Previous work showed that at soil P concentrations below 57 mg 0.5 M NaHCO3
-extractable P (Olsen P) kg(-1) soil, little P was found in drainage waters
collected from tile drains set 65 cm below the soil surface in soils from
the Broadbalk Continuous Wheat Experiment. Above this soil P concentration
(termed the Change-Point) both total P and molybdate-reactive P (MRP) in dr
ainage waters were linearly related to soil Olsen P concentrations. We now
need to know if the Change-Point measured on Broadbalk occurs on other soil
s, and if so, whether a common value applies or if it varies depending upon
soil type, management, and site hydrology. We investigated the possibility
of 0.01 M CaCl2-extractable P being an indicator of the Change-Point. In a
ll the soils studied, we found that the dynamics of P solubility in CaCl2 c
losely resembled the dynamics of P solubility in drainage waters of Broadba
lk, since very distinct Change-Points occurred under both conditions. Howev
er, Change-Points measured following extraction with CaCl2 varied widely be
tween soils, from 10 to 119 mg Olsen P kg(-1) soil. Lysimeter studies showe
d, with some exceptions, good agreement between Change-Points measured in d
rainage water and in 0.01 M CaCl2. We therefore suggest that this approach
may provide a valid indicator of the soil Olsen P concentration at which si
gnificant amounts of P begin to leach from soil to water, provided preferen
tial pathways exist in the subsoil to permit P leaching down the soil profi
le in drainage water.