THE INFLUENCE OF ORGANIC RESIDUALS ON SOIL PHYSICAL
PROPERTIES
INTRODUCTION
In the last three decades, land application of organic residuals has increased due to evidence
of their ability to enhance crop growth and because it is proving to be an environmentally sound
alternative to traditional disposal methods. In addition to their fertilizing value, organic residuals
are demonstrating to be an advantageous soil amendment by increasing the soil organic matter
content. The gradual decrease in the organic matter content in cultivated soils, which may lead to
degeneration of the soil physical status and erosion, is particularly worrying in warmer climates
where organic matter decomposes rapidly (Metzger and Yaron, 1987). The application of
residuals is a means of maintaining sufficient levels of organic matter in soils as well as
enhancing the productivity of historically low fertile soils. Understanding the effects residuals
have on soil physical properties allows managers, regulators, scientists and land owners to
practice the best known methods for residual use and disposal.
Many studies have examined the effects residuals have on soil physical properties (Epstein et
al., 1976; Guidi and Hall, 1984; Sabrah et al., 1993; Lal and Nor, 1994; and others). Virtually all
of them note the interrelationship the properties have with one another. Each physical property
seems to affect another. As an example, a study conducted in Alabama used a municipal compost
consisting of residential garbage and up to 20 percent biosolids to examine the effects on crop
yields and select soil properties (Mays et al., 1973). Compost was plowed under in the fall of
1968 and disked in during the spring of 1969. These combined applications ranged from 9 to 183
Mg ha-1. Following two harvests of sorghum, the same field received additional applications of
compost in 1970 ranging from 9 to 144 Mg ha-1, for a two-year total application rate ranging
from 23 to 327 Mg ha-1. Following the final harvest of sorghum, bulk density and compression
strength significantly decreased, while soil moisture content and moisture holding capacity
increased. As organic matter rates increased, bulk density and the unconfined compression
strength consistently decreased, while moisture capacity and moisture content consistently
increased. Researchers found that the finer textured compost was more favorable to soil physical
characteristics than the coarser material.
The following is a summary of the content of various literature on issues of concern
regarding land application of biosolids and other organic residuals as they relate to the following
soil physical properties: organic matter, soil moisture and porosity, bulk density, hydraulic
conductivity, mechanical properties and aggregation. Abstracts of the articles that are referenced
in this summary are included and are available for further review by contacting the Northwest
Biosolids Management Association.
NBMA DRAFT Summary - Soil Physical Properties
ORGANIC MATTER RETENTION
Results from literature indicate that the fate of organic matter is dependent on incubation time
and the constituent composition of the organic material (Metzger and Yaron, 1987). The
following authors primarily focused on general trends of soil organic matter (SOM) following
residual application, and, for the purpose of this review, in-depth examinations of organic
fractionation and decomposition rates of organic carbon have not been included.
An overall increase in levels of organic matter following the application of residuals occurs
with a variety of soil types (Hinsley et al., 1979; Bevacqua and Mellano, 1994; Tsadilas et al.,
1995; Droogers and Bouma, 1996; and Paino et al., 1996). Martens and Frankenberger (1992)
found that several residuals increased SOM over a 25 month period. SOM increased by 57, 84,
37, and 13 percent with poultry manure, biosolids, barley straw, and alfalfa additions,
respectively. The greatest increase was with biosolids application. Hinsley et al. (1979) noted that
significant levels of organic carbon remained in the soil during 3 years of liquid biosolids
applications and for the following 4 years.
The use of residuals in land reclamation has proven to have made substantial improvements
to severely disturbed soils while aiding in revegetation and erosion prevention. Joost et al. (1987)
found that biosolids-amended coal refuse increased the SOM 2 to 3 times greater than that of
limestone-amended and unamended sites. The SOM of biosolids-amended plots increased 300
percent initially, then dropped by 40 percent after the first 2 years. A significant increase over
other treatments remained despite the decomposition that had taken place. Roberts et al. (1988)
noted that accurate measurements of SOM in mine soils can be complex and varies tremendously
due to chemical processes occurring in mine soil.
