Journal of Environmental Protection, 2011, 2, 280-286
doi:10.4236/jep.2011.23031 Published Online May 2011 (http://www.scirp.org/journal/jep)
1
Impact of Feedlot on Soil Phosphorus
Concentration
Nicolás Wyngaard, Liliana Picone, Cecilia Videla, Ester Zamuner, Néstor Maceira
Facultad de Ciencias Agrarias (U. N. M. P.)-Estación Experimental Agropecuaria Balcarce (I. N. T. A.), Unidad Integrada Balcarce,
Balcarce, Argentina.
Received December 27th, 2010; revised January 30th, 2011; accepted March 10th, 2011.
ABSTRACT
Raising cattle in feedlots is becoming more common in Argentina, but there is little information available about the
effects of this practice on soil phosphorus (P) dynamics. In this study, concentrations of water and Bray-1 extractable
soil P were quantified in a feedlot (upper and lower slope pos itions in the feedlot pen), runoff area and pa sture during
2006-2008. The feedlot showed higher P concentrations in all P forms, soluble reactive P, soluble nonreactive, total
soluble P, and Bray-P1 when compared to runoff area and pasture. Most of the to tal soluble P was soluble reactive P
from the soil in the feedlot and the runoff area, while soluble nonreactive P predominated in the pasture. Concentra-
tions of total soluble P were elevated in the feedlot at the soil surface, ranging on average from 158.71 to 245.86
mg·P·kg−1, and had a rapid decrease within the first 20 cm and remained relatively low, about 25.53 - 27.33 mg·P·kg−1.
The evidence tha t total soluble P concen tration in the feedlot was significa ntly (p < 0.05) higher than that registered in
the pasture at 20 - 40 and 40 - 60 cm soil depth suggests a potentia l transfer of P through the soil from the surface. Al-
though the feedlot had a moderate increase of 23.05 to 24.55 mg·P·kg−1 from the background concentration, it may
represent a long-term source of in creased nutrient loading to groundwater. So il extractable Bray -1 P in the top 0 - 2.5
cm fluctuated from 659.26 to 45.36 mg·P·kg−1 in the feedlot and pasture, respectively. The relationship between soil P
extracted by test and TSP was linear, and correlation coefficient was r2 = 0.95.
Keywords: Feedlot, Water Soluble P, Bray-1 P, Ca ttle Manure, Phosphorus, Runoff
1. Introduction
Although in Argentina grazing is the beef production
system most extensively used, in recent years concen-
trated bovine feeding, especially in the Pampa region, is
more frequently used [1]. Concentrated animal opera-
tions have advantages in terms of animal growth rate and
uniformity in meat quality; however they may be a po-
tential risk of environmental pollution because of the
high animal stocking that produces a large volume of
manure. This situation creates an imbalance in soil nu-
trients that could lead to contamination of water sources
and atmosphere, either by accumulation of nutrients or
transfer of them to other systems. This condition is espe-
cially true for phosphorus (P) which is one of the nutria-
ents excreted by ruminants, mostly through feces [2], and
its management is very important from the agricultural
and environmental standpoint. Low soil P concentrations
affect crop production while high soil P concentrations
can produce the release of this nutrient to surface waters
and accelerate the eutrophication, affecting water quality.
In an increasing number of areas, the potential for P
losses through runoff has been increased by the continu-
ous application of fertilizer and/or manure from intensive
livestock operations [3]. Many studies have reported that
the loss of dissolved P in surface runoff is dependent on
the available P content of surface soil as measured by
soil P test extractants [4-6]. Pote et al. [7] considered that
water extractable soil P should be the best predictor of P
concentration in water runoff, showing that soluble reac-
tive P and bioavailable P in runoff were better correlated
with water soluble P than with P extracted by Mehlich-3,
Bray-Kurtz 1 and Olsen from soil. However, soil test P
levels alone have little meaning in P loss potential unless
they are used in conjunction with an estimate of potential
surface runoff, erosion and leaching. To this end, a P
indexing system was developed to identify the vulner-
ability to P loss of areas or fields [8].
