Vol.2, No.3, 364-374 (2011)
doi:10.4236/as.2011.23048
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
Agricultural Scienc es
Land application of spent gypsum from ditch filters:
phosphorus source or sink?
Karen L. Grubb1, Joshua M. McGrath2, Chad J. Penn3, Ray B. Bryant4
1USDA-National Institute of Food and Agriculture, Washington, D.C., USA;
2Department of Environmental Science and Technology, University of Maryland, College Park, PA, USA; Corresponding Author:
mcgrathj@umd.edu
3Department of Plant a nd Soil Sciences, Oklahoma State University, Stillwater, USA;
4USDA-Agricultural Research Service, College Park, PA, USA.
Received 26 May 2011; revised 13 July 2011; accepted 29 July 2011.
ABSTRACT
Agricultural drainage ditches can provide a di-
rect connection between fields and surface wa-
ters, and some have been shown to deliver high
loads of phosphorus (P) to sensitive water
bodies. A potential way to reduce nutrient loads
in drainage ditches is to install filter structures
containing P sorbing materials (PSMs) such as
gypsum to remove P from ditch flow. The objec-
tive of this study was to determine the effect of
land-application of gypsum removed from such
filters on soil P forms and concentrations.
Gypsum was saturated at two levels on a mass
basis of P and applied to two soils of contrast-
ing texture, a silt loam and a sandy loam and
applied at both a high and low rate. The treated
soils were incubated in the laboratory at 25˚C,
and samples were collected at 1, 7 and 119 days
after initiation. Soil type, time after application,
gypsum rate, and P saturation level all had a
significant impact on soil P forms and concen-
trations. However, it appears that land applica-
tion of spent filter gypsum at realistic rates
would have little effect on soluble P concentra-
tions in amended soils.
Keywords: Phosphoru s S o r b i ng Materials;
Gypsum; Ditch Filter
Abreviations: BMP, Best Manageme nt Practices;
ICP-OES, Inductively Coupled Plasma–Optical
Emission Spectroscopy; LG, Low Gypsum Rate;
HG, High Gypsum Rate; LP, Low P Saturation
Level; HP, High P Saturation Level; FGD, Flue-Gas
Desulfurization; PSM, Phosphorus Sorbing
Material
1. INTRODUCTION
Globally, phosphorus (P) and nitrogen (N) loading to
surface water contribute to accelerated eutrophication,
resulting in water quality degradation. The Chesapeake
Bay is the largest estuary in the United States and has
been declared a “National Treasure” by a White House
Executive Order [1]. However, the water quality of the
Chesapeake Bay has been significantly impaired due to
sediment and nutrient contributions from point and non-
point sources. The Chesapeake Bay Program Phase 4.3
Watershed Model 2007 Simulation estimates that 3756
mg of P or 45% of the total P load delivered to the
Chesapeake Bay in 2007 originated from agriculture [2].
The Delmarva Peninsula is located on the eastern shore
of the Bay and is home to a large concentration of poul-
try production; leading to a regional surplus of P. Many
of the soils surrounding the poultry producing areas of
the Peninsula have become saturated with P after long-
term P application beyond crop requirements [3].
The southern Delmarva Peninsula is dominated by
coarse textured soils and shallow groundwater tables.
Agricultural drainage ditches dominate the landscape
and are used to move water away from the rooting zone
in order to allow crop production. These conditions con-
tribute to P movement through subsurface pathways to
ditches and ultimately surface water and the Chesa-
peake Bay [4-6]. Vadas et al. [7] found that environ-
mentally significant concentrations of N and P moved to
ditch waters from soils with high P concentrations and
shallow groundwater. This rapid lateral movement of P
through shallow subsurface pathways represents a by-
pass of most traditional best management practices
(BMP’s) designed for P control, which typically focus on
overland transport of dissolved and sediment bound P.
Recently, research has been conducted on passive P fil-
ters which can be installed in agricultural drainage
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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365365
ditches and used to capture dissolved P once it reappears
in the ditch [8]. These structures use head pressure gen-
erated by elevation or flow control devices to force ditch
water through a P sorbing material (PSM). Often these
PSM’s are byproducts of other industrial processes and
they provide a substrate for precipitation with metals
and/or adsor pt i on o nt o m e t a l oxides or hy d r oxides [9-11].
Phosphorus sorbing materials can be applied directly to
soil or manure, broadcast into ditches, or used in flow-
through structures [12]. One readily available PSM is
byproduct gypsum, typically produced through flue-gas
desulfurization (FGD) at coal-fired power plants. The
primary mode of action of byproduct gypsum is dissolu-
tion of the CaSO4 and precipitation of calcium phos-
phates from dissolved Ca in solution [13]. Several PSM
filters are in place and being evaluated on the Delmarva
Peninsula to determine their efficiency as a potential
BMP in these landscapes. Therefore there is a need to
determine how to handle the residual PSM’s after the
filters either physically or chemically fail. The benefits
of soil amendment with gypsum are well known. Gyp-
sum has been shown to improve soil physical properties
by alleviating surface crusting and compaction, increas-
ing water infiltration and holding capacity, improving
aggregate stability, and reducing water runoff and ero-
sion [14]. However, there is a need to determine how P
applied with gypsum residuals from PSM filters will
behave in regards to soil P availability. Therefore, a
laboratory incubation study was conducted to evaluate
soil application of spent gypsum from a ditch filter. The
objective of this study was to determine how spent gyp-
sum application would affect soil P forms and concen-
trations over time.
