PHOSPHORUS AVAILABILITY AND SPECIATION IN LONG-TERM NO-TILL AND
DISK-TILL SOIL
Essington, Michael E.; Howard, Donald D.
Soil Science
Issue: Volume 165(2), February 2000, pp 144-152
Abstract
Conservation tillage results in the concentration of plant-available P near the soil surface.
We studied the effects of conservation tillage on P speciation by examining the distribution
of P in inorganic and organic chemical pools. Depth-incremented soil samples were
collected from long-term (9- and 10-yr) no-till (NT) and disk tillage (DT) systems cropped
in corn (Zea mays L.) with a wheat (Triticum aestivum L.) cover crop. Rates of P were 0, 20,
and 60 kg P ha-1 yr-1. Total P (PT), organic P (PO), and available P (Mehlich-3, M3-P; Olsen
NaHCO3-pH 8.5, Olsen-P) were determined. P was also extracted from the following
chemical pools: non-occluded Al-bound (Al-P), non-occluded Febound (Fe-P), occludedreductant-soluble (CBD-P), and Ca-bound (Ca-P). Total P did not vary with depth, but was
greater in NT than in DT and increased with P rate. Organic P increased with P rate in the 0to 8-cm depth. Organic P was greater in NT plots in the 8- to 60-cm depths, averaging 75 mg
kg-1 for NT and 48 mg kg-1 for DT plots. Mehlich 3-P and Olsen-P were greatest in the
surface 4 cm and in the 60-kg P ha-1 plots, with higher levels observed in NT plots. On
average, the forms of P (as a % of total P) in NT soil was 6.2% Al-P, 33.9% Fe-P, 33.9%
CBD-P, and 4.7% Ca-P. Average P distribution in DT soils was 5.4% Al-P, 35.6% Fe-P,
31.3% CBD-P, and 5.1% Ca-P. The influence of tillage on P distribution was primarily
limited to the soil surface, with the exception of Al-P, which was greater in the 8- to 30-cm
depths of the NT plots. Because the impact of tillage was limited to a thin, soil surface layer
(<4 cm), soil P-test rating would not be affected by tillage practice. However, the improper
collection of soil samples from NT (i.e., too shallow) for P-testing may provide erroneous Ptest results and fertilizer recommendations.
The scientific literature is replete with studies that illustrate the significant impact of no-till
(NT) and other conservation tillage management systems on nutrient and organic C
stratification with soil depth. Under long-term NT management, the availability of immobile
nutrients, such as P, is greater in a thin layer (0 to 5 cm or less) at the soil surface, relative to
tilled soils (Weil et al. 1988; Eckert 1985 and 1991; Tracy et al. 1990; Karlen et al. 1991;
Ismail et al. 1994; Pierce et al. 1994). These studies have also illustrated that the effect of
conservation tillage is localized near the soil surface, as differences in P availability as a
function of tillage management below the 5-cm depth are generally not significant. Although
the significant tillage management effect on available P has been routinely reported, a limited
number of studies have reported no differences between the P status of NT and tilled surface
soils (Follett and Peterson 1988; Grant and Bailey 1994), indicating that cropping systems,
time in NT, and soil type can also influence the accumulation of available P near the soil
surface.
Because NT-managed surface soils tend to accumulate organic C, the distribution of P into
organic forms has been a topic of interest. Organic P, if significant, may play an important
role in the maintenance of a readily plant-available source of P and slow the cycling of P in
NT-managed soils. The general hypothesis is that the higher concentrations of organic matter
will result in high concentrations of organic P, thus affecting P cycling. However,
contradictory results have been obtained. Follett and Peterson (1988) observed significantly
higher organic P levels in the 0 to 5 cm surface of two Mollisols (relative to organic P at
greater depth), one under native sod and the other under winter wheat. Weil et al., (1988)
examined the influence of tillage practice on the organic P levels in three Ultisols cropped in
long-term NT and moldboard plowed corn. Although organic C levels were significantly
higher in the NT 0 to 2 cm surface soil than at greater depths, the concentration of organic P
near the surface was not affected by tillage practice.
