Photo by Kimberly Stockwell-Morrison.

Trees use about one-tenth as much phosphorus as nitrogen, yet it is no less important in overall nutrition. Its primary role is metabolic by helping plants mediate energy transfers. Phosphorus is also involved in photosynthesis, and it is a major component of important organic compounds. An adequate supply of phosphorus in trees and all green plants is associated with vigor, hastening of plant maturity, flowering and seed set and good root development.

Virtually all phosphate in the United States is obtained from the mineral apatite, formed in sedimentary rocks found in geological formations that are actually uplifted ancient sea bottoms. Large deposits are located along the central spine of Florida and other parts of the Southeast and in the western United States. Apatite is treated with heat and acid to break strong chemical bonds that exist with fluorine, chlorine or carbon. Most phosphate fertilizer materials are reaction products of rock phosphate and sulfuric or phosphoric acid. Although sources of phosphorus are classified according to the amount of phosphate, or available P2O5 (either soluble in water or in a weak acid), the principal forms of phosphorus found in soils and used by plants are quite different. In other words, fertilizer phosphorus is formulated and sold as P2O5, yet phosphorus does not exist in soil in the phosphate form, nor do trees use phosphates.

There are two principal forms of soil phosphorus available to trees: H2PO4 and HPO4; the former is known as the primary orthophosphate, and the latter is called a secondary orthophosphate. The primary orthophosphate is more prevalent under acidic soil conditions (the extra hydrogen atom is a reflection of the fact that more H+ is floating around looking for anions to mate with when conditions are acidic), and the secondary orthophosphate is more prevalent under basic conditions.

Even slightly acid soils tend to bind phosphorus as insoluble phosphates of iron (Fe) and aluminum (Al), while basic soil conditions cause the formation of calcium (Ca) and magnesium (Mg) phosphates that are equally insoluble. Of the two forms, trees use H2PO4, the primary orthophosphate, to a greater extent than the other form.

Common Phosphorus-Based Fertilizers
Compound Formula Guaranteed Analysis (N – P2O5 – K2O)
Ordinary Superphosphate A Ca-primary orthophosphate 0-20-0
Triple Superphosphate A Ca-primary orthophosphate 0-45-0
Ammonium Phosphates An NH4-primary orthophosphate  
Monoammonium phosphate (MAP)
11-52-0
Diammonium phosphate (DAP)
18-46-0

For the reasons mentioned above, phosphorus does not leach readily through soil, a characteristic that presents a double-edged sword. On one hand, surface applications of fertilizer phosphorus almost never leach through soil even after the heaviest rains, but on the other hand, if soils are even slightly too acidic or too basic, even massive broadcast applications of phosphorus will not penetrate soil more than a centimeter or two before is gets locked up and unavailable to roots. The optimum pH for phosphorus availability is about 10 times more acidic than a neutral soil, or a pH value of about 6. Even under optimum conditions, usually only a small portion of total soil phosphorus is available to trees at any given time. The balance is either only slowly available or tied up and unavailable until soil chemistry changes.

In virtually all landscapes, total native supplies of phosphorus are more than adequate. However, its low solubility—when soil pH is too high or too low—can lead to shortages that are hard to detect. In most tree species, by the time symptoms show up on foliage the tree is already severely deficient in phosphorus, but often this is the exception rather than the rule. There are few instances of phosphorus deficiencies in trees.

Soil tests for phosphorus usually report total amounts divided into two fractions: available or soluble phosphorus, and unavailable, reserve or labile phosphorus, which is locked up in mineral forms and not available to trees. Available phosphorus in soils can range from 0 to 20 parts per million (PPM); values of 8 PPM and above indicate a high level of soluble phosphorus, obviating the need for fertilizer sources. Soil test values of 0 to 2 PPM may benefit from an application of 2 pounds of fertilizer phosphate (P2O5) per 1,000 cubic feet of soil in the tree’s rooting zone.