2. MATERIALS AND METHODS
2.1. Soil Incubation Study
A soil incubation study was initiated to determine and
compare the effect of adding phosphorus saturated flue
gas desulfurization gypsum (CaSO4·2H2O) on soil che-
mical properties. Treatments consisted of two soil types
(silt loam and sandy loam), two gypsum rates (high rate
and low rate), and two P saturation levels (25% and 75%
of the gypsum sorption maximum; see below for details),
and sampled at three dates. The treatments were assigned
in a two (soil type) by two (gypsum rate) by two (P
saturation level) by three (sampling date) factorial de-
sign, resulting in 24 treatment co mbinations. Total P was
only extracted at two sampling dates (day 1 and 119),
while the sequential P fractionation was performed at
day 1, 7 and 119. Each treatment combination was ran-
domly assigned within four replicate incubators, with
each incubator serving as one block. The incubators used
were VWR Scientific Model 2020 Low Temperature
Incubators set at 25˚C.
Soils used in the study were collected to a depth of 30
cm from cropped fields located on the Delmarva Penin-
sula. The sandy loam was a Galestown siliceous, mesic
Psammentic Hapludult located in Quantico, MD, USA
that was planted in corn when collected. The silt loam
was a Mattapex fine-silty, mixed, active, mesic Aquic
Hapludult and was collected from the edge of culti-
vated field located in Chestertown, MD, USA planted in
soybeans when collected. The soils were air-dried at
20˚C, passed through a 20-mm wire screen (to remove
detritus), and then 200 g (dry weight) of soil was added
to 96 plastic cups. Each cup had a snap cap with four
3.97 mm holes drilled in the lid to allow fo r air exchan g e.
Prior to amendment, a pre-incubation was conducted
where each cup was brought to a moisture content
equivalent to 70% of field capacity and incubated at
25˚C for 14 days. Field capacity was determined by the
method of Tan [15] .
The FGD gypsum was collected from US Gypsum
Company in Baltimore. In order to determine the amount
of P to add to the gypsum for the incubation study, P
sorption isotherms were conducted. Gypsum was air-
dried and sieved (2 mm) and 2 g of sample was weighed
into 50 ml centrifuge tubes. Phosphorus solutions were
made at 12 concentrations (0, 1, 5, 10, 50, 100, 800,
1600, 2400, 3200, 6400, 10,000 mg P/L) using KH2PO4
and deionized water. Four tubes with gypsum were
amended with 30 ml of solution at each concentration
for a total of 48 tubes. The P solution and gypsum were
reacted in an end-over-end shaker for 24 hours, then
centrifuged at 1163 × G, and filtered through 0.45 µm
filters. The supernatant was analyzed for total dissolved
P using inductively coupled plasma-optical emission
spectroscopy (ICP-OES). This P addition process was
repeated three more times on the same sample to deter-
mine sequential P sorption. A cumulative sorption curve
was created by plotting the sum of P adsorbed during the
four sequential sorption experiments versus the initial P
concentration in solution. The point at which the cumu-
lative curve leveled off (i.e. slope = 0) was assumed to
represent the potential sorption maximum. At the sorp-
tion maximum, approximately 24.7 mg·g–1 of P was
sorbed. The amount of P sorbed at 25 and 75% of this
maximum was approximated at 6.25 and 18.75 mg·g–1,
requiring initial P concentrations of 550 and 1550
mg·L–1.
In order to partially saturate the gypsum at the target
25 and 75% levels, 200 g of gypsum was reacted for 24
hours with either 2 .27 L of 550 mg·L–1 or 2.42 L of 1550
mg·L–1 P solution, respectively. After the reactions were
complete, the excess solution was poured off and the
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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366
gypsum was allowed to air dry. After completion of the
pre-incubation, each cup was amended with either 0.5 g
or 2.0 g in order to approximate a total application rate
of 5.6 or 22.4 Mg·ha–1 gypsum, assuming 2244 Mg·ha–1
of soil. The two levels of P saturation and two rates of
gypsum resulted in four treatment combinations: low P
and low gypsum (LP-LG), high P and low gypsum (HP-
LG), low P and high gypsum (LP-HG), and high P and
high gypsum (HP-HG). The resulting P and Ca applica-
tion rates associated with each treatment combination
are presented in Table 1. In addition, unamended con trol
samples were included in the study, but were not com-
pared statistically to the amended soils because they
would have caused an unbalanced design, complicating
statistical analysis. During the incubation study, samples
were weighed every 7 days, and sufficient deionized
water was added to maintain the moisture content at
70% of field capacity. Cups were destructively sampled
1, 7, and 119 days after amendment. When removed,
samples were oven dried at 60˚C for 24 hours an d siev ed
using a 2-mm sieve prior to sample analysis. Samples
from day 1, 7, and 119 were analyzed for chemically
defined P fractions and total P and Ca concentrations
were determined in samples from day 1 and 119 using
methods presented below.