The influence of tillage practice on plant available P, and, to a limited extent, on organic P,
has been examined, but very little information is available about the speciation of inorganic P
as influenced by tillage practice and depth. It is well established that the solid-phase
speciation of an element dictates, to a large extent, the fate and behavior of the element in the
environment. Further, it has been shown that plant available P, as determined by various soil
test procedures (e.g., Mehlich-1 and -3, Bray-1 and -2), can be underestimated in loessderived soil (Hardin et al. 1989; Howard et al. 1990), a clear indication that a more detailed
understanding of rhizosphere chemistry is necessary to achieve an accurate prediction of crop
response to fertilizer P. It has also been established that the primary mechanism by which P is
lost from surface soils is through sediment-laden surface runoff. Therefore, knowledge of the
distribution of P in the organic and various inorganic phases is necessary to predict the impact
of runoff-P on aquatic environments.
The objectives of this study were to determine the influence of tillage management (NT vs
disk tillage (DT)) on plant available P, the partitioning of P between inorganic and organic
phases, and the speciation of inorganic P into operationally defined chemical pools as a
function of P fertilization rate and depth in long-term tillage plots. The sequential P extraction
procedure of Petersen and Corey (1966) was used to partition inorganic P into the following
operationally defined chemical pools: loosely bound and soluble; non-occluded Al-bound;
non-occluded Fe-bound; occluded-reductant-soluble; and Ca-bound. As with any selective
dissolution procedure, P extracted by each reagent in the sequential selective dissolution
procedure may not actually reside in the indicated solid phase. Rather, the P-bearing phases
have similar susceptibility to the reagent employed and may have reactivity similar to that of
the indicated phase in the soil environment.
MATERIALS AND METHODS
Continuous NT and DT plots, cropped in corn (Zea mays L.) with a wheat (Triticum aestivum
L.) cover crop, were established on a loess-derived, Loring silt loam soil (fine-silty, mixed,
active, thermic, Oxyaquic Fragiudalf) in 1983 at the Milan Experiment Station, Milan, TN
(35°59'N, 88°50'W; elevation 124 m). The soil tested low in P. The mean annual rainfall at
Milan is 138.4 cm, and the average annual temperature is 14.3°C. Individual plots measured
3.04 by 9.12 m. There is essentially no slope in the experimental area, and no attempt was
made to isolate the treatments. The experimental design was a randomized, complete block
with a split-plot arrangement of treatments and four replications. The main plots were tillage
(NT and DT), and the subplots were P fertilization rate (0, 20, and 60 kg P ha-1 yr-1). Several
depth-incremented soil samples (0 to 8, 8 to 15, 15 to 30, 30 to 45, and 45 to 60 cm) were
obtained from random locations in each plot in the early Spring of 1993 using a hand auger.
The soil samples were composited, broken-up while moist, allowed to air-dry, passed through
a 2-mm sieve, and stored in sealed plastic bags. The surface soil (0 to 8 cm) is approximately
15% clay, primarily composed of hydroxy-interlayered vermiculite-vermiculite, kaolinite, and
mica, with minor amounts of smectite. Soil pH (1:1 soil to water extract) of the 0- to 8-cm
samples averaged 4.9 for NT and 5.4 for DT treatments. Soil pH did not vary with depth
below 8 cm, averaging 5.8 for NT and 6.6 for DT treatments.
Total P (PT) in the soil samples was determined using the H2SO4-H2O2-HF extraction
procedure of Bowman (1988) followed by the colorimetric analysis of an aliquot of extract
(Watanabe and Olsen 1965). Soil organic P (PO) was determined by extraction with
concentrated H2SO4, followed by extraction with 0.5 M NaOH (Bowman 1989). The H2SO4
and NaOH extracts were combined for analysis of total soluble P using a Thermo Jarrell Ash
Model 61 (Franklin, MA) inductively coupled argon plasma-optical emission spectrometry
(ICP-OES). Inorganic P (PI) in the combined extracts was determined colorimetrically
(Watanabe and Olsen, 1965). The computed difference between PT and PI is PO. Plant
available P was determined using two soil test procedures: the Mehlich-3 (M3-P) extraction
procedure (Mehlich, 1984) and the 0.5 M NaHCO3 (pH 8.5) (Olsen-P) extraction procedure
(Olsen and Sommers 1982). In addition, the soil samples were subjected to total C analysis by
dry combustion using a Leco C analyzer (CR-12, Leco Corp., St. Joseph, MI). Because of the
absence of inorganic C in these soils, total C is assumed to represent soil organic C. This
assumption has been validated for similar loess-derived soils through a comparison of dry
combustion and dichromate oxidation results (unpublished data).
