Phosphorus Importance in a Soil Test Report
Professor DeBacco
Impact of Phosphorus on Plants
Phosphorus (P) is essential for root development and the production of flowers and fruit.
Regulating Phosphorus (CT example)
Native Connecticut soils are generally low in phosphorus and much of what is present is bound in both organic and inorganic forms not readily available to plants.
However, over-application has led to a 2013 law regulating the use of phosphorus on established lawns.
Phosphorus is a serious pollutant of inland waterways leading to algal blooms and accompanying environmental and human health risks.
Phosphorus Levels
Phosphorus is most available at a soil pH of around 6.5 and during moist, warm conditions.
Soil tests provide an estimate of the amount of readily available phosphorus and recommendations are made accordingly.
Factors Impacting P Availability in Soils
Organic Matter:
Mineralization of organic matter releases plant available forms of phosphorus into soils.
Organic molecules can also reduce phosphorus retention increasing the amount of available forms
Clay Content:
Soils with higher clay content have high phosphorus retention capacity because clay particles have very large surface area per unit volume, which can adsorb phosphorus easily.
Soil Mineralogy:
The mineral composition of the soil influences the phosphorus adsorption capacity.
Example, soils with a high content of aluminum or iron ions tend to have the greatest phosphorus adsorption capacity which will decrease its availability to plants
Soil pH:
Optimum soil pH between 6 and 7 will result in maximum phosphorus availability.
At low pH (acidic soils), soils have greater amounts of aluminum and iron, which form very strong bonds with phosphate.
At high pH when calcium is the dominant cation, phosphate tends to precipitate with calcium.
Other factors:
Temperature, moisture, and soil aeration can affect the rate of P mineralization from organic matter decomposition. So in warm, humid climates organic matter decomposes faster compared to cool dry climates.
Control How Much Phosphorus You Add
Addition of phosphorus beyond the agronomic need of crops has minimal effect on crop yield.
However, the excess phosphorus is susceptible to loss through runoff and erosion and can promote algal growth in freshwater systems causing the degradation of water quality.
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