Soil testing for different phosphorus pools in cropland soils of the Great Plains

ABSTRACT:

Knowledge of soil phosphorus (P) pools and their measurements is important in nutrient cycling studies. An overview of the P cycle, with emphasis on calcareous cropland soils of the Great Plains, was prepared to meet this objective. Phosphorus pools can broadly be placed into four main groups: primary P minerals, secondary P minerals, occluded P minerals, and organic P. The dominant groups and their amounts in the soil are highly dependent upon soil pH, weathering intensity, P fertilization, cropping history, and crop-residue management. 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 calcium, iron, and aluminum, or from reactive clay surfaces. Generally, the first three groups exist in inorganic forms, and the fourth as part of the soil organic matter. These pools require combustion or oxidation to orthophosphate before analysis. The primary and secondary P minerals, through 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 (acid soils, calcareous soils, etc.) usually use the same soil test or P-availability index. Thus, in arid western states, the sodium bicarbonate (Olsen) procedure is used, 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. Other P procedures exist, but regardless of the procedure used, all have to show good correlation with soil P levels and P uptake and crop yields. Thus, categories (low, medium, high) are established for soil-test values based upon the probability of response of a crop to added fertilizer P. Knowledge of the cycling and fate of this nutrient, the second-most-important soil element for plant growth, is essential for good soil management and productivity.