Soil and Water Science
University o F Ii o F and Ag
DEGREE OF PHOSPHORUS SATURATION (DPS) IN LAKE
OKEECHOBEE BASIN SOILS
V. D. Nair and D. A. Graetz
Phosphorus (P) becomes a pollutant if it
moves from the site of its intended use to
surface waters either via surface runoff or
subsurface drainage. The Spodosols of the
Lake Okeechobee Basin of Florida receiving
significant loadings of animal manure would
be one of the most likely sources where
subsurface leaching of P would adversely
affect the quality of surface waters.
The "Degree of P Saturation" (DPS) is
expressed as the percentage of extractable P
(in moles) to the P sorption capacity (PSC)
of a soil. The PSC will be the maximum
amount of P that can be retained by soils as
determined by laboratory experiments and is
the sum of oxalate-extractable Fe and Al,
expressed in moles. The potential of using
DPS as a tool for quantifying the
environmental soil quality is now being
examined in various parts of the US.
The use of DPS in preference to extractable
P concentrations, such as Mehlich 1-
extractable P for identifying soils vulnerable
to P loss can be best illustrated by the use of
an example. If two soils have the same
Mehlich 1-P, say 50 mg P kg-', but different
adsorption capacities, say 200 mg P kg-' and
500 mg P kg-', then the DPS for the first soil
will be 25% while that for the second soil
will be 10%. It is obvious that the first soil is
more likely to release P than the second one,
though they both have the same Mehlich 1-P
concentration. Also, the second soil will
have greater capacity to receive additional P
compared to the first soil.
Manure loadings in the Florida Spodosols
range from very high total P (TP)
concentrations in the A horizon: from 2300
mg P kg-' from areas near the barn
(intensive and holding areas or high manure-
impacted soils, HMS) to about 25 mg P kg-'
in the low manure-impacted pasture, forage,
and unimpacted or native area soils (LMS).
For DPS evaluation, soil profiles were
selected from three active and three
abandoned dairies in the Okeechobee Basin.
All four dairy components were selected
from the active dairies, and only the
intensive and holding components from the
abandoned dairies. Native areas were also
sampled to obtain background conditions.
DPS was calculated as the percentage of
Mehlich 1-extractable P to the P sorbing
capacity of the soil, expressed as the sum of
oxalate-extractable Fe and Al.
The HMS for both active and abandoned
dairies generally had higher DPS values
compared to LMS. The HMS had surface
DPS ranging from 100% to 600% indicating
that the P sorption capacity of the horizon
had exceeded its maximum potential. The
LMS had DPS values ranging from 20% in a
beef pasture to 1% in a native area.
Abandoned dairies generally had higher
DPS values than active dairies throughout
the soil profile, suggesting P movement
through the soil profile with time.
The use of any parameter for assessing the
potential for off-site movement of P should
logically be related to some form of
available P, usually determined by
traditional soil test procedures. Previous
research on Lake Okeechobee Basin soils
had shown that the available P fraction
depended on the total P concentration.
Hence, we examined the relation between
DPS and water-soluble P, the most readily
available P form.
The HMS had exceeded their P sorption
capacity and these soils certainly will
contribute to off-site P movement. DPS
calculations for LMS on Florida Spodosols
(all horizons) gave a highly significant
relationship with water soluble P (WSP).
WSP = 0.3430 + 0.0.0728 [DPS]
R2 = 0.3336, n = 194, P < 0.0001
High (Intensive + Holding)
207 mg kg-1
TP = 2300 mg kg
Low (Pasture and Forage)
20 ma ka -1
TP = 165 mg kg1
NH Cl-P NaOH-lnorg. P NaOH-Org. P
SHCI-P Residual P
A method of DPS determination that is
simple and yet applicable to all Florida
soils including those sandy soils of the lower
coastal plain of the U.S. is needed. The
possibilities of developing a suitable DPS
method is currently being explored with the
aid of a FDEP grant (PIs: D.A. Graetz and
V.D. Nair), and the possibility of
introduction of the DPS factor in the Florida
P-Index matrix is being explored with the
aid of a recent USDA-IFAFS grant (PIs:
W.G. Harris and V.D. Nair).
By the end of these studies, we hope to
establish a threshold soil P level (establish a
critical DPS value) and define critical
sources within watersheds to protect water
quality by reducing surface and subsurface
runoff of P inputs to surface waters.
V.D. Nair and D.A. Graetz
Soil and Water Science Dept.
P.O. Box 110510, University of Florida
Gainesville, FL 32611-0510
DPS for HMS, %
0 200 400 600