Fertilizer: All About Phosphorus

N-P-K, Part II – Phosphorous by Steve Jones, Master Rosarian


Unknown-1The second number of the “big three” nutrients is phosphorous. The number on the bag or bottle of fertilizer refers to the amount of phosphoric acid is available, expressed as P2O5. Like nitrogen, it is a vital element for plant growth and development.


Phosphorous is a naturally occurring element in the soil. It occurs in plants only in a small amount, 0.5%, however, it is the one element that is greatly affected by other factors which will greatly affect the availability for uptake. Phosphorous stimulates root and plant growth, plus increases bloom size. It is essential for bloom and seed formation, and is an important constituent of nucleic acids. It also promotes new growth and causes early maturation in plants. Phosphorous is also essential for photosynthesis, conversion of carbohydrates and metabolism, energy transfer, and other plant functions. The most important is the formation of adenosine triphosphate (ATP) which is required for energy transfer in the plant.


Some regions of the United States are naturally high in soil phosphorous. Generally the west, northwest, and through the northern United Sates are highest. The low concentrations are in the deep south. In high clay soils, phosphates will form with clay to form insoluble complexes.


There are inorganic and organic sources of phosphorus. Some phosphorous is a by-product of decomposition of animal and plant matter. However, all organic phosphorous must be broken down to be absorbed by the plant. Plants take up phosphorus as the phosphate ions, H2PO4-, and to a lesser degree, HPO4-2.. High or low pH conditions will convert these ions to other forms, some of which are insoluble.


As with nitrogen, deficiencies of phosphorous is shown in the older leaves first as the plant takes the phosphorous from them for the new leaves. Deficiencies of phosphorous has an overall stifling affect on the plant. Leaves will drop fast without turning yellow and may form dead areas on the leaf. They turn a dull gray green color and the leaf edges may curl under. Purplish discoloration will also appear on leaves. Buds and blooms will be slow to develop.


Excess phosphorous will cause interference with the uptake of minor nutrients such as zinc, copper, and iron, which will cause deficiencies of those elements, even if large amounts are available. Most of these deficiencies involves the development of cane tips and blooms, plus chlorosis. In all honesty, I have yet to see excess or deficiencies of this element.


Phosphorous availability is strong affected by pH, high amounts of iron and aluminum, calcium, anion exchange with clays, and presence of microorganisms. As the soil pH drops below 4.5 and above 7.0, which is neutral, phosphorous will not be available to the plant as the phosphates are either bound to clay, or becomes an insoluble form. High concentrations of aluminum and iron will cause the phosphate to react and form insoluble salts. The same applies to calcium and magnesium, however, the insoluble forms are only present at pHs of 7 and greater. In pH soils above 7, phosphates will also bind with clay micelles. As mentioned before, some organic phosphorous require microorganisms to convert it to a usable form.


Phosphorous is a very slow moving compound in the soil, about an inch or so a year. This is the reason many rosarians will place superphosphate or bone meal in the hole near the roots when planting the rose. Phosphorous is generally added to the soil by using ammonium phosphates (48%), triphosphates (45%), or bone meal (25%). There is a liquid product called Hi Bloom which contains 10% of phosphoric acid. It is combined with potassium to help create larger blooms. Superphosphate is rated at 0-10-10.


In the next article, we will cover the last member of the “big three,” Potassium.