Reserve phosphate test values are usually considerably higher than available values, sometimes by a wide margin, especially on former agricultural soils. Expect to see test values ranging from 10 to 400-plus PPM of reserve-phosphorus. High values are indicative of a history of fertilizer phosphate applications on soils with excessively low or high pH. High reserve-phosphorus values should be looked upon as a phosphorus “mine,” easily tapped into with good practices such as liming soils to an optimum pH before planting landscape trees (if possible). Yes, reserve phosphorus is locked up in the mineral fraction of a soil and generally not available, but as soil chemistry changes (e.g. increasing the pH of an acid soil) and as available sources of phosphorus are used up, reserve sources become more available, especially to tree roots.

Photo courtesy of Gabriel Doyle/www.stock.xchng.com
On well-established landscape trees, the best way to deliver phosphorus to specimens that appear deficient is directly to the rooting zone within a few feet either side of the crown’s drip line.

If soil tests call for phosphate applications, the most effective use of phosphorus is when it is mixed with a soil (preferably after pH has been adjusted to near-optimum levels). Since phosphorus does not move appreciably in most soils, the idea is to put it in places where roots can find it. Generally, the best strategy for managing soil phosphorus around landscape trees is to apply virtually all necessary phosphorus before trees are planted. Apply ordinary superphosphate (0-20-0) in a soil band near the bottom of a planting hole—or to the side—separated from roots by a few inches of soil. This will allow tree roots to grow into an enriched band of soil, supplying all the phosphorus that tree will need for many years.

Unlike nitrogen, which has three different forms, fertilizer phosphorus is available in only one form: the phosphate salt (P2O5). It is combined with either calcium or ammonium to form superphosphates and ammonium phosphates. When phosphorus is applied as a preestablishment soil amendment, choose one of the superphosphates (of the two, ordinary superphosphate 0-20-0 is probably preferable to the more concentrated ‘triple’). Where availability or costs are significant factors, use the cheaper of the two sources (which is almost always triple when figured as cost per pound of elemental phosphorus). The ammoniated phosphates are excellent fertilizers, but when used preestablishment, the nitrogen is almost always lost before tree roots can use it, and they are usually a more expensive source of phosphorus than the superphosphates.

Where soils have high reserve-phosphorus levels, consider taking advantage of mycorrhizae-forming fungi, applied to tree roots as an inoculum immediately before establishment or after. The presence of mycorrhizal fungi on roots has been associated with adequate phosphorus nutrition even in nutrient-poor soils. Mycorrhizae fungi form nonpathogenic infections on roots that increase surface area and ability to use native soil nutrients, especially phosphorus. In exchange for a small share of photosynthate, mycorrhizae-infected roots are more efficient than roots without infections. Healthy mycorrhizae on tree roots are also associated with drought resistance, nematode and pathogenic disease resistance, accelerated root growth and faster establishment.

On well-established landscape trees, the best way to deliver phosphorus to specimens that appear deficient is directly to the rooting zone within a few feet either side of the crown’s drip line. Using a water-soluble form of phosphate is a little more expensive than dry fertilizer, but roots will assimilate the phosphorus-solution and correct the deficiency within days. An added benefit of pressure nozzles is that the hole will also effectively aerate the soil, which can be a huge benefit on clay soils.

If pressure sprays are not an option, use a soil auger to a depth of at least 12 inches, and then divide the total phosphate application into equal amounts mixed with soil and backfilled. Response to applications of dry phosphate is not as rapid as solutions, but the effect is more long lasting. Also, of the primary nutrients—nitrogen, phosphorus and potassium—phosphorus is the only element that needs to be applied directly to the soil. Nitrogen and potassium applications can be topdressed, since they move readily into the rooting zone with the next rain or irrigation cycle.

Do trees really need phosphorus? Absolutely, but not every soil is deficient. In fact, chances are there are more than adequate supplies of phosphorus already in the soil.

The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.