SOIL MOISTURE RETENTION AND POROSITY
The ability of an organically amended soil to retain water is inherent in the properties of the
organic fraction of the soil. Generally all organic amendments consist of large surface to mass
ratios and thus provide for greater water retention than coarse material such as sand or wood
chips. Water holding capacity of soils is primarily controlled by: 1) the number of pores and pore
size distribution and 2) the specific surface area of soils (Khaleel et al., 1981). With different
types of amendments come various degrees of water retention. Epstein (1975) found that raw and
digested biosolids increased the total water retained by the soil, with the greatest increase in the
raw biosolids-amended soil. The two digested biosolids-amended treatments (high and low pH)
retained 10 times more water than the controls, while the raw biosolids treatments retained twice
as much as the digested treatments. Over the course of 180 days of incubation the raw biosolidssoil mixtures showed the most significant differences relative to controls. Maintaining digested
biosolids-soil mixtures at two different water potential values (-0.51 and -0.051 Mpa) did not
affect their retention values. The influence of dewatered biosolids on soil moisture retention was
seen in a study by Wei et al. (1985). At lower tensions of less than 5 kPa, the volume of water
retained by an amended silty clay loam was 112 > 44.8 > 0 Mg ha-1 indicating larger pores were
present with increasing biosolids application. For applications greater than 10 kPa, the order was
0 > 44.8 > 112, indicating smaller pores for the control and lower levels of biosolids application.
Webber (1978) indicated that carbon resulting from the decomposition of residuals affect the
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NBMA DRAFT Summary - Soil Physical Properties
moisture properties of an amended soil, not necessarily the residuals per se. He found that the
soil moisture at 1.5 Mpa for biosolids (B), unsorted shredded domestic garbage (W), manure and
W, B and W, and double W and B treatments increased with increments of soil carbon. In a study
examining different residuals and their effect on soil moisture, barley straw showed the greatest
increase in gravametric moisture content by 25 percent over controls compared to 3, 4, and 9
percent increases with poultry manure, alfalfa, and biosolids additions (Martens and
Frankenberger, 1992).
A decrease in moisture retention was seen in a biosolids-amended bauxite refining residue
(red mud). Wong and Ho (1991) reported a decrease in soil water content with an increase in
biosolids application. This decrease was due to the relatively low density of the biosolids
compared with the red mud. The addition of biosolids increased the total porosity of the soil and
hence reduced the amount of pores able to store water. This was seen as a benefit to the red mud
because it increased aeration and reduced the risk of an anaerobic environment.
Giusquiani et al. (1995) found that total porosity (pores > 50 mm) increased with compost
rates regardless of soil depth and sampling time. The average increase of porosity with compost
rates was, 0.074, 0.075, and 0.079 percent for each Mg of compost added, in November 1991,
June 1992, and November 1992, respectively. The greater porosity in the amended plots was due
to an increase in the abundance of elongated pores. This increase in porosity positively correlated
with an increase in water retention, especially with higher application rates. Kladivko and Nelson
(1979) also found that the volume of large pores (pores > 50 mm) increased with the application
of liquid biosolids on a silty loam. Micropores (0.2-50 mm) and macropores (> 50 mm) were
increased by the addition of animal slurries, municipal compost and biosolids (Pagliai and
Antisari, 1992). Researchers have shown that seasonal conditions can affect soil porosity
following organic additions (Guidi et al., 1981). Guidi and Poggio (1987) showed differences in
total porosity at separate sampling times, June and September, but not within replicates of the
same time periods. A reduced total porosity was accompanied by a loss of pores greater than 3
mm.
BULK DENSITY
Residuals tend to be less dense than the mineral fraction of soils (Metzger and Yaron, 1987).