*This work was supported by the projects FCA-UNMdP 15/A240 and
INTA-PNECO 1302 Similar to cropped soils, high concentrations of easily

Impact of Feedlot on Soil Phosphorus Concentration 281
soluble P in soil surface from intensive beef production
systems are associated with high concentrations of solu0
ble reactive P and total P in percolated water [9]. Klein-
man et al. [10] found that the concentration of dissolved
reactive P in water runoff was positively correlated with
water soluble P concentration in surface-applied manure.
To study the dynamics of P from feedlots can be com-
plicated compared with agricultural soils due to the
forms and concentrations of P in animal feces vary
widely because variations in P nutritional physiology and
P content in animal diets. There is high variability in
nutrients within and among feedlots, and fresh manure is
continuously deposited on the pen surface.
During the 1990s, Argentina has experienced a signi-
ficant growth in the number of feedlots. According to
National Animal Health Service, there are now about 2
278 active, registered feedlots that contain 1,600,890
head of cattle and range in size from small farms to
large-scale commercial livestock operations, but most of
them have 500 animals. In addition, unlike developed
countries, no rules have been promulgated to apply in
most aspects of animal production including location,
construction, operation and management of feedlots and
manure handling facilities, except for incipient legisla-
tions in some provinces. Clearly, livestock production in
Argentina is not as intensive in the USA or Europe, thus
P inputs are often lower. However it is important to un-
derstand the transformations undergone by nutrients
deposited in significant amount on the pen surface at
open cattle feedlots, and if they represent a potential risk
threat to water quality in order to develop effective
management practices for sustainable animal production.
Consequently, the objectives of this study were to 1)
measure the amount of water soluble P in soil surface
and in depth from two areas within feedlot, and in adja-
cent fields, runoff area and pasture, and 2) determine if
there is a relationship between agronomic test P (Bray-1)
and water soluble P.
2. Materials and Methods
2.1. Area of Study
The study was carried out in a farm located at Balcarce
county (37˚S; 58˚13 W), in the southeastern area of
Buenos Aires province (Argentina) that has an open
feedlot established on even area, but slightly sloping to-
wards the back of the pen (1% slope). Two areas were
selected within the feedlot pen: in the upper (UP) and
lower (LP) slope positions of the feedlot. Outside of the
feedlot, on the lower side of the farm, there is a depress-
sion that collects the runoff water from feedlot and adja-
cent fields and this area was identified as runoff area
(RA). An additional site evaluated was the pasture (PA)
or reference area composed by grasses, with no animal
occupancy. Soil chemical properties of these areas are
summarized in Table 1.
48 
The soil was classified as a Chelforó Series, a fine,
mixed, thermic, Typic Natraqualf characterized by the
presence of a surface horizon with loam texture, and clay
loam to clay subsoil horizons with low permeability due
to high Na concentration. Climate of the area is humid-
subhumid mesothermal with an annual precipitation of
879 mm (period 1971-2007). Mean annual maximum
and minimum temperature are 19.6 and 8.1˚C, respect-
tively, with annual mean thermal amplitude of 12.8˚C.
Cattle were placed at a rate of 50 to 165 m2·head−1 and
were fed a corn-based diet. The animals also received
250 g·day−1 of mineral supplement that had 60% of
non-protein nitrogen such as urea (19%) and ammonium
sulphate (5%).
2.2. Soil Sample Analysis
At each area, three composite soil samples were taken at
0 - 2.5 cm depth in November and December 2006, and
February and May 2007. Soil samples were also col-
lected at 0 - 10, 10 - 20, 20 - 40 and 40 - 60 cm to evalu-
ate changes in water soluble P concentrations with depth
in July and September 2008. Soil samples were homoge-
nized, air dried (25˚C - 30˚C) and passed through a
2-mm mesh sieve.
Water soluble P from soil was measured following
the technique described by Kuo [11]. Two g of soil with
20 mL of distilled water were shaken in an end-over-end
shaker for 1 h. The water extract was centrifuged at 2500
x g for 10 min, filtered through a 0.45 µm membrane
filter and then analyzed for soluble reactive P (SRP).
Same filtrate was digested in an autoclave (121˚C) for 60 -
90 min with ammonium persulfate and 0.9M H2SO4.