2.2. Laboratory Anal yses
Samples from day 1, 7, and 119 were extracted using
the phosphorus fractionation method modified from
Hedley et al. [16] by Warren et al. [17]. Samples were
extracted sequentially by deionized H2O, 0.5 M Na-
HCO3, 0.1 M NaOH, and 1.0 M HCl at a solid to solu-
tion ratio of 1:60 (0.5 g to 30 ml). Samples (0.5 g) were
weighed into 50 ml centrifuge tubes and equilibrated for
24 h with 30 ml of the respective ex tractant at low speed
on a reciprocating shaker. After shaking, samples were
centrifuged for 15 minutes at 1538 × G. The supernatant
was passed through a 0.45 µm Millipore filter, diluted
tenfold with deionized H2O, and analyzed for P using
ICP-OES. The Hedley fraction separates P i nto chemi cal ly
defined pools. Typically the H2O-P is defined as labile P,
Table 1. Phosphorus (P) and calcium (Ca) application rates
associated with each P saturation and gypsum treatment com-
bination.
Treatment
Combination P Saturation
Level Gypsum Appli-
cation Rate P Applica-
tion Rate Ca Applica-
tion Rate
% of maxi-
mum Mg·ha–1 kg·ha–1
LP-LG 25% 5.6 35 1232
HP-LG 75% 22.4 105 1232
LP-HG 25% 5.6 140 4928
HP-HG 75% 22.4 420 4928
NaHCO3-P represents P loosely bound by Ca, Fe, Al, or
organics, NaOH extracts Fe and Al-P or P associated
with humic compounds, and the HCl fraction represents
Ca-P, P held in internal structures, or tightly held P in
organic forms such as phytate-P.
Total P was determined in soil samples from days 1
and 119 using EPA method 200.2 [18]. Soil (0.5 g) was
digested using an Environmental Express hot block
model SC154 with 2 ml of 1:1 HNO3 and 5 ml of 1:4
HCl at 95˚C. The samples were removed from the hot
block after 30 minutes, cooled, and diluted to 50 ml total
volume using deionized H2O and analyzed using the
ICP-OES. Soil pH was determined in each sample via
pH probe using 10g of air-dried soil and 10 ml of deion-
ized H2O.
2.3. Statistical Analysis
Statistics were conducted using SAS version 9.1. Al-
though the experimental design was a randomized in-
complete block design, the results were analyzed as a
randomized complete block design (background samples
were not considered in statistical analysis, but were in-
dicated as comparisons). Incubators served as blocks,
and the blocks were treated as a random factor. Proc
mixed was used as the data analysis model. Tukey’s
Multiple Mean Comparison Test was used to make pair
wise comparisons [19]. Significant differences in means
were determined at α < 0.05. Due to the complex facto-
rial design of the experiment with treatment factors of
gypsum rate, P saturation, soil type, and time interac-
tions between factors prohibited evaluation of the main
effect of treatment factors in most cases. However,
wherever a variable was determined to be significantly
affected by a treatment factor or multiple factor interac-
tion the means for the other variables (even if they were
not significantly affected) are presented for the sake of
comparison.
3. RESULTS AND DISCUSSION
Soil type, time of incubation, gypsum rate, and P
saturation level of the gypsum were the main treatment
factors evaluated. The complex interactions between soil,
gypsum, and P chemistry over time made statistical ana-
lysis of the data complex. However, several clear trends
emerged from the data.
3.1. Effect of Soil Type on Soil P Forms in
Unamended Soil: Silt Loam Had Higher
P Concentrations in Each Fraction
Except for H2O
The control soils, which were not amended with gyp-
um or P, were considered separately during statistical s
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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367367
Table 2. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extraction, and total phosphorus and calcium extracted by soil type averaged across incubation time for unamended soils*.
Soil Type pH H2O NaHCO3 NaOH HCl Cumulative Extracted Total P Total Ca
- mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1
sandy loam 4.39 11.23a 23.73b 46.30b 11.81b 93.06b 180.21 229.65
silt loam 6.26 8.59b 42.46a 127. 77a 42.19a 221.01a 412.71 1583.88
*Means fo llowed by d ifferent letter s within the same col umn are sig nificantl y different at P < 0.05. Means not followed by a letter could not be compared sta-
tistically due to a significant intera ction (P < 0.05) between the main effects of soil type and time.
Table 3. Mean pH, phosphorus (P) concentrations in chemically defined fractions, the cumulative extracted phosphorus from the
sequential extraction, and total phosphorus and calcium extracted for th e two-way interaction b etween soil type and day in unamended
soils*.
Soil Type Day pH H2O NaHCO3 NaOH HCl Cumulative Extracted Total P Total Ca
- -
mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg –1 mg·kg–1
sandy loam 1 4.55b 10.68 21.29 45.19 12.5789.73 199.25c 260.88b
sandy loam 7 4.44b 10.16 20.50 45.17 10.2386.05 Nd** nd
sandy loam 119 4.18c 12.84 29.40 48.53 12.62103.39 161.18c 198.43b
silt loam 1 6.29a 8.81 41.40 127.72 53.00230.92 367.78b 1544.25a
silt loam 7 6.27a 8.99 45.96 126.15 39.55220.65 nd nd
silt loam 119 6.22a 7.97 40.03 129.46 34.02211.48 457.65a 1623.50a
*Means fo llo wed by di fferen t let ters wit hin th e same co lu mn are s ig nifi cant ly di fferen t at P < 0.05. Means not followed by a letter were determined to not have
a significant interaction between soil type and time; **No data (nd) was available for day seven total phosphorus or calcium concentrations.
analysis. There were four replications or blocks of each
unamended soil included in the study. However, in order
to balance the design we would have had to include two
levels of zero gypsum and two levels of P saturation of
the gypsum or we would have had to include a gypsum
only treatment (no P) and P only treatment (no gypsum).