The decrease in bulk density (BD) of soils as a result of organic applications, such as biosolids, is
due to the dilution effect resulting from the mixing of the added organic matter with mineral soil
(Khaleel et al., 1981). Reduced bulk densities were measured in biosolids-amended sites,
regardless of the soil texture (Mays et al., 1973; Gupta et al., 1977; and Kladivko and Nelson,
1979). Biosolids-amended soils with coarser textures have resulted in dramatic changes in bulk
densities. A 45 percent reduction was seen when biosolids compost was applied on an Evesboro
loamy sand (97 percent sand) at a rate of 240 Mg ha-1 (Tester, 1990). Similar results were noted
with the addition of 450 Mg ha-1 yr-1 of an anaerobically digested biosolids on a Hubbard coarse
sand. Following two consecutive years of applications there was a 28 percent decrease in BD
relative to controls (Gupta et al., 1977). Logan et al. (1996) found that the addition of olestra (a
fat replacement product developed by Proctor and Gamble) to a biosolids-amended loamy sand
had no effect on BD. However, biosolids alone significantly decreased the BD. Webber (1978)
found significant decreases in BD of soil amended with an unsorted shredded domestic garbage
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NBMA DRAFT Summary - Soil Physical Properties
and biosolids mixture but found no differences in soil amended with biosolids only. Differences
in BD in the 0-17 cm depth were noted by Harrison et al. (1994) but none between 17-185 cm
depths 15 years after biosolids application in a forested watershed. Four different types of
residuals applied to a coarse loamy alfisol resulted in significant decreases in BD (Martens and
Frankenberger, 1992). Poultry manure, biosolids, barley straw, and alfalfa additions decreased
BD by 7, 10, 11, and 7 percent when compared to unamended plots during a 2 year period.
HYDRAULIC CONDUCTIVITY
Hydraulic conductivity is essentially the ease with which soil pores allow water movement
(Brady, 1990). The size and configuration of soil pores determine the conductivity of a soil.
Residuals greatly influence pores by changing the magnitude at which pores exist in the soil. The
addition of organic matter can be seen as the addition of small and large pores and the accretion
of various shapes of soil pores. Consequently, hydraulic conductivity can increase or decrease
when the soil porosity is affected. Measurements of hydraulic conductivity of biosolids-amended
soils have shown conductivity significantly increases relative to controls (Epstein 1975; Wei et
al., 1985; and Wong and Ho, 1991). However, differences tend to vary with application rates,
time, and other environmental factors. In a short-term laboratory study Wei et al.(1985) found
that low annual dewatered biosolids application rates of 11.2 and 22.4 Mg ha-1 did not
significantly affect conductivity of saturated soil cores of a silty clay loam, while higher
applications of 44.8 and 112 Mg ha-1 did. Epstein’s (1975) incubation study of three types of
biosolids (one raw and two digested) indicated that after 27 days all biosolids treated soils
showed significant increases in hydraulic conductivity. By 54 days, conductivity had decreased in
the raw biosolids-amended soils, but the digested biosolids-soil mixtures remained high. After 79
days, all treatments showed no significant differences in conductivity. Logan and Harrison (1995)
examined the effect of an alkaline stabilized form of biosolids (N-Viro soil) upon the saturated
hydraulic conductivity of a silt loam. A five-fold difference was seen between controls (0.0008
cm s-1) and the 500 Mg ha-1 treatment (0.0041 cm s-1). Even greater increases were seen in a
study by Wong and Ho (1991). They amended a fine bauxite refining residue (red mud) with
38.5, 77, and 177 Mg ha-1 of municipal biosolids and found that there was an increase in
conductivity of up to 20 times that of untreated plots.
MECHANICAL PROPERTIES
The influence of residuals on engineering aspects of soils is highly prevalent due to the
compositional make-up of the organic matter present in these materials. The change in the
mechanical properties of soils tends to be dependent on the type and application rate of the
materials used. No significant differences in liquid limit, plastic limit, and plasticity index were
seen when pig slurry and cattle slurry were applied on four soils in Italy (Mgbagwe et al., 1991).
However, biosolids applied to a sandy loam significantly increased the liquid limit, plastic limit,
and plasticity index, especially at the 100 Mg ha-1 rate. At this rate, relative increases over the
control were 23, 24 and 21 percent, respectively. The addition of two types of sphagnum, a peat
muck, and composted biosolids showed increases in a compression index used to examine their
effectiveness as sports turf rootzone mixes (McCoy, 1992). While all residuals exhibited positive
results, none were statistically significant.
