Land application of spent gypsum from ditch filters: phosphorus source or sink?

doi:10.4236/as.2011.23048

Paper Menu >>

Journal Menu >>

Vol.2, No.3, 364-374 (2011)

doi:10.4236/as.2011.23048

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

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

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

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

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

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

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

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

372

100

120

140

160

180

17 119

Incubation Time (days)

NaOH-P (mg kg

-1

)

sand controlsand LGsand HGsilt controlsilt LGsilt HG

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

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

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

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.

REFERENCES

[1] Obama, B. (2009) Executive order: Chesapeake Bay

protection and res to r at i on.

[2] CBPO (2009) Sources of phosphorus loads to the Bay.

www.chesapeakebay.net/status_phosphorusloads.aspx?m

enuitem=19801

[3] Pote, D.H., et al. (1996) Relating extractable soil phos-

phorus to phosphorus losses in Runoff. Soil Science So-

ciety of America Journal, 60, 855-859.

doi:10.2136/sssaj1996.03615995006000030025x

[4] Buda, A.R., et al. (2009) Effects of hydrology and field

management on phosphorus transport in surface runoff.

Journal of Environmental Quality, 38, 2273-2284.

doi:10.2134/jeq2008.0501

[5] Sims, J.T., Simard, R.R. and Joern, B.C. (1998) Phos-

phorus loss in agricultural drainage: Historical perspec-

tive and current research. Journal of Environmental

Quality, 27, 277-293.

doi:10.2134/jeq1998.00472425002700020006x

[6] Kleinman, P.J.A., et al. (2007) Dynamics of phosphorus

transfers from heavily manured coastal plain soils to

drainage ditches. Journal of Soil and Water Conservation,

62, 225-235.

[7] Vadas, P.A., et al., (2007) Hydrology and groundwater

nutrient concentrations in a ditch-drained agroecosystem.

Journal of Soil and Water Conservation, 62, 178-188.

[8] Penn, C.J., et al. (2011) Trapping phosphorus in runoff

with a phosphorus removal structure. Journal of Environ-

ment Quality, In Press.

[9] Leader, J.W., Dunne, E.J. and Reddy, K.R. (2008) Phos-

phorus sorbing materials: Sorption dynamics and phys-

icochemical characteristics. Journal of Environmental

Quality, 37, 174-181. doi:10.2134/jeq2007.0148

[10] Penn, C.J., et al. (2007) Removing dissolved phosphorus

from drainage ditch water with phosphorus sorbing ma-

terials. Journal of Soil and Water Conservation, 62, 269-

276.

[11] Moore, P.A. and Miller, D.M. (1994) Decreasing phos-

phorus solubility in poultry litter with aluminum, calcium,

and iron amendments. Journal of Environmental Quality,

23, 325-330.

doi:10.2134/jeq1994.00472425002300020016x

[12] Penn, C.J., McGrath, J. M. and Bryant, R.B. (2010) Ditch

drain age manage ment fo r water quality improvement. In:

Moore, M.T. and Kroger, R., Eds., Agricultural Drainage

Ditches: Mitigation Wetlands for the 21st Century, Re-

search Signpost, Kerala.

[13] Penn, C.J., et al. (2011) Use of industrial by-products to

sorb and retain phosphorus. Communications in Soil

Science and Plant Analysis, 42, 633-64 4.

doi:10.1080/00103624.2011.550374

[14] Clark, R.B., Ritchey, K.D. and Baligar, V.C. (2001)

Benefits and constraints for use of FGD products on ag-

ricultural land. Fuel, 80, 821-828.

doi:10.1016/S0016-2361(00)00162-9

[15] Tan, K.H. (2005) Soil sampling, prep ar at io n, and analysis.

Taylor & Francis, New York.

[16] Hedley, M.J., White, R.E. and Nye, P.H. (1982) Plant-

Induced changes in the rhizosphere of rape (brassica

napus var. emerald) seedlings. New Phytologist, 91,

45-56. doi:10.1111/j.1469-8137.1982.tb03291.x

[17] Warren, J.G., et al. (2008) The impact of alum addition

on organic P transformations in poultry litter and litter-

amended soil. Journal of Environmental Quality, 37,

469-476. doi:10.2134/jeq2007.0239

[18] Martin, T.D., Creed, J.T. and Brockhoff, C.A. (1994)

EPA method 200.2 sample preparation procedure for

spectrochemical determination of total recoverable ele-

ments. US Environmental Protect ion Agency , Cincinnati.

[19] Tukey, J.W. (1953) The problem of multiple comparisons.

Princeton University.

[20] Penn, C.J., et al. (2011) Alternative poultry litter storage

for improved transportation and use as a soil amendment.

Journal of Environmental Quality, 40, 233-241.

doi:10.2134/jeq2010.0266

[21] Marsh, B. H. and J.H. Grove (1992) Surface and subsur-

face soil acidity: Soybean root response to sulfate-bear-

ing spent lime. Soil Science Society of America Journal,

56, 1837-1842.

doi:10.2136/sssaj1992.03615995005600060031x

[22] Sumner, M.E., et al. (1986) Amelioration of an acid soil

profile through deep liming and surface application of

gypsum1. Soil Science Society of America Journal, 50,

1254-1258.

doi:10.2136/sssaj1986.03615995005000050034x

[23] Edgell, K. (1986) Method 3050: Acid digestion of sedi-

ments, sludges, and soils. USEPA, Washington.

[24] Penn, C.J. and Bryant, R.B. (2006) Application of phos-

phorus sorbing materials to streamside cattle loafing ar-

eas. Journal of Soil and Water Conservation, 61, 303-

310.