Soil aggregate size affects phosphorus desorption from highly weathered soils and plant growth

Because plant absorption of P depends on the desorption of P from soil, understanding P desorption from soils may improve the precision of P diagnosis and fertilization recommendations. Many soils with high P retention due to high levels of Fe and Al are also highly aggregated, Extractable P is sometimes higher on larger size aggregates, which sill probably result in increased P release from aggregates. The effects of aggregate size on P availability of three highly weathered soils were quantified with a column-leaching study and a pot experiment. Phosphorus desorption by leaching from small aggregates was greater than that from large aggregates when P had been added to the bulk soil (Kapaa and Leilehua soils) and a mixture of different-sized aggregates (Leilehua soil). When aggregates were separated and then P added, however, P desorption was greater from large aggregates (4-6 mm) than from small aggregates (<0.5 mm), Conformity of the P desorption data to the parabolic diffusion and expanded Elovich equations suggests that P desorption is probably controlled by diffusion processes. A pot experiment showed that total P in lettuce (Lactuca saliva L.) and soybean [Glycine max (L,) Merr,] shoots, and the root dry weights of plants grown in the large aggregates (2-6 mm) were higher than for plants gown in the small aggregates (<0.5 mm) after equal amounts of P were added to the separated aggregate fractions. Increased P uptake with increased aggregate size was attributed to increased P release from aggregates because of reduced P fixation, The results suggest that soil management that favors soil aggregation may, in some cases, increase availability of applied P, Perhaps the distribution of soil aggregates should be considered in making P management decisions.