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NBMA DRAFT Summary - Soil Physical Properties
SOIL AGGREGATION AND AGGREGATE STABILITY
Soil aggregation is the grouping together of various soil particles (organic and/or mineral)
into various sized and shaped units; this is also frequently referred to as the soil structure. The
stability of aggregates can be looked at as the ability to withstand a given disruptive force or as
the strength of the cohesive forces holding the particles together. The two forces most frequently
considered in the literature as leading to aggregate disintegration are water and wind. Soil
aggregation is a complex process that is not entirely understood, and trying to isolate the
mechanisms resulting in the stability of soil aggregates is equally daunting. However, the benefits
of the presence of stable aggregates in a given soil are easily discernible. There exists
considerable evidence that the existence of stable aggregates in a soil positively influences
numerous other soil physical properties such as the infiltration rate, porosity and bulk density.
Additionally, increased aggregation and aggregate stability have been positively correlated with
root growth and negatively correlated with the rate of erosion due to both rain and wind. An
analysis of relevant literature leads to a clear conclusion that the addition of biosolids and other
organic amendments increases soil aggregation and aggregate stability, and this has been shown
to be particularly true in coarse textured soils. However, the length of the effects is often only
short-term.
There are numerous theories on how aggregation and the stabilization of aggregates occurs. It
seems clear from the literature that the roles of organic matter and microorganism activity are
paramount. According to Brady (1974), any action that leads to the development of lines of
weakness, shifts the particles back and forth and/or forces contact between individual particles
will encourage aggregation. Tisdale and Oades (1982) contended that aggregates are formed
primarily by three organic components: 1) transient organic materials--mainly polysaccharides;
this is organic material which is decomposed rapidly by microorganisms, 2) temporary organic
materials such as roots and fungal hyphae and 3) persistent organic materials such as aromatic
components associated with polyvalent metal cations and strongly sorbed polymers. Dengens et
al. (1996) suggested that increasing fungal hyphae length can have a positive effect on both the
wet- and dry-stability of soil aggregates and further suggested that the involvement of microbial
polysaccharides was minimal. However, Roldan et al. (1996) reported a significant correlation
between mycelium length and aggregate stability and suggested that this was likely due to the
existence of readily decomposable carbohydrates and its positive effect on stimulating fungal
growth.
Given the theoretical basis of soil aggregation, many researchers have explored the role that
organic residuals have in enhancing the aggregation of soils. The addition of biosolids and other
organic residuals increases the organic matter of soils and typically an increase in microorganism
activity follows. Fortún and Fortún (1996) concluded that the most favorable aggregation effect
(occurring after a 15-day contact period with MSW and a composted biosolids) may be due to the
formation of cation bridges of Fe and Al and to Ca as an agent flocculating particles. They also
found that, with regards to a composted biosolids, Fe and Al were the elements primarily
involved in aggregation processes because they are likely to interact with clay and result in stable
organo-mineral complexes. Guidi et al. (1983) reported that the water-stability of the aggregates
of two soils examined increased significantly following treatment with ethyl ether-soluble
fractions and ethyl alcohol-soluble fractions taken from an aerobic and an anaerobic biosolids.
