Understanding Soil Test Phosphorus

Phosphorus (P) is a key nutrient for crop production, and keeping adequate levels of P in the soil is important for maximizing plant growth and development. However, understanding the various analytical methods for determining soil phosphorus can be challenging. The greatest confusion often lies in understanding why there are different analytical methods for determining soil P. The key to understanding this is to differentiate between total, available, and extractable levels of a soil nutrient.

Total P is the total amount of phosphorus in the soil. This can be P contained in organic materials, P in soil solution, exchangeable P, and P contained in insoluble mineral forms, and can be quite high in many soils. This information generally has limited agronomic use, however, because the amount of P that is actually plant available is generally only a small amount of the total P in the soil.
 
Of much greater benefit from an agricultural perspective is what is referred to as extractable P. Extractable P is the amount of phosphorus that can be extracted, or removed, from the soil by using one of a number of different types of chemical extractants. These extractants have been developed to remove certain forms of P from the soil, and this can be a more accurate index of what might be actually available to a growing crop The ultimate goal of an extractant is to reliably and consistently determine levels of the nutrient that correlate with the amount of that nutrient that might be available to a growing plant.
 
Bray-Kurtz P1 (Bray P1) has long been utilized in the Great Lakes region as the “standard” P extractant. It was developed in 1945 at the University of Illinois to correlate with the plant-available P fraction of the soil in slightly acid soils. Many of the P recommendation models, including the Tri State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, still utilize Bray P1 soil test values in their equations due to the widespread use of the extractant when these models were developed.
 
Bray P2, or strong Bray, is a more acidic solution that extracts forms of P that are less soluble than those extracted by the Bray P1 method. This extractant was commonly used when rock phosphate was the major P fertilizer product used in agriculture. It is still utilized by many to measure less soluble forms of P, what is commonly referred to as “active reserve” P in the soil, although most P recommendation models do not consider Bray P2.
 
Olsen P, or bicarbonate P, is a procedure that was developed in the 1950’s for determining P levels in neutral to alkaline soils. These soils are more commonly found in areas west of the Great Lakes region, so this test is only performed by request.
 
Mehlich-3 is the most commonly used extractant currently employed by soil testing laboratories in the region. It is a relatively safe extractant to work with and can be used to determine levels of other nutrients in addition to P, which makes it a more efficient method than others. Mehlich-3 is effective on the same types of soils as the Bray P1, but Mehlich-3 soil test P values are somewhat higher than those obtained by a Bray P1 extraction. However, the Mehlich-3 values correlate well with Bray P1 values, so Mehlich-3 values can be regressed into a Bray P1 equivalent number by using a mathematical operation. This allows soil test P values to be reported as a Bray P1 equivalent, which is necessary for making P fertilizer recommendations.
 
For any type of laboratory analysis to be useful, interpretations must exist in order for the data to be utilized to make decisions on a field scale. While different extracts have been developed to target different forms of P in the soil that may be plant available, this does not mean that the values determined by an extraction are absolute quantities of that nutrient in the soil. Much research has been done to correlate these soil test levels with crop response to a fertilizer material, and it is that correlation that is necessary for interpreting this information and making decisions.