The size of the incubators available precluded these
treatment options; therefore, in order to keep the ex-
periment balanced for statistical analysis the unamended
soils were considered separately and presented only for
comparison. Only the main effect of soil type was de-
termined to be significant for H2O-P, NaHCO3-P, NaOH-
P, HCl-P, and cumulative P extracted in the sequential
fractionation. Therefore the mean concentration of each
of these variables is presented across incubation time in
Table 2. The silt loam soil had significantly higher P
concentrations than the sandy loam in each fraction ex-
cept for H2O. These results indicate that the silt loam,
while it had higher overall P concentrations, had a higher
P buffer capacity than the sandy loam, resulting in higher
H2O-extractable P in the sandy loam than the silt loam.
This trend persisted through most of the gypsum-P
treatments discussed later.
3.2. Effect of Soil Type and Incubation Time
on Total Soil Phosphorus and Calcium
and pH in Unamended Soil
There was a significant interaction between soil types
and incubation time for the total P and Ca extracted by
EPA method 200.2 [18]. Total P concentrations were
significantly lower in the sandy loam samples than the
silt loam for all sample dates (Table 3). Total P increased
with time in the silt loam samples; however, these are
unamended soils therefore no P was added over the
course of the incubation. One possible explanation for
this statistical increase would be loss of C through mi-
crobial respiration over time. The higher organic matter
content of the silt loam compared to the sandy loam soil
would explain why this increase in total P was not seen
in the sandy loam samples. This mechanism of total P
concentration increase via organic C degradation has
been observed among organic materials containing P
[20].
3.3. Main Effect of Soil Type on Amended
Soil Phosphorus Forms: Soil Type
Determined Differences in NaHCO3 and
HCl Extractable Phosphorus
Soil type did not interact with any other treatment
factors (Ta b l e 4). The silt loam soil averaged across all
gypsum and P application rates and incubation times had
significantly more NaHCO3-P and HCl extractable P
then the sandy loam soil. In addition, the cumulative P
extracted through the sequential fractionation was sig-
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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368
Table 4. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extraction, and total phosphorus and calcium extracted for each main ef f ect of day, phosphorus satu ration, g ypsum r ate, or soil type
in soils amended with gypsum and incubated for 119 days*.
Sequential Extractions
pH H2O NaHCO3 NaOH HCl Cumulative Extracted Total P Total Ca
- mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·k g–1 mg·kg–1 mg·kg–1
Incubation time
Days
1 5.26 24.69a 45.40b 94.73 27.18192.00a 323.93 2544.71
7 5.20 24.95a 50.14a 93.19 26.14194.41a
119 5.21 16.46b 44.40b 98.72 23.75183.32b 334.38 2086.85
Phosphorus Saturation
%
25 5.23 14.26 40.97 92.21 24.92172.36 318.98b 2206.98
75 5.21 29.76 52.25 98.75 26.49207.24 338.85a 2428.32
Gypsum Rate
Mg·ha–1
5.6 5.13 14.13 38.67 91.52 25.91170.24 307.27b 1544.56b
22.4 5.31 30.22 54.84 99.59 25.50210.15 351.57a 3119.26a
Soil Type
sandy loam 4.29 27.64 34.91b 48.20 12.66b123.41b 201.64 1656.45
silt loam 6.13 16.66 58.18a 141.84 38.49a255.17a 452.52 2961.66
*Means within a column, under the same main effect followed by different letters are significantly different at P < 0.05. If no letter follows the mean no sig-
nificance was determined or that treatment factor was involved in an interaction that prohibited statistical comparison.
nificantly higher in the silt loam soil than the sandy soil.
The NaHCO3 fraction represents potentially labile P
loosely held by Fe, Al, or in organic fractions. In com-
parison, the HCl fraction represents strongly held Ca-P,
likely as phytate P or mineral P held in internal struc-
tures. The combination of gypsum and P did not signify-
cantly affect these P fractions in the soil. Instead these
fractions were influenced by soil type. For each P-gyp-
sum treatment combination each soil type was amended
with the same amount of P; however, total P concentra-
tions in the silt loam were much higher than the sandy
loam as a result of the n ative soil P which overshadowed
treatment effect. Total P concentrations in the sandy
loam and silt loam soils prior to amendment were 180.2 1
and 412.71 mg·kg–1, respectively.
3.4. Effect of Soil Type, Gypsum Rate, and
Phosphorus Saturation Level on
Phosphorus Forms in Amended Soil:
Phosphorus Application Rate and Soil
Phosphorus Buffer Capacity Controlled
Differences Expressed in H2O and
NaHCO3 Extractable Phosphorus
Soil type had a significant interaction with gypsum
rate for the H2O-P fraction (P < 0.05). Significantly
more H 2O-P was extracted from both the sandy loam and
silt loam soil amended at the highest gypsum rate of 22.4
Mg·ha–1 (HG) compared to their low gypsum rate coun-
terparts (5.6 Mg·ha–1; LG) within soil type (Tab le 5). In
addition, the HG treated sandy loam had significantly
higher H2O-P than the HG silt loam treatment combina-
ion. At first it may seem counterintuitive that the high t
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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369369
Table 5. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extract ion, and t otal phospho rus and calcium e xtracted for th e two-way interaction soil type and gypsum rate in soils amended with
gypsum and incubated for 119 days*.