5
NBMA DRAFT Summary - Soil Physical Properties
The authors believed the results were due to a modification of the “wettability” of the aggregate
surfaces caused by waxes, fats, oils and resins; they suggested that the low decomposition rates
of these fractions lead to an increase in the total organic matter and a resultant decrease in the
wettability of the aggregates following biosolids application. Kinsbursky et al. (1989) stated that
a biosolids-treatment (5 percent of soil [dry wt.]) resulted in significant increases in the waterstability of aggregates for four of five soils examined after 30 days. The authors concluded that
their work indicated that the addition of biosolids to soils possessing low organic C levels can
lead to significantly greater stabilization of the macroaggregate fraction (those aggregates >250
g) primarily by microbially synthesized carbohydrates (water-soluble carbohydrates) and
suggested that extracellular polysaccharides, probably of fungal origin, likely served as
cementing agents responsible for stability with mycelia entanglement playing a less important
role. Metzger et al. (1987) found that changes in the amount of water-stable aggregates after the
addition of biosolids proceeded through the following phases: 1) a phase of water-stable
aggregate formation lasting about 10 days, 2) a phase of decreasing water-stable aggregation
content leading to 3) a constant water-stable aggregate level. The authors submitted that fungal
activity was best correlated with structural stability in biosolids-amended soils, suggesting that
cementing by fungal carbohydrates and physical entanglement by mycelium may have acted as
binding mechanisms in forming water-stable aggregates when biosolids was applied. Pagliai et
al. (1981) reported that among the treatments they tested, an anaerobic biosolids produced the
most significant results on stabilization, resulting in aggregates with a higher level of waterstability and with a longer lasting stability. The authors theorized that this might be due to the
existence of greater levels of stable organic compounds such as lignin, cellulose, lipids and
humic-like material not modified during the anaerobic digestion.
Most researchers studying soil aggregation and aggregate stability following organic residual
applications have discovered a positive influence where one or both variables increase, and
typically the increases are at statistically significant levels. Sort and Alcañiz (1996) performed
experiments upon plots along a 28 degree slope to which either 200 or 400 Mg ha-1 biosolids (dry
weight) had been applied. Significant reductions in both sheet erosion rates and runoff rates were
found on treated plots compared to non-treated plots (both with and without vegetation). Also,
aggregate stability increased with biosolids dose. Additionally, the mean weight diameter was 3.5
to 7.5 times greater in biosolids-treated plots compared to the control plot (an increase in the
mean weight diameter represents an increase in the percentage of macroaggregates [those > 250
µm]). Jordahl and Karlen (1993) reported a significantly greater water-stability of soil aggregates
on an “alternatively” managed farm using a 5-year, 4-crop rotation and a fertilization regime of
animal manure and a municipal biosolids when compared to an adjacent farm that used a
“conventional” management scheme of a 2-year, 2-crop rotation plan and utilization of a
commercial, anhydrous ammonia fertilizer. Glauser et al. (1988) assessed the size distributions of
soil aggregates and the aggregate stability of agricultural soils that had received eight years of
anaerobically digested biosolids applications of 72 Mg ha-1 yr-1. The percentage of water-stable
aggregates in the treated soils was 85 percent compared to only 45 percent of the untreated soil’s
aggregates. Wei et al. (1985) applied a Zimpro®-processed biosolids (35 percent solids) at rates
of 0, 11, 22, 45 and 112 Mg ha-1 (dry wt. basis) and an annual 22 Mg ha-1 for 6 years (134 Mg ha1 total input) to a silty clay loam to study the long-term effect of biosolids applications.
Aggregate stability significantly increased following the 112 and 134 Mg ha-1 treatments by 9
and 12 percent, respectively. The authors suggested that annual, small applications of biosolids
6
NBMA DRAFT Summary - Soil Physical Properties
improved soil physical properties as well as single, large applications. To examine the influence
of increasingly greater application rates of biosolids on soil structure, Borchert (1983) performed
a field experiment by applying 130 m3 ha-1 yr-1, 400 m3 ha-1 yr-3 and 800 m3 ha-1 yr-3 over a sixyear period from 1973-1978 to four separate soil types. After five years, the biosolids
applications resulted in a highly significant increase in the percentage of aggregates within the 62 mm sized fraction at all application rates in the Spring 1977 sampling period for two of the
soils. The author concluded that the influence of biosolids on soil structure was insignificant for
most application rates but resulted in a positive influence on sandy soils. Epstein (1975)
performed a lab study exploring the effects upon various soil physical properties due to the
incorporation of biosolids into a silt loam subsoil at a rate of 5 percent (dry weight). A raw solids
typically resulted in the greatest percentage of stable aggregates during the first 118 days
compared to a digested biosolids of high pH and a digested biosolids of low pH. However, after
175 days, the percentage of stable aggregates for all biosolids treatments were statistically the
same but remained significantly greater than the control (a mean of 34 percent vs. 17 percent).
