it encourages the growth of weeds and algae. Test your soil every two to three years to make sure you are not overfertilizing your garden.
Using balanced plant food or switching to well-rotted manures and compost provides plants not only their phosphorus needs, but also nitrogen, phosphorus, trace nutrients, good soil structure and high beneficial microbe activity. To help your plants develop healthy roots and plentiful blooms, plant them in rich, well-drained soil in a sunny location. Rotate crops and annual flowerbeds with plants that do not require as much phosphorus so you will not need to apply high phosphorus plant foods as often.