Sequential Extractions
Soil Type Gypsum
Rate pH H2O NaHCO3 NaOH HCl Cumulative
Extracted Total P Total Ca
Mg·ha–1 - mg·kg–1 mg·kg–1 mg·kg–1 mg·k g–1 mg·kg–1 mg·kg–1 mg ·kg–1
sandy loam 5.6 4.15c 16.39bc 27. 65 45.44 1 2.66 102.14 184.80 937.68
sandy loam 22.4 4.45b 39.38a 42.49 51.08 12.67 145.61 219.61 2423.13
silt loam 5.6 6.11a 11.88c 49.68 137.61 39.17 238.34 429.74 2151.44
silt loam 22.4 6.14a 21.43b 66.68 146.07 37.80 271.99 475.29 3771.88
*Means within a column followed by dif ferent letters are significantly different at P < 0.05. If no letter follows the mean no significance was determined
or that treatment factor was involved in an in te raction that prohibited statistical comparison.
gypsum rates resulted in higher H2O extractable P con-
centrations, regardless of P saturation level. However,
gypsum rate is tied directly to final P application rate.
The LP-HG and HP-HG treatment combinations resulted
in the two highest P application rates of 140 and 420 kg-
P·ha –1, respectively (Ta b le 1). By comparison the lower
gypsum rate resulted in P application rates of 35 and 105
kg-P·ha–1 for the LP and HP saturation levels, respec-
tively (Table 1). Therefore the increase in H2O-P exhib-
ited in each of the HG-soil combinations reflects the
higher P application rates in combination with the P
buffering capacity of the soil. The sandy loam has a
lower P buffer capacity than the silt loam and therefore
the H2O fraction in this soil is more susceptible to
changes induced by P application at a given P rate.
Similarly, there was a significant interaction between
soil type and gypsum rate and this combination had sig-
nificant effect on soil pH. The pH of the silt loam soil
was significantly higher than the pH of the sandy loam
soils regardless of gypsum rate. Gypsum rate did not
affect the pH of the silt loam soil, but the HG rate sig-
nificantly increased soil pH co mpared to the LG rate. As
with H2O-P the sandy loam soil exhibited a lower pH
buffer capacity and therefore was more easily affected
by increased gypsum rate. Gypsum applications to soils
considered more weathered (i.e. mineralogy dominated
by 1:1 minerals and Al/Fe oxyhydroxides) have been
shown to increase in pH more than less weathered soils
(i.e. mineralogy dominated by 2:1 miner als). Th is occurs
mainly from a ligand exchange reaction of sulfate with
terminal hydroxyl groups on variable charged minerals
which releases a hydr o xide to solution [2 1,22].
The effect of soil mineralogy and P buffer capacity is
also demonstrated by the interaction between P satura-
tion level and soil type, which was significant at P < 0.05.
Averaged across time and gypsum rate the sandy loam-
HP treatment combination resulted in the highest H2O-P
concentration relative to the other treatments, which
were all equivalent (Ta b l e 6). Once again the P satura-
tion level cannot be separated from the total P amend-
ment rate which was 105 and 420 kg-P·ha–1 for the HP-
LG and HP-HG treatment combinations respectively.
Therefore, only at the highest P application rate, on the
least buffered soil was there a significant effect of soil
type-P saturation. It is interesting to note that a different
trend was seen in NaHCO3-P for the soil-P saturation
treatment combination (Table 6). There were no statisti-
cal differences between the sandy loam soil and HP or
LP treatments, which had less NaHCO3-P than either silt
loam combination. However, the HP-silt loam combina-
tion resulted in higher NaHCO3-P concentrations than
the LP-silt loam combination. This is likely a result of
the higher clay and silt content of the silt loam soil re-
taining more of the added P in this fraction than in the
sandy loam where the added P resided in the H2O ex-
tractable fraction.
3.5. Effect of Soil Type and Time on Total
Phosphorus and Calcium and pH in
Amended Soils
Soil type also had a significant interaction (P < 0.05)
with incubation time for both total extractable P and Ca
(determined by EPA method 200.2) and soil pH. Total
extractable P was significantly higher on day 119 (471
mg·kg –1) in the silt loam soil than on day one (434
mg·kg –1) and the silt loam soils had more total P on ei-
ther day than the sandy loam on either day (Table 7).
However, there was no statistical difference between the
sandy loam soil on day 1 (214 mg·kg–1) and day 119
(189 mg·kg–1). No additional P was added during the
incubation so the in crease in total P seen in the silt loam
soil is likely due to loss of C through microbial respire-
tion as previously discussed with the unamended soils.
This loss would be expected to be more pronounced in
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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370
Table 6. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extracti on, and total phos phorus and calcium extracted for the two-way inter action soil type and phosphorus saturatio n level in soils
amended with gypsum and incubated for 119 days*.