Numerous studies attempting to determine the type of organic material most suited to
enhancing soil aggregation and stability have been conducted. Martens and Frankenberger (1992)
found that barley straw, alfalfa, biosolids and poultry manure applied at 25 Mg ha-1 each resulted
in statistically significant increases in the percentage of stable aggregates compared to the
control. Among the treatments, the barley straw was found to significantly increase the aggregate
stability relative to all other treatments and the control. Pagliai et al. (1981) studied the effect on
soil porosity and aggregation following the application of biosolids and co-composts at rates
equivalent to 50 and 150 Mg ha-1 of manure (the calculations were based on the level of organic
carbon). The authors found that the stability of soil aggregates was significantly increased by an
aerobic biosolids, an anaerobic biosolids and a 40:60 percent co-compost (biosolids:organic
fraction of municipal solid wastes); the co-compost was found to be effective only at the 150 Mg
ha-1 rate. The anaerobic biosolids tended to produce the most significant results. Guidi and
Poggio (1987) reported similar findings while conducting similar research. Likewise, Pagliai and
Antisari (1992) performed studies of similar materials at the lower application rate and found
very similar results. Most importantly, the authors reported that all organic materials applied
significantly increased the total number of micropores and particularly enhanced the elongated
pores (elongated micropores are thought to suggest the existence of soil microaggregates
responsible for allowing the growth of root hairs). Among the treatments, the manure and the
20:80 percent co-compost significantly outperformed the other three treatments in producing
micropores. Roldan et al. (1996) also examined the effects upon aggregate stability following the
application of biosolids, uncomposted MSW, composted MSW, and horse manure and found that
uncomposted MSW was the most effective treatment resulting in stable aggregates. Both
application rates (0.5 percent and 2 percent total carbon content) of uncomposted MSW resulted
in a significantly higher level of stable aggregates compared to all other treatments and rates,
while the 0.5 percent rate was statistically higher than the 2 percent rate. Additionally, the 2
percent application rate of biosolids and composted MSW resulted in an aggregate stability that
was statistically greater than the control, the manure at both rates, and the composted MSW at
the lower rate. Webber (1978) studied the effect on soil physical properties following the
application of numerous organic treatments. The treatments examined were the following: 1) 2.3
cm liquid biosolids, 2) 188 Mg ha-1 shredded MSW, 3) 188 Mg ha-1 + 2.3 cm liquid biosolids, 4)
188 Mg ha-1 MSW + 1.4 cm liquid poultry manure and 5) 376 Mg ha-1 MSW + 4.6 cm liquid
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NBMA DRAFT Summary - Soil Physical Properties
biosolids. Treatments 4 and 5 resulted in a significantly higher percentage of water-stable
aggregates (72 and 70 percent, respectively) compared to the control and the other three
treatments.
Two studies (Hulugalle, 1996; Furrer and Staugger, 1983) indicated no significant effects
upon soil aggregation or aggregate stability following the application of biosolids, however, in
both cases the authors theorized that the results found were due to the type of management prior
to the initiation of the experiments. Hulugalle (1996) examined the effect of applying pelletized
mixtures with ratios of 1:1 biosolids:peat or 2:2:1 biosolids:peat:cotton gin residue upon the soil
physical and chemical properties of an Australian clay. The results indicated no statistically
significant effects upon any of the soil physical properties analyzed following the application of
the pellets at any application rate (0-250 g kg-1 soil). The author suggested that the lack of
response of the soil physical properties may have been partially due to the fact that during the
previous 10 years all crop residues were retained, and the soil was under a no-till agricultural
scheme during the previous one year. Thus, the effect of the addition of organic matter contained
within the biosolids that may have enhanced the development of aggregates would have been
minimized. The work of Furrer and Staugger (1983) reported findings of a study in which 5 Mg
ha-1 yr-1 organic matter provided by biosolids were applied for five years to field plots. The
authors found no significant differences in aggregate stability among samples of a clayey soil
amended with a mineral fertilizer and those amended with biosolids, though both had
significantly higher aggregate stability than did the control. They submitted that on this type of
soil, the existence of crop growth was the most important factor in developing aggregate stability
and found that grass had an especially strong influence on aggregate stability with the most stable
aggregates being found on plots with a permanent grass-clover-mixture. Additionally, in a further
lysimeter experiment where biosolids at rates of 4, 8 and 12 Mg ha-1 yr-1 dry matter were applied
to a sandy loam for five years, there was a significant increase in soil aggregate stability
following the biosolids applications. The latter results indicate the enhanced effect of biosolids
on coarser soils than upon fine-grained soils.