Sequential P extraction from the following operationally defined chemical pools was
performed using a modified Chang and Jackson procedure (Petersen and Corey 1966): loosely
bound and soluble (1 M NH4Cl-extractable); non-occluded Al-bound (Al-P; 0.5 M NH4Fextractable); non-occluded Fe-bound (Fe-P; 0.1 M NaOH-extractable); occluded-reductant
soluble (CBD-P; citrate-bicarbonate-dithionite-extractable); and Ca-bound (Ca-P; 0.25 M
H2SO4-extractable). The P content of the CBD extracts was determined by ICP-OES; the P
content of all other extracts was determined colorimetrically.
In the spring of 1994, the experimental plots were resampled to better resolve the effects of
tillage practice proximate to the soil surface. Soil samples from the 0- to 4- and 4- to 8-cm
depths were collected and handled according to the procedure established above. However,
these samples were only subjected to a limited battery of analyses: PT, M3-P, Olsen-P, and
soil organic C.
RESULTS AND DISCUSSIONS
The organic C content of a soil is a chemical property that is usually influenced by tillage
practice and residue management, particularly near the soil surface. Analysis of variance
indicates that both tillage practice and sample depth had a significant influence ([alpha] =
0.05) on organic C content, with a significant depth-by-tillage interaction at [alpha] = 0.10.
Across all treatments, organic C decreased from an average 10.07 g kg-1 in the 0- to 8-cm
samples to 2.08 g kg-1 in the 45- to 60-cm samples. Upon closer examination, however (Table
1), organic C was not influenced by tillage practice in the soil surface (0- to 8-cm depth).
Significant differences were observed in the 8- to 15-cm and 15- to 30-cm depths, with the
DT treatments averaging 6.50 g kg-1 compared with 5.62 g kg-1 for the NT treatments for the
8- to 15-cm depth, and 4.11 and 3.70 g kg-1 for the 15- to 30-cm depth for the DT and NT
treatments, respectively. Higher organic C concentrations with depth in the DT treatments
may simply be a result of soil mixing associated with tillage. More clear evidence of this
effect, as well as the propensity for NT management to affect only a very thin layer near the
soil surface, is indicated in Table 2. The analysis of variance that considers all variables
indicates that only depth and a depth-by-tillage interaction significantly influenced organic C.
However, the data in Table 2 clearly illustrate a significantly greater amount of organic C in
the 0- to 4-cm depth under NT (averaging 18.1 g kg-1) compared with DT (averaging 14.3 g
kg-1). However, the opposite influence is evident in the 4- to 8-cm depth. The NT samples
averaged 7.97 g kg-1 compared with 10.4 g kg-1 for DT samples, indicating that the effect of
NT is localized near the soil surface and that, as expected, tillage distributes materials in the
plow-layer. Fertilizer P application rate, which may influence organic C indirectly through
improved crop response (and thus residues produced), had no effect on organic C content
(Tables 1 and 2).
Total P did not vary significantly with depth in either the 1993 sampling (Table 1) or the 1994
sampling (Table 2). However, the 1993 data indicate that PT was significantly influenced by
tillage, primarily in the 8- to 30-cm depths, (averaging 364 and 356 mg kg-1 for NT and DT,
respectively), and fertilizer P rate (averaging 316, 339, and 424 mg kg-1 for 0, 20, and 60 kg P
ha-1). The rate-by-tillage and depth-by-rate interactions were also significant, the former
indicating that the higher fertilizer P rates tended to result in higher PT levels in the NT
treatments and the latter indicating that the effect of P rate is greater at the surface than at
depth (a result of P uptake by crops not receiving fertilizer P).