Sequential Extractions
Soil Type Phosphorus
Saturation pH H2O NaHCO3NaOH HCl
Cumulative
Extracted Total P Total Ca
% - mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1
sandy loam 25 4.31 17.35b 30.16c 46.50 12.65 106.66 187.83 1640.03
sandy loam 75 4.28 37.50a 39.46c 49.82 12.68 139.46 214.59 1671.83
silt loam 25 6.1 1 11.30b 51.33b 136.01 36.68 235.31 441.93 2738.50
silt loam 75 6.14 22.02b 65.04a 147.67 40.30 275.02 463.11 3184.81
*Means wit hin a column fol l o wed by different l et t er s are signif i can t l y different at P < 0.05. If no letter follows the mean no significance was determined or that
treatment factor was involved in an interaction that prohibited statisti cal comparison.
Table 7. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extraction, and total phosphorus and calcium extracted for the two-way interaction between soil type and incubation time in soils
amended with gypsum and incubated for 119 days*.
Sequential Extractions
Soil Type Day pH H 2O NaHCO3 NaOH HCl Cumulative
Extracted Total P Total Ca
- - mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1
sandy loam 1 4.39b 28.94 32.97 46.61 14.08 122.60 213.58c 2076.29b
sandy loam 7 4.28c 32.66 38.37 49.30 12.21 132.53 Nd** nd
sandy loam 119 4.20d 20.90 33.29 48.71 11.64 114.55 188.91c 1208.61c
silt loam 1 6.12a 20.44 57.83 142.85 40.28 261.41 434.28b 3013.13a
silt loam 7 6.11a 17.24 61.90 137.07 40.08 256.29 nd nd
silt loam 119 6.15a 12.29 54.81 145.59 35.10 247.80 470.76a 2910.19a
*Means wit hin a column fol l o wed by different l et t er s are signif i can t l y different at P < 0.05.If no letter follows the mean no significance was determined or that
treatment factor was involved in an interaction that prohibited statistical comparison; **No data (nd) available as total phosphorus and calcium were only
measured on day one and 119.
the silt loam soil due to higher organic matter content of
the soil. Total Ca remained cons tant over time in the silt
loam soil and was significantly higher than in the sandy
loam soil at either date. However, total Ca concentra-
tions decreased over time in the sandy loam soil. There-
fore, it appears that gypsum application in the sandy
loam caused the formation of recalcitrant Ca minerals
after 119 days of incubation that were not extracted by
the EPA 200.2 method. This is not surprising since this
digestion method is less rigorous compared to the EPA
3050 digestio n [23].
3.6. Combined Effect of Soil Type, Gypsum
Rate, and Incubation Time on NaOH
Extractable Phosphorus: Increasing
Gypsum Rate Decreased P Solubility
The final significant effect of soil type was a three-
way interaction with gypsum rate and time (i.e. day;
Table 8). The combination of soil typ e, gypsum rate, and
incubation time only had a significant effect on NaOH-P.
Overall, the silt loam soil, for every gypsum rate and
incubation time, h ad higher NaOH-P concentrations th an
the sandy loam soil. Within the silt loam soil incubation
time did not have a significant affect on NaOH-P con-
centrations at the LG rate. For the sandy loam at the LG
rate, NaOH-P decreased significantly between day one
and seven, and then stayed the same out to 119 days of
incubation. At the HG rate the opposite trend was seen
where NaOH-P concentrations increased significantly
from day one to seven and then remained the same out to
119 days of incubation. Figure 1 shows these trends
alongside the control soils that were incubated without
mendment, while statistical comparisons were not made a
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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371371
Table 8. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extraction, and total phosphorus and calcium extracted for the thre e-way interaction between soil type, gypsum rate, and d ay in soils
amended with gypsum*.
Sequential Extractions
Soil Type Gypsum
Rate Day pH H2O NaHCO3NaOH HCl
Cumulative
Extracted Total P Total Ca
Mg·ha–1 - - mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1
sandy loam 5.6 1 4.25 17.72 28.90 47.68d 14.73 109.03 204.10 1292.8 3
sandy loam 5.6 7 4.14 19.43 28.32 44.87e 12.03 104.66 Nd** nd
sandy loam 5.6 119 4.06 12.01 25.74 43.76e 11.21 92.72 165.50 582.53
sandy loam 22.4 1 4.54 40.16 37.04 45.54e 13.42 136.17 223.06 2859.75
sandy loam 22.4 7 4.43 45.88 48.42 53.73c 12.38 160.41 nd nd
sandy loam 22.4 119 4.36 31.06 41.92 53.72c 12.13 139.49 215.66 1924.14
silt loam 5.6 1 6.10 12.04 48.84 136.85b 40.72 238.45 410.04 2095.75
silt loam 5.6 7 6.11 15.37 52.50 137.46b 41.29 246.62 nd nd
silt loam 5.6 119 6.13 8.25 47.70 138.50b 35.50 229.95 449.45 2207.13
silt loam 22.4 1 6.13 28.85 66.83 148.86a 39.83 284.36 458.53 3930.50
silt loam 22.4 7 6.11 19.12 71.30 136.68b 38.87 265.96 nd nd
silt loam 22.4 119 6.18 16.34 61.92 152.69a 34.71 265.65 492.06 3613.25
*The three way interacti on for soil type, gypsum rate, and day wa s o nly found to be significant for the NaOH (P < 0.05); Means for the other variables are given
for reference. **No da ta (nd) available for total phosphorus or calcium on d ay seven.
between control soils and amended soils they are useful
in interpreting the findings of the incub ated soils relative
to NaOH-P. With 7 to 1119 d o f incub ation, the high rate
of gypsum application (HG) resulted in higher NaOH-P
concentrations relative to the low rate (LG) applications
for the sandy loam. This was also true after 119 d of in-
cubation for the silt loam soil (Figure 1). These in-
creases in NaOH-P appear to be at the expense of a de-
crease in H2O-P (Table 8). In other words, the greater Ca
additions associated with the HG treatment (Table 1) are
shifting P from the most soluble pool (H20-P) to a less
soluble pool (NaOH-P), which is positive from the per-
spective of reducing risk of P loss to surface waters.
