Ba. Linquist et al., AGGREGATE SIZE EFFECTS ON THE SORPTION AND RELEASE OF PHOSPHORUS IN AN ULTISOL, Soil Science Society of America journal, 61(1), 1997, pp. 160-166
Despite extensive research on soil-P reactions, little is known of the
effect of soil aggregation on these processes. This study was conduct
ed to determine the role of aggregate size on the sorption and release
of P in a Haiku clay (Typic Palehumult). A range of aggregate sizes u
p to 4 mm in diameter were used in this study. Autoradiography of sorb
ed 32P and P sorption studies indicated that added P was initially sor
bed to a 0.188-mm layer around aggregates and remained in this periphe
ral layer for up to 28 d. We defined this layer as the reactive mass.
Analysis of P in aggregates from a 4-yr P management field experiment
on the same soil indicated that diffusion of P from this peripheral la
yer into the aggregate was slow, requiring >2 yr for aggregate P to re
ach equilibrium. When P was added to a combined mix of aggregate size
fractions, P sorption increased from 55 to 245 mg P kg(-1) as mean agg
regate diameter decreased from 3.4 to 0.375 mm, and was linearly relat
ed to the aggregates' reactive mass (r(2)=0.96). Similarly, the reacti
ve mass of aggregates was linearly correlated to P release from aggreg
ates (r(2)=0.99) and to the linear buffer coefficients derived from P-
sorption isotherms (r(2)=0.98). Since aggregate size is related to P s
orption, soil buffering capacity, and P release from soil, it is likel
y that aggregation affects short- and long-term plant P availability.
The effect of aggregate size, however, needs to be investigated for ot
her soils.