The difference in concentration between TSP and SRP
was considered the concentration of SNRP. The SRP
consists of inorganic orthophosphate extractable with
water, while SNRP may contain organic and some con-
densed forms of P.
Phosphate in extracts and digests were quantified colo-
rimetrically with the molybdenum-blue method of Mur-
phy and Riley [12]. Previously, the digests were neutral-
lized using p-nitrophenol indicator and dropwise addition
of either 0.5M H2SO4 or 1M NaOH.
Besides SRP, SNRP and TSP determinations, P was
determined using the agronomic soil test of Bray and
Kurtz [13] in soil samples from 0 - 2.5 cm depth.
For soil chemical characterization, total organic car-
bon (TOC) was determined by wet combustion with po-
tassium dichromate and concentrated sulphuric acid [14],
totl nitrogen (TN) by digestion with sulphuric acid in a

Impact of Feedlot on Soil Phosphorus Concentration
282
Table 1. Selected soil characteristics of the four areas evaluated in the study.
Area Total Organic Carbon Total Nitrogen pH Total Phosphorus
g kg−1 g·kg−1
Upper feedlot 63.78 6.85 7.52 1.56
Lower feedlot 73.59 7.90 7.25 1.80
Runoff area 69.63 5.10 6.36 1.37
Pasture 33.86 2.43 6.37 0.56
presence of potassium sulfate-catalyst mixture at 370˚C
[15] and total P (TP) by ignition at 550˚C and dilution in
1N H2SO4 [16]. Soil pH was measured using a 1:2.5 soil
to water ratio (w/w).
2.3. Statistical Analysis
The effect of sampling date and area on SRP, SNRP and
TSP concentrations at 0 - 2.5 cm soil depth was analyzed
statistically using a SAS/MIXED procedure [17] with
REML estimation method. The same procedure was also
used to test the differences in TSP concentrations be-
tween sampling dates and areas, at different depths (0 - 10,
10 - 20, 20 - 40 and 40 - 60 cm). The sampling depths
were analyzed separately. The Tukey-Kramer test was
used for mean comparison with a 5% level of signifi-
cance. The relationship between TSP levels and Bray P1
concentrations in the soil surface layer (0 - 2.5 cm depth)
was determined using PROC CORR [17].
3. Results and Discussion
According to statistical analyses, the SRP, SNRP and
TSP forms from the surface layer of soil were significant
affected (p < 0.05) by area, sampling date, and the inter-
action between area and date, except for SRP that was
not affect by sampling date (p > 0.05) (Table 2).
The concentration of SRP was significantly greater in
the UP as well as in the LP compared with the other two
areas, representing 77 and 66% of TSP; respectively
(Table 2). This is probably due to P in feces is mainly
under the dissolved inorganic form and its concentration
can be as high as 75% of total P depending on animal
diet [18]. Mineralization of organic P during feces deg-
radation could also have generated orthophosphate ions
that contributed to a predominance of SRP form in the
soil from feedlot. Long-term field experiments had
shown that soils with large manure applications have a
higher percentage of inorganic P [19,20]. The RA had a
similar behavior to the feedlot, the majority of TSP was
present in the reactive form (66%). The higher SRP con-
centration in the RA compared with the PA was reflected
in the higher amount of TSP because there were no sig-
nificant differences (p > 0.05) in SNRP concentrations
between these both areas (Table 2). Contrary to what
was observed in the UP, LP and RA, SNRP was the pre-
dominant form in the PA being this fraction 64% of TSP
(Table 2). Pasture growth promotes the release of or-
ganic phosphate compounds as labile monoesters through
root and microorganism turnover [21] that could contri-
bute to increase the concentration of SNRP. Similar to
our results, McDowell and Koopmans [22] reported that
70% - 90% of the total dissolved P found in water ex-
tracts from soils under unfertilized pastures was organic
P.
