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Phosphorus Sorption, Desorption and Availibility in Oxisols and Utisols as Influenced by Soil Aggregate Size
|Title:||Phosphorus Sorption, Desorption and Availibility in Oxisols and Utisols as Influenced by Soil Aggregate Size|
|Abstract:||Phosphorus (P) limits food and fiber production in most regions of the tropics. The diagnosis and prediction of nutrient P requirements continues with low precision and high uncertainty. Some recent estimates suggest that predictions may be in error by as much as 50%. Initial data suggest that the high degree of soil aggregation common in highly weathered soils may affect P sorption. This study was conducted to determine the effects of soil aggregate size on P availability in order to improve P requirement prediction. The soils studied were highly weathered Typic Kandihumult (Leilehua), Anionic Acrudox (Kapaa), and Rhodic Eutrustox (Wahiawa) that represent high P sorption and a range in soil aggregation. Samples were collected from field plots of P experiments, where P had been applied 3-7.5 years before. For each soil, eight aggregate size fractions of < 0.053, 0.053-0.125, 0.125-0.25, 0.25-0.5, 0.5-1, 1-2, 2-4, and 4-6 mm were obtained using a dry-sieving method. For the Kapaa and Leilehua soils, sodium bicarbonate extractable P in aggregates increased up to 5-fold with decreasing aggregate size when P had been applied to soils. The extractable P did not increase with decreasing aggregate size on all soils where no P had been applied and even where P had been added to the Wahiawa soil. An incubation study showed that the increased extractable P was due to more sorbed P in small aggregates after P was added to a mixture of aggregates of different size. In contrast, P sorption decreased, and extractable P increased with increasing aggregate size after P was added to the separated aggregate fractions. The total P in soybean and lettuce shoots grown in larger aggregates (2-6 mm) was greater than in smaller aggregates (<0.5 mm) after P was added to the separated aggregate fractions. The reduced P sorption and increased P desorption with increasing aggregate size suggests that an improved prediction of the P buffer coefficients and P requirements in crop production systems could be achieved considering soil aggregate size distribution.|
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Ph.D. - Agronomy and Soil Science|
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