Normal Level of Phosphorus in Garden Soil

Phosphorous appears as the letter P in the periodic table of elements.

Phosphorus helps drive chemical reactions in plant cells, making it integral to growth and development. The relatively large amount of phosphorous required by plants qualifies the element as a macronutrient. Normal phosphorous levels in garden soil run a relatively large gamut. Understanding phosphorous levels and plant requirements requires an understanding of phosphorous types, fertilizers and the relationship between phosphorus and soil pH.

Types of Phosphorous

Understanding phosphorous types requires and understanding of the three forms the element takes in the soil and the difference between phosphorous and phosphate. When in soil, phosphorous appears as solution phosphorous, active phosphorous and fixed phosphorous. Phosphorous rarely appears in soil as pure phosphorous, but rather as phosphate, which is a mixture phosphorous and oxygen. Various types of phosphate exist, depending upon the ratio of phosphorous to oxygen in the substance’s molecules. Orthophosphate consists of 1 part phosphorous to every 4 parts of oxygen.

Phosphorous Levels

According to a fact sheet on phosphorous in soil published by the University of Minnesota, normal soil phosphorous levels range from several hundred to several thousand pounds per acre. In Missouri, normal levels range from 800 to 2,000 pounds per acre. If you amend the soil in your garden, the level of phosphorus in the soil depends upon natural levels and levels in amendments. Plants only take one type of phosphorous from the soil, solution phosphorous and usually only use phosphate, rather than pure phosphorous. Solution phosphate represents a small fraction of the actual phosphorous in soil.

Testing Levels

Soil tests reveal the levels of various elements in soil, including phosphorous. Tests for gardeners and commercial growers only measure the amount of phosphorous or phosphate in soil available to plants, not the actual pounds per acre. According to a University of Arkansas publication of phosphorous in soil, levels of 80 to 100 pounds per acre produce fine results for growing. Any measurement of more than 300 pounds per acre is potentially damaging to plants.

Adding Phosphorous to Garden Soils

Standard garden fertilizers contain three elements: nitrogen, phosphorous and potassium, which are represented by the letters N-P-K. Phosphorous in fertilizer usually appears as a phosphate of some kind. If a test of your garden soil indicates a need for more phosphorous, add a phosphorous-heavy fertilizer. Ratios on fertilize packaging indicate the amount of each element present. Confer with a greenhouse or garden supply store employee if you express doubts about adding phosphorous to your soil through fertilizer. Keep in mind that too much fertilizer damages plants more than not enough phosphorous.

Phosphorous and pH

The amount of phosphorous plants take from soil depends upon the soil’s pH level. Acidic soils, or those with a pH range less than 7.0, provide poor conditions for phosphorous absorption. The optimal pH level for phosphorous absorption is 6.0 to 7.0. Overly alkaline soils impede phosphorous absorption by plants. Lowering soil pH with aluminum sulfate or raising it with lime helps achieve maximum pH levels for phosphorous absorption.

References

• University of Minnesota Extension: The Nature of Phosphorous in Soils
• University of Arkansas Extension: Soil Phosphorous Levels
• University of Missouri Extension: Phosphorous in Missouri Soils
• Clemson Cooperative Extension: Changing the pH of Your Soil