Note: Descriptions are shown in the official language in which they were submitted.
CA 02551943 2007-06-13
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TITLE OF THE INVENTION: A metnod tor ameliorating sodicity in soil.
BACKGROUND OF THE INVENTION:
Field of invention:
This invention relates to a method for improving soil adversely affected by
sodium, by means of adding a chelating agent to said soil to solubilize
calcium and at the same time restrict the resulting increase in soil salinity.
Prior art:
It is well known that soils containing excessive sodium can thereby be made
unsuitable for crop production, landscaping or engineering purposes, due to
two separate adverse properties:
(a) salinity, often expressed in terms of the soil's electrical conductivity
(EC), and
(b) sodicity, often expressed in terms of the soil's sodium adsorption ratio
(SAR).
A significant percentage of the world's agricultural land base is adversely
affected by salinity and sodicity, due to either naturally occurring sodium
contamination or man-made contamination arising from activities such as
mining, petroleum extraction, dam construction and irrigation.
Effects of salinity and sodici on plant growth and soil quality:
(a) Salinity (high EC) directly affects plant growth by hindering or even
preventing root uptake of water which must occur against an osmotic
pressure gradient. The greater the concentration of dissociated, ionized
salts in a soil's pore water, the greater the water's charge-carrying
capacity and hence the higher the soil's EC.
The EC is often measured in soil extracts, derived by filtering a paste of
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water-saturated soil (a "saturated paste"). It is expressed in units such as
deciSiemens per meter (dS/m). Below EC = 2 dS/m, soils are considered
non-saline and few plant species are affected, but at salinity levels above
12 dS/m most plant species cannot grow.
(b) Sodicity (high SAR) can cause soil plasticity, leading to difficulties in
soil cultivation and to slow rates of water infiltration and drainage.
These effects occur with sodic soils containing much clay, and in soils
with naturally-occurring sodic subsoils such as solonetzic soil. SAR
values in saturated paste extracts of non-sodic soils are usually less than
1 SAR unit. Sodicity problems typically arise when SAR values exceed
6-10 units, depending on clay content. The SAR is a measure of the
influence of sodium Na ions (positively charged cations) in the pore
water, relative to that of calcium Ca and magnesium Mg cations.
The SAR value is calculated using the equation:
SAR = [Na+] / sqrt { [Ca2+] + [Mg a+] } . . . (1)
where [Na+] etc. are cation concentrations in a filtrate of a saturated soil
paste. Na cations are monovalent (carrying a single positive charge)
whereas Ca and Mg cations are divalent (two charges). In naturally
occurring sodic soils, the SAR is correlated with the percentage of cation
exchange sites, on clay and organic matter, occupied by sodium cations.
As a result of these adverse effects, environmental guidelines are in place in
various jurisdictions, regulating permitted levels of EC and SAR in soil and
subsoil. For example, the two parameters are regulated in current guidelines
for drilling waste disposal on soil in western Canada (e.g. AEUB, 1996).
Remedial treatment of sodium-affected soils:
Traditional ameliorants are salts such as calcium sulfate (gypsum), calcium
nitrate, calcium chloride, and magnesium sulfate (Epsom salts) which
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dissolve in soil pore water to yield Ca or Mg cations that are dissociated
(widely separated) in solution from attendant negatively charged anions.
Alternatively, acids have been applied such as sulfuric acid, which reacts
with calcium or magnesium carbonate (forms of lime present in alkaline
soil) to release Ca or Mg cations in situ, again dissociated from the
negatively charged anion (sulfate).
Such approaches were developed many years ago (Richards, 1954) and
remain in widespread use (e.g. Naidu et al. 1993; Ashworth et al., 1999).
Said ameliorants are applied in order to increase the concentration of Ca or
Mg cations in the soil's pore water, thus lowering the soil's sodicity (the
SAR value, obtained using Eqn. 1 above).
Unfortunately as an unavoidable and inconvenient side effect, the salinity
(EC) of the treated soil increases, due to the charged Ca or Mg cations (and
related anions) either added or produced in situ, thus possibly damaging
plant growth as well as potentially exceeding EC guideline thresholds.
Chelating A eg nts:
Chelating agents (also known as complexing or sequestering agents) form
stable complexes with many cations, in which the cation is enveloped by the
molecular structure of the chelating agent, which serves as the anion.
Such agents have been used to deliver to plant roots micronutrients such as
copper, manganese and iron, which otherwise can precipitate out or be
adsorbed to soil and thus made unavailable to plants (e.g. Allison and
Hewitt, U.S. # 2,813,014). Chelated iron has been used as an ameliorant for
acidic soil (Sasaki and Mitsunaga, J.P. # 89,197,591).
Chelating agents have also been used as sequestering agents to remove toxic
metals such as lead and cadmium from contaminated soil (e.g. Redwine et
al., US patent # 6,210,078). They have also been used to sequester pollutants
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in soil and render them harmless through reaction with a colloidal matrix
(Newton, E.P. # 756,904).
Chelating agents were used in pot experiments by Panov et al. (Dokl. Vses.
Akad. S-kh. Nauk im. V.I. Lenina, 12: 2-4, 1982) to improve plant uptake of
micronutrients. An abstract of their work (Chemical Abstracts 99: 174785u,
1983) refers to increased solubility of Ca compounds and improved quality
of irrigation water.
Chelating agents have been added to fluids used for well-drilling in the
petroleum industry so as to bring insoluble barium compounds into solution,
as well as for affecting the properties of bentonite clay used in said fluids.
Spent fluids may afterwards be disposed of to soil, but the resulting presence
of chelating agents in the fluid-treated soil is incidental and not
intentionally
for the purpose of affecting the properties of said soil.
Chelating agents have been used to treat soils for well over half a century
(Chaberek & Martell 1959), and the electrically neutral nature of chelated
cations has also long been known (Ayres 1968). However, despite the length
of this period of related knowledge, the applicant has not uncovered any
reference to the objects of (i) amelioration of soil SAR, in conjunction with
(ii) limiting soil EC increase.
This combination of low SAR along with low EC (improving sodicity as
well as curbing salinity increase) has been very difficult to achieve in much
previous work (e.g. Ashworth & Webster 2004) using traditional ameliorants
that contain, or generate in situ, soluble Ca or Mg.
On the basis of art cited above, and other published work on chelating agents
considered but not cited, in the opinion of the applicant the possibility does
not obviously follow that said agents can be used to improve soil quality by
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releasing Ca and Mg cations to counter sodicity, while at the same time
largely avoiding undesirable increase in salinity.
Objