EVALUATION OF PHOSPHORUS INDICES IN RECENTLY ACIDIFIED SOIL OF THE COLUMBIA BASIN, WASHINGTON STATE

Kyle Edward Bair

Continual application of ammoniacal sources of nitrogen fertilizer can result in decreased bulk soil pH. Many soils of the Columbia Basin (CB), Washington State are weakly buffered and therefore susceptible to acidification. This change in pH can result in altered phosphorus (P) chemistry and puts into question the validity of using certain P extraction methods that were intended for use on high pH soils. The purpose of this dissertation is to 1) identify the species of P that exist in acidified CB soils, 2) determine what differences in P sorption and availability extraction occur across a range of soil pH's and 3) evaluate if plant uptake and production differences arise with soils that have become acidified. Several CB soils were collected that represent a range of soil pH (5.2-8.4) and soil test P (3-331 mg P kg-1, Olsen P extraction, 0.5M NaHCO₃, OP). These samples, together with other reference and comparative soils, were analyzed by sequential extraction and 31P NMR to determine predominant P species. Phosphorus sorption isotherms were also conducted to compare P sorption and binding energies. Further, fertilizer additions were made and incubated soils were extracted to determine the effectiveness of several P availability indices. Lastly, a greenhouse study was completed with various fertilizer treatments to compare plant P uptake and biomass production for soils with a range of pH values. Soil P speciation revealed that even at very low soil pH (5.2), the form of P that predominated was as calcium (Ca)-P (60% or more). 31P NMR spectroscopy was less useful for making this determination than the sequential P extraction method for these samples; however, clear differences in NMR spectra and extractable amounts could be seen between very calcareous soils and native acidic soils containing large amounts of iron (Fe) or aluminum (Al)-P forms. Comparison of extraction methods following incubation with fertilizer showed that across a range of soil pH, variability was least with the OP method. Differences in sorption maximum and binding energy existed, but in limited instances, suggesting CB soils changed little in these two parameters as a result of lower soil pH. Similarly, results from the greenhouse study where P uptake and biomass production were compared across the soil pH range suggest that while some differences in the measured greenhouse parameters occurred, generally one can expect that a soil at low pH will react reasonably close to a high pH soil following fertilizer application. From this data we conclude that differences in P speciation, sorption, extraction indices, and plant growth are not large enough to merit changing extraction methods from what is currently preferred. The OP test and fertilizer recommendations currently made using OP can be utilized on acidified CB soils reliably.

EVALUATION OF PHOSPHORUS INDICES IN RECENTLY ACIDIFIED SOIL OF THE COLUMBIA BASIN, WASHINGTON STATE

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