By comparing P data among areas, it was observed
that the SRP concentration for the RA was significantly
(p < 0.05) higher than in the reference area or PA while
the concentration of SNRP was not statistically different
between these both sites (p > 0.05), being the concentra-
tion of soluble P much higher in the UP and the LP
compared with the RA (Table 2). This seems to indicate
that there was a surface lateral P movement, especially
under SRP form, from the feedlot to the RA. The pre-
sence of subsoil horizons with low permeability, and the
high availability of water soluble P in the soil from feed-
lot makes it susceptible to move toward a lower area. As
the solid phase is P saturated as in the feedlot soil, the
sorption of P is limited and any orthophosphate present
in the manure remains as soluble P being this P more
easily transported [23]. In addition, organic acids re-
leased during the decomposition of manure not only re-
place P from the binding sites thus enhancing the release
of P to the soil solution, but also form complexes with
Ca, Al and Fe preventing them to precipitate with P [24].
Sampling date interacted with area which indicated
that there were significant differences between areas,
depending on the date. The TSP concentration was sig-
nificantly (p < 0.05) different among areas, in most of
the sampling dates; but no significant differences (p >
0.05) were found between the RA and the PA in No-
vember 2006, and between the UP and the LP in Febru-
ary 2007 (Figure 1). The higher TSP concentration in
the UP and LP, again shows the greater amount of ma-
nure that caused a larger composition of soluble P.
Without taking into account the amount of soluble P in
the urine, that could be significant [25], 56% - 64% of
total P from feces is water soluble P [26]. Mean concen-
tration of TSP in the adjacent PA was 18.50 mg P kg−1
which indicates the low background level and initial soil

Impact of Feedlot on Soil Phosphorus Concentration 283
P status. In general, concentrations of TSP did not fluc-
tuate greatly among dates; except for the UP which de-
creased sharply in February 2007 because the animal
waste was removed from the soil surface before sam-
pling of the soil (Figure 1). The concentration of SRP
was significantly (p < 0.05) higher in the UP compared
with the LP, RA and PA for all the sampling dates; and
the differences in this fraction P were not significant (p >
0.05) between the RA and the PA in November and De-
cember 2006 (Figure 2). As pointed out above, the
presence of manure together with mineralization of or-
ganic P could explain the higher SRP concentration in
the feedlot. The increase in soil pH (averaged 7.41) in
the feedlot may also have a bearing on P release. In con-
trast to SRP, the concentration of SRNP presented sig-
nificant (p < 0.05) differences between the UP and the
LP only in May 2007. Concentration of SRNP was lower
in the RA and the PA compared with that observed in the
UP and the LP, except for February where there were no
differences (p > 0.05) among the RA, PA and LP (Fig-
ure 3).
In relation to changes of TSP with depth, the data
showed that the concentration of this fraction was only
affected by area (p < 0.05); however, area interacted with
sampling date at 40 - 60 cm depth (Table 3). This inter-
action indicated that there was significant difference (p <
0.05) in TSP concentration between the UP and the LP
(48.7 and 4.82 mg·P·kg−1, respectively) in July while in
September the TSP was unaffected by areas. The con-
centration of TSP was high in the feedlot (158.71 to
245.86 mg·P·kg−1), low in the RA (26.61 mg·P·kg−1) and
still lower in the PA (4.16 mg·P·kg−1) on the soil surface.
Compared with the RA and the PA, TSP concentration in
the feedlot rapidly decreased within the upper 20 cm of
soil and remained relatively low, 25.53 - 27.33 mg P kg−1.
The presence of a compacted layer of manure and soil,
and the reduced water infiltration [27] possibly restricted
the P movement into deeper soil layers. However, the
evidence that TSP concentration in the feedlot, especially
in the upper slope position of the pen, was significantly
(p < 0.05) higher than in the reference area at 20 - 40 and
40 - 60 cm soil depth, suggests a potential transfer of P
through the soil from the surface in the feedlot. This
process could have been favored not only by the high P
concentrations in the soil surface but also by dry-
ing-wetting cycles that generate fissures that allow a
preferential flow of P or the urine infiltration. In Ne-
braska, during most summers, 2 - 3 cm wide cracks were
observed in feedlots, especially those with soils having
high clay content [28].
The concentration of Bray P1 in the top 2.5 cm of the
Table 2. Mean concentrations of total soluble phosphorus
(TSP), soluble reactive phosphorus (SRP) and soluble non-
reactive phosphorus (SNRP) from the surface layer of soil
(0 - 2.5 cm) in the evaluated areas, and a summary of
analysis of variance results.
Area TSP SRP SNRP
mg·P·kg−1
Upper-feedlot 233.43 a 178.84 a 54.58 a
Lower-feedlot 180.83 b 119.81 b 57.15 a
Runoff area 44.68 c 29.40 c 15.04 b
Pasture 18.50 d 6.87 d 11.77 b
Pr > F values
Area 0.0001 0.0001 0.0001
Sampling date 0.0001 0.1203 0.0001
Area*sampling date0.0001 0.0027 0.0001
Within each column, means in TSP, SRP and SNRP followed by the same
letter do not differ significantly (p > 0.05).
Table 3. Concentration of total soluble phosph orus (TSP) at
different soil depths in the evaluated areas.
Depth Upper
feedlot Lower feedlot Runoff areaPasture
cm mg·P·kg−1
0 - 10 245.86 a 158.71b 26.61 c 4.16 c
10 - 20150.26 a 54.98 b 15.11 c 2.76 c
20 - 4025.53 a 10.87 b 10.79 b 2.48 c
40 - 6027.33 a 6.73 b 6.46 b 2.78 b
Within each depth, means in TSP among sampling areas followed by the
same letter do not differ significantly (p > 0.05).
Figure 1. Concentration of total soluble phosphorus (TSP)
as influenced by sampling dates and areas during the study
periods. Means in TSP among sampling areas, for each date,
followed by the same letter do not differ significantly (p <
0.05).

Impact of Feedlot on Soil Phosphorus Concentration
284
soil profile increased drastically in the feedlot (average
659.26 mg·P·kg−1) compared with the RA and the PA
(154.66 and 45.36 mg·P·kg−1, respectively). The higher
values of Bray P1 in the RA in comparison to the refer-
ence area confirm the observation that there was a P
transport from the feedlot to the RA, where soluble P
from manure interacted with soil minerals and ions such
as Ca, Al and Fe forming compounds not soluble in wa-
ter that are extracted by the acidity of the Bray extractant.
According with these results, an increase in concentra-
tion of Mehlich-3, Olsen and Bray-1 extractable P in soil
with animal manure were reported in other studies
[29,30]. The correlation of TSP with Bray P1 was sig-
nificant (p < 0.0001) with r2 = 0.93 (Figure 4), in agree-
ment with the findings of other researchers who found
that soil test P was linearly related with water soluble P
[29,31]. Although the relationship between Bray P1 and
water soluble P is significant, to be able to use the agro-
nomic soil P test as indicator of the risk of P loss to water
bodies it is necessary to have information on other fac-
tors that affect the transport of P to water such as the
hydrology, topography and nutrient management prac-
tices. Adjacent fields with similar levels of soil test P but
with different susceptibilities to surface runoff and ero-
sion due to topographic and hydrologic variables will
have different potentials for P loss [32]. Therefore, soil
test P levels are not the most effective tools to identify
the risk of land P to water quality unless they are used
along with an estimation of potential surface runoff, ero-
sion, and leaching.
In summary, our results indicate that feedlot generates
high levels of water soluble P in the soil surface with a
prevalence of the SRP form that is highly bioavailable.
Compared with feedlot, the concentration of SNRP in-
Figure 2. Concentration of soluble reactive phosphorus
(SRP) as influenced by sampling dates and areas during the
study periods. Means in SRP among sampling areas, for
each date, followed by the same letter do not differ signify-
cantly (p < 0.05).
Figure 3. Concentration of soluble nonreactive phosphorus
(SNRP) as influenced by sampling dates and areas during
the study periods. Means in TSP among sampling areas, for
each date, followed by the same letter do not differ signifi-
cantly (p < 0.05).
Figure 4. Relationship between concentration of total solu-
ble phosphorus (TSP) and extractable Bray-1 P from the
surface layer of soil (0 - 2.5 cm depth).
creased in the PA as percentage of TSP. The large pro-
portion of SRP in the feedlot could be the cause of in-
creased P levels with depth compared to the PA, which
indicate some mechanism of P transfer from the soil sur-
face. Then, the feedlot may represent a long-term source
of increased nutrient loading to groundwater.
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