The bioavailability and fate and behavior of an element in the soil is primarily a function of
speciation, both in the solid and solution phases. One form of P that figures significantly in P
cycling is PO. This form is labile and is transformed, slowly, into sparingly soluble (nonlabile)
inorganic forms. According to the analysis of variance, PO was influenced significantly by
tillage, fertilizer P rate, and sampling depth. Significant rate-by-tillage and depth-by-rate
interactions were also evident. Organic P averaged 79.7 and 57.3 mg kg-1 for NT and DT,
respectively, and 56.7, 63.6, and 85.3 mg kg-1 for 0, 20, and 60 kg P ha-1. Organic P was
significantly higher at the 0- to 8-cm depth (98.1 mg kg-1) relative to the lower depths
(averaging 61.1 mg kg-1 in the 8- to 60-cm increments). Examined as a function of depth,
however, it is apparent that tillage practice has no impact on PO in the surface 8-cm sample
(Table 1). Tillage practice significantly affected PO at depths below 8 cm, but only for the 0
and 20 kg ha-1 P rates (rate-by-tillage interaction). On average, for depths below 8 cm, PO
with the 0 kg ha-1 P rate was 72.8 mg kg-1 for NT and 43.5 mg kg-1 for DT. With the 20 kg ha1
P rate, PO was 79.0 mg kg-1 for NT and 34.5 mg kg-1 for DT. It is also evident that the
disparity between the PO content of the surface 8-cm samples and that of the 8- to 60-cm
samples increased as fertilizer P rate increased (depth-by-rate interaction). For example, for
the 0 kg P ha-1 treatment, PO in the 0- to 8-cm NT sample was similar to that in the 8- to 15cm sample (75 vs 73 mg kg-1). For the 20 kg P ha-1, NT treatment, PO was 106 mg kg-1 in the
0- to 8-cm depth and 77 mg kg-1 in the 8- to 15-cm depth (a significant difference). Finally,
for the 60 kg P ha-1, NT treatment, PO was 135 mg kg-1 at 0- to 8-cm depth and 66 mg kg-1 in
the 8- to 15-cm depth (also a significant difference). A similar pattern occurred in the DT
treatment.
Several states in the southeastern United States (Arkansas, Kentucky, North Carolina, and
Oklahoma) use the Mehlich-3 extract for soil-test P (Hanlon 1996). According to our analysis
of variance, M3-P was influenced significantly by fertilizer P rate and sampling depth, with a
significant depth-by-rate interaction. Tillage management did not influence M3-P. At
individual depths, only fertilizer P rate significantly influenced M3-P (Table 1). Below a
depth of 8 cm, M3-P was generally not influenced by any of the independent variables,
averaging 6.9 mg kg-1. Mehlich 3-P was significantly higher in the 0- to 8-cm samples,
irrespective of P rate (except the 0-kg ha-1 P rate, NT treatment). Further, M3-P was
significantly greater with the 60-kg ha-1 P rate than with other P rates. The 1994 sampling
further illustrates the vertical stratification of M3-P (Table 2). In all cases, M3-P was greater
in the 0- to 4-cm than the 4- to 8-cm layer, and it increased with increasing P rate. M3-P in the
NT surface 4-cm was also significantly greater than that in the DT surface, at the highest P
rate.
Another measure of plant-available P is obtained through the use of the pH 8.5, NaHCO3
extraction procedure (Olsen-P). Although a significantly linear regression equation can be
generated, relating 1993 Olsen-P to M3-P (Olsen-P = 0.859*M3-P + 2.872; r2 = 0.758***),
only 76% of the variability in Olsen-P values could be predicted using the regression equation
(Fig. 1). Further, since the data are highly skewed toward low extractable P levels, the validity
of the regression expression is questionable. Indeed, a logarithmic relationship would seem to
be more fitting. Still, either test would likely provide a similar soil P test result, even though
the Olsen-P soil test results have not been correlated with crop response to fertilizer P
application on the Loring or other loess-derived soils. Similar to M3-P, Olsen-P was
influenced significantly by P rate and sampling depth but not by tillage practice (Table 1).