3.7. Effect of Incubation Time on Soil
Phosphorus Forms in Amended Soils:
Added Phosphorus Became Less
Available With Time
Incubation time had a significant effect on all vari-
ables measured except HCl-P. As described previously
time had a significant interaction with soil type for pH,
total P, and total Ca and was part of the three way inter-
action that influenced NaOH-P concentrations. In addi-
tion, incubation time had a significant effect on H2O and
NaHCO3 extractable P concentrations without interact-
ing with other treatment factors . Therefore th e following
results are presented as the main effect of time averaged
across gypsum rate, P saturation level, and soil type. As
shown in Ta ble 4, H2O-P concentrations did not change
over the first seven days of incubation, but decreased
thereafter. Initially, for all gypsum rate—P saturation
combinations a large pool of highly soluble P was ap-
plied to each of the soils. It is not surprising that with
time this P was repartitioned into the other soil P pools.
Similarly, it is not surprising that time had no effect on
HCl-P, ev en in combination with other treatment factors,
as HCl would represent one of the most recalcitrant
pools and therefore would be most resistant to change
with time. Sodium bicarbonate extractable P showed a
slight, but significant increase after seven days of incu-
bation but then return ed to its initial concentrations after
119 days. The slight increase after one week of incuba-
tion could be due to repartitioning of P as the system
equilibrates. It is interesting to note that while time
proved to be a significant factor for most variables meas-
ured in the amended soil it was not as important in the
control soils that were incubated without added gypsum
or P. As discussed previously time only affected pH and
total P and Ca in the unamended soils. This indicates that
most of the P forms are relatively stable with time in
these soils or in other words were in equilibrium relative
o P forms and the addition of P with the spiked gypsum t
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
372
0
20
40
60
80
100
120
140
160
180
17 119
Incubation Time (days)
NaOH-P (mg kg
-1
)
sand controlsand LGsand HGsilt controlsilt LGsilt HG
de
a
b
c
e
bb
c
e
a
b
Figure 1. Effect of the combination of soil type, incubation time, and gypsum rate on sodium hydroxide extractable phosphorus con-
centrations in amended soils. Means labeled with different letters were considered statistically different (P < 0.05). Mean concentra-
tions in unamended, control soils are presented for reference only and not included in statistical analysis.
caused changes to occur over time as this P was reallo-
cated between the chemically defined fractions.
3.8. Effect of Gypsum Rate and Phosphorus
Saturation Level on Soil Phosphorus
Forms: The Ratio of Phosphorus Added
to Calcium Added Dominated
Phosphorus Partitioning in the Soil
The effects of gypsum rate and P saturation level were
of the most interest to this study. As previously discussed
gypsum rate interacted with soil type to significantly
affect soil pH and H2O-P and the three-way interaction
between gypsum rate, soil type, and incubation time had
a significant influence on NaOH-P. Likewise, P satura-
tion level in combination with soil type significantly
influence d H2O and NaHCO3-P concentrations.
As a main effect gypsum rate had a significant effect
on total soil P. This is not surprising since P rate was tied
directly to each gypsum rate. The LG rate corresponded
with P application rates of either 35 or 105 kg P/ha,
while the HG rates resulted in application of 140 or 420
kg P/ha (Ta b l e 1 ). As a result, the HG rates resulted in
an average of four times more P being applied. This
crossed factorial of gypsum rate with P rate resulted in
the HG treatments having significantly more total soil P
averaged across soil type, incubation time, and P satura-
tion level. The total P concentrations in soils amended
with the LG and HG rates were 307.27 and 351.57 Mg
P/ha, respectively (Ta ble 4). Similarly, the P saturation
treatment factor was crossed with the resulting P appli-
cation rate and had a significan t main effect on total soil
P concentrations. The LP treatment resulted in either 35
or 140 kg P/ha and the HP treatment represented P ap-
plications of either 105 or 420 kg P/ha (Ta b l e 1 ), or an
average of three times more P being applied. As a result
total soil P concentrations for the LP treatment, averaged
across all other treatment factors, were 318.98 Mg·ha–1
compared to 338.85 Mg·ha–1 for the HP treatments.
While P application rate was tied to gypsum rate and P
saturation level, the Ca application rate was only tied to
the gypsum rate and not confounded with P saturation
level. Table 1 shows that the average total Ca rate for
the LP and HP saturation levels was the same (3080
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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373373
Table 9. Mean pH, phosphorus concentrations in chemically defined fractions, the cumulative extracted phosphorus from the sequen-
tial extraction, and total phosphorus and calcium extracted for the two-way interaction of gypsum rate and phosphorus saturation in
soils amended with gypsum*.