There exists some evidence that liming biosolids may also contribute positively to
aggregation. Logan and Harrison (1995) studied the impact of the N-Viro process (an alkaline
stabilization of biosolids) upon various soil physical properties of the resulting “N-Viro Soil” and
compared the properties to those of four mineral soils. N-Viro Soils were found to possess a
greater number of larger aggregates and fewer smaller aggregates than the mineral soils (this
indicates a higher aggregate stability of the N-Viro Soils) as well as a greater overall aggregation
percentage. An additional experiment was performed by the authors where N-Viro Soil was
incorporated into a degraded soil at a rate of 500 Mg ha-1 (dry solids) and analyzed one year after
incorporation. The aggregate stability percentage increased at a highly significant rate in all
aggregate size fractions >0.5 mm; 88 percent of the treated soil possessed water stable
aggregates, compared to only 47 percent for the control soil. Another study examining the
influence of limed biosolids upon soil physical properties was undertaken by Morel and Guckert
(1983). In laboratory and long-term field experiments, the authors studied stable aggregate
development in soil amended with an aerobic biosolids and an anaerobic limed biosolids applied
at similar rates based upon their respective organic carbon content. The limed anaerobic biosolids
resulted in a significantly higher percentage of water-stable aggregates relative to the aerobic,
non-limed biosolids. Moreover, limed biosolids applied during six years at a total rate of 200 Mg
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NBMA DRAFT Summary - Soil Physical Properties
ha-1 dry matter resulted in significantly greater percentage of water-stable aggregates than limed
biosolids applied during six years at a total level of 100 Mg ha-1 dry matter. According to the
authors, this latter result suggested that increasing levels of limed biosolids increased the soil
stability and decreased the soil wettability due to an enhanced liming effect and polysaccharide
production and possibly an increasing level of soil and grease carbon.
SIGNIFICANCE OF RESIDUALS EFFECTS ON SOIL PHYSICAL PROPERTIES
Research to date indicates that residuals as a whole provide positive changes to soils physical
status by increasing the organic matter content, enhancing water holding capacity and porosity,
decreasing bulk density, supporting aggregation, and increasing hydraulic conductivity (Gupta et
al., 1977; Mays et al., 1973; Unger and Stewart, 1974; Weil and Kroontje, 1979). While high
application rates have resulted in dramatic changes in physical properties, the risk of
environmental degradation and toxicity is possible (Guidi, 1981). However, current operational
regulations prevent this risk by assuring suitable application rates. Lower application rates have
proven to sustain positive changes over longer time periods and even with one time small
applications (Pagliai et al., 1981 and Khaleel et al., 1981). When managed properly, residuals
provide beneficial changes to the soils physical properties and can result in heightened soil
productivity.
Reference: Henry, C., and R. Harrison. 1998. Environmental Effects of Biosolids Management.
Trace Metals: Potential for Movement and Toxicity from Biosolids Application, Effects on
Wildlife and Domestic Animals from Biosolids Application, Air Emissions and Ash
Resulting from Incineration of Biosolids, Nitrogen Cycle and Nitrate Leaching from
Biosolids Application, Microbial Activity, Survival and Transport in Soils Amended with
Biosolids, The Fate of Trace Synthetic Organics in Biosolids Applied to Soil, Runoff Water
Quality from Biosolids Application, Effects of Organic Residuals on Poplars. Northwest
Biosolids Management Association.
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