Below the surface 8 cm, Olsen-P did not vary as a function of depth for any given tillage
practice or P rate. However, at the highest P rate, Olsen-P was significantly greater at all
depths under NT than DT. As was noted for the M3-P data, the vertical stratification of OlsenP was more pronounced in the 1994 sampling (Table 2). In all cases, Olsen-P was greater in
the 0- to 4-cm than the 4- to 8-cm layer, and it increased with increasing P rate. Further, at the
highest P rate, Olsen-P in the surface 4-cm was significantly greater with NT than with DT.
The higher concentrations of both M3-P and Olsen-P, and the broader range of concentrations
observed, produced a much more satisfying relationship between the two plant-available P
indicators (Olsen-P = 0.005*(M3-P)2 + 0.146*M3-P + 5.974; r2 = 0.967***; Fig. 1).
Inorganic soil P is found predominantly in the non-occluded Fe and occluded-reductant
soluble fractions (Table 3). On average, the forms of P as a percentage of total P in NT soil
was 6.2% Al-P, 33.9% Fe-P, 33.9% CBD-P, and 4.7% Ca-P. The P forms in DT soil averaged
5.4% Al-P, 35.6% Fe-P, 31.3% CBD-P, and 5.1% Ca-P. The distribution of P in the various
chemical pools varied with sampling depth and P rate, with the exception of CBD-P (Table 3).
Numerous interactions were significant, and Al-P and PO were influenced significantly by
tillage practice at the [alpha] = 0.10 level. The effect of tillage on these two chemical pools
was not observed in the 0- to 8-cm sample but was, however, in the 8- to 30-cm samples
(Table 1). In general, only the more labile forms of P (Al-P and Fe-P) were influenced
significantly both by P rate and sampling depth. Both Al- and Fe-P were usually depleted
from the 0- to 15-cm depths of the 0-kg ha-1 P rates, relative to the 15- to 60-cm depths. This
trend is also evident for Fe-P in plots with 20-kg ha-1 P rates. However, Al-P in the 20- and
60-kg ha-1 P plots and Fe-P in the 60-kg ha-1 P plots were more likely to be more concentrated
near the surface. Interestingly, in the 8- to 30-cm layers of the 0- and 20-kg ha-1 P rates, Al-P
was more concentrated in the NT plots than in the DT plots, a condition not observed for M3or Olsen-P.
As mentioned previously, selective sequential dissolution techniques provide a general
indication of the chemical pool(s) (operationally defined) in which an element resides in the
solid phase. If one assumes that the M3 and Olsen extracts are accessing plant-available P,
then a comparison of available P to the amounts of P extracted from each chemical pool may
provide an indication of the chemical pools that are accessible by the plant. The relationships
between the available P forms (M3-P and Olsen-P) with the three more obvious choices for
chemical pools that may contain available P (Po, Al-P, and Fe-P) are illustrated in Fig. 2. Nonoccluded Al-P is related significantly to both M3-P: Al-P = 15.56 + 0.54*(M3-P), r2 =
0.556***, and Olsen-P:Al-P = 14.34 + 0.57*(Olsen-P), r2 = 0.620***. This observation
suggests that Al-P is wholly or fractionally plant available, although the true test of
availability would be positive and significant correlation to a measure of bioavailability (e.g.,
plant uptake). On the other hand, both Fe-P and Po vary extensively, with little variance in
available P, particularly when available P is low (<20 mg kg-1).