Sequential Extractions
Gypsum Rate Phosphorus
Saturation pH H2O NaHCO3 NaOH HCl Cumulative
Extracted Total P Total Ca
Mg·ha–1 % - mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1 mg·kg–1
5.6 25 5.13 13.77b 36.53c 90.23b 25.18 165.72b 300.29 1480.72
5.6 75 5.13 14.50b 40.80bc 92.81b 26.64 174.76b 314.25 1608.39
22.4 25 5.33 14.77b 45.60b 94.27b 24.64 179.28b 338.91 2981.67
22.4 75 5.30 45.02a 63.70a 104.68a 26.33 239.72a 363.44 3248.25
Control** 5.32 9.91 33.10 87.03 27.00 157.04 296.46 906.76
*Means wit hin a column fol l o wed by different l et t er s are signif i can t l y different at P< 0.05. If no letter follows the mean no significance was determined or that
treatment fa ctor was i nvolved in an inte rac tion that pr ohibite d statis tical c ompa rison; **Me an value s for unam ende d soil ave raged across inc u
b
ation tim e and soi l
type are presented for reference only and not compared statistically to results for amended soils.
kg-Ca·ha–1), but four times more Ca was applied with
the HG rate than the LG rate. Therefore, total Ca con-
centration in soil amended at the HG rate was signifi-
cantly higher than in soil amended at the LG rate, with
soil Ca concentr ation s of 3119.26 and 1544.56 Mg Ca/ha,
respectively.
As would be expected due to the crossed nature of the
experimental design, where Ca rate and P rate were con-
founded within the P saturation and gypsum rate treat-
ments, the interaction of P saturation and gypsum rate
was significant. The four treatment factors resulting
from this interaction represented four P rates and two Ca
rates (Table 1) and had a significant effect on H2O,
NaHCO3, and NaOH extractable P as well as the cumu-
lative P extracted through the sequential extraction. The
HP-HG treatment, which represented the highest P ap-
plication rate, resulted in significantly higher H2O, Na-
HCO3, and NaOH extractable P as well as cumulative P
concentrations than the other three treatment combina-
tions (Ta b l e 9 ). The other three treatment combinations
did not differ statistically for th e H2O or NaOH fractions
or the cumulative extractable P. However, the LP-HG
treatment combination resulted in statistically higher
NaHCO3 concentrations than the LP-LG treatment. The
NaHCO3 fraction most closely reflected the P applica-
tion rates of the four treatment combinations, so was
apparently most responsive to P added with the spiked
gypsum.
The P concentrations in each chemically defined frac-
tion for the unamended soils (averaged across incubation
time and soil type) are presented in Table 9 for reference.
No statistical analysis was performed comparing the
control soil to the amended soils; however, it is evident
that only the highest P rate, which was delivered by the
HP-HG treatment, resulted in a substantial increase in
any fraction relative to the control. Additionally, we see
that the HCl fraction was unaffected by the P added with
the gypsum. Overall, the HP-HG treatment increased
cumulative P extracted by the sequential fractionation
53% relative to the control soils, while the other three
treatment combinations in creased cumulative P extracted
only 6% - 14%. The greatest increases were seen in the
H2O and NaHCO3 fractions, where the HP-HG treatment
increased P concentrations 35.4% and 92% relative to
the control, respectively. The lower three P rates in-
creased the H2O-P 39% - 49% and the NaHCO3-P 10% -
38% relative to the control.
4. CONCLUSIONS
This study was conducted to see the effect of adding P
with gypsum on soil P forms and amounts as might oc-
cur when spreading spent gypsum from in-situ gypsum
filters on adjacent fields. The 22.4 Mg·ha–1 gypsum rate
would be considered high and would likely result in
other fertility issues (such as Mg leaching) due to exces-
sive Ca application rates [24]. It appears that time after
application and soil type would be significant factors in
determining the availability of P added with gypsum.
Moreover, total P rate, as a combination of gypsum rate
and P saturation of that gypsum, had the most pro-
nounced effect on soil P forms and concentrations. These
findings indicate that spent gypsum from in-situ “ditch
filters” could be applied at realistic rates to nearby fields
without substantially changing the soil P concentrations
and forms. The spent gypsum does not appear to be a
viable fertilizer source of P since it does not appear to be
very labile, but it would not pose an environmental risk
of highly soluble P available for loss in surface or sub-
K. L. Grubb et al. / Agricultural Science 2 (2011) 364-374
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374
surface discharges. In fact, at P saturation levels that
would be expected in actual spent gypsum, which would
be much lower than those tested in this study, it is possi-
ble that the spent gypsum would continue to react with P
in the soil solution in high P soils, reducing the avail-
ability of P for losses in runoff. However, this hypothesis
should be evaluated in the future using actual residuals
from prototype filters currently being evaluated. Fur-
thermore, this study was conducted in a laboratory set-
ting. The conditions in the laboratory differ significantly
from what would be experienced in the field, most nota-
bly in biotic processes that would be tied to climatic
variation in the field. Therefore, it would be prudent to
duplicate this experiment in a long term field study.
5. ACKNOWLEDGEMENTS
This project was funded in part by a USDA-NRCS Conservation
Innovation Grant (Award number NRCS 69-3A75-7-116). In addition,
the authors wish to give special thanks to Dr. Bahram Momen for his
guidance in statistical analysis of the data.
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