Technical Abstract: Soil phosphorus (P) pools can broadly be placed into four main groups: primary P minerals, secondary P minerals, occluded P minerals, and organic. The dominant groups are highly dependent upon soil pH and P inputs. Chemical procedures for the first three groups are usually based upon the ability of the extractant to separate P from the dominant cations such as Ca, Fe, and Al, or from reactive clay surfaces. Generally, the first three groups exist in inorganic forms and the fourth as part of the soil organic matter. The primary and secondary P minerals through its equilibrium with the soil solution P, contribute to the plant-available P, and consequently, to the extractable P in a soil test. The organic P requires mineralization to orthophosphate first before it becomes a part of the available P, and the occluded P requires dissolution of an outer protective coating. Areas of similar soil mineralogy and pH usually have the same soil test. Thus, in narid western States, the sodium bicarbonate (Olsen) procedure is utilized, and in areas of greater rainfall where acid soils exist, the Bray-1 procedure is used. More universal methods include the Mehlich 3 procedure which is acceptable for both acid and basic soils. Thus, categories (low, medium, high) are established for soil test values based upon the probability of response of a crop to added fertilizer P. As the second most important soil element, knowledge of the cycling and fate of this nutrient is essential for good soil management and productivity.