A great deal of effort has been spent trying to identify the physical and chemical
modifications imparted to the soil environment through the implementation of conservation
tillage. The most significant finding has consistently been that the effect of conservation
tillage is localized in the surface few centimeters of soil (see previous citations). From a soil
fertility perspective, the effect of conservation tillage may be of little significance as the
influence of a thin surface soil layer on a standard soil sample (a depth of 15 cm) may be
immeasurable. Indeed, as the data in Table 3 illustrate, a 0- to 15-cm soil sample (data
obtained by averaging 0-8 cm and 8-15 cm values) does not influence available P (M3-P or
Olsen-P) significantly. However, there is a significant rate-by-tillage interaction for the OlsenP data. This interaction is caused by a significant effect of tillage on available P in the 60 kg P
ha-1 treatment (32.7 mg kg-1 for NT vs. 22.5 mg kg-1 for DT). Because these Olsen-P test
values are not correlated with a measure of predicted crop response to fertilizer P for loessderived soils, it is impossible to determine the practical significance of the differing test
values. Although not influenced significantly by tillage practice, M3-P values in the 60 kg P
ha-1 (24.1 mg kg-1 for NT and 27.8 mg kg-1 for DT) indicate a low soil P-test rating. This
study suggests that conservation tillage may not influence soil P fertility assessments
significantly. However, localization of the influence of tillage on P speciation near the soil
surface indicates that the form of P, which has the potential to be lost through surface runoff,
is an important soil chemical characteristic. Because speciation influences bioavailability, any
modification in P speciation may potentially affect aquatic environments that receive runoff.
Of the more bioavailable forms of P, only Olsen P in the 0- to 8-cm depth of the NT treatment
(52 mg kg-1) was greater than that in the DT treatment (39 mg kg-1). This characteristic was
specific to the 60 kg P ha-1 treatment. In the 0- to 4-cm depth, both M3-P and Olsen-P were
greater in NT (118 mg kg-1 and 97 mg kg-1) than in DT (104 mg kg-1 and 77 mg kg-1). Again,
these significant differences were specific to the 60 kg P ha-1 rate. Although not assessed,
differences in P bioavailability should become more pronounced as the depth of sampling is
decreased.
The results of this study are rather surprising. After 9 and 10 years of continuous cropping in
corn with a winter wheat cover crop, tillage type (NT vs. DT) had very little effect on the
status of P in the soil profile. Only P fertilization rate was found to significantly influence PT,
PO, available P, and the distribution of P into various operationally defined chemical pools. As
expected, the influence of tillage practice was restricted to the surface few centimeters: the
more shallow the sample, the more significant the effect. It is predicted that as surface
samples become more shallow than those examined in this study (<4 cm in depth), the
differences in P availability and speciation will become more evident. It is also clear that
tillage practice may have no practical significance on the fertility status of the soil with
respect to P. However, this is only true when soils are sampled to the depth prescribed by soil
test protocol. Soil test correlations, calibrations, and interpretations are normally based on a 0-
to 15-cm sample, irrespective of tillage practice. It is clear that a NT soil sample more shallow
than that used for soil P-test calibration will favor an erroneously high soil test-P result.
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Key words: Fertility; solid-phase speciation; selective dissolution; fertilizer rate; sampling
depth
TABLE 1 The influence of tillage practice, P fertilization rate, and soil sampling depth on
total P, available P, operationally defined P pools, and soil organic carbon for surface soil
samples collected in 1993
TABLE 2 The influence of tillage practice, P fertilization rate, and soil sampling depth on
total P, available P, operationally defined P pools, and soil organic C for surface soil samples
collected in 1994
TABLE 3 The influence of tillage practice, P fertilization rate, and soil sampling depth on
total P, available P, operationally defined P pools, and soil organic C for 0- to 15-cm soil
samples collected in 1993+
Fig. 1 . Relationship between Mehlich-3 P (M3-P) and NaHCO3-pH 8.5 P (Olsen-P) for soil
samples collected in 1993 to a depth of 60 cm and in 1994 to a depth of 8 cm (solid lines
represent regression equations).
Fig. 2 . Relationships between Mehlich-3 (M3-P) and (a) organic P and (b) non-occluded Aland Fe-P, and NaHCO3-pH 8.5 P (Olsen-P) and (c) organic P and (d) non-occluded Al- and
Fe-P for soil samples collected in 1993 to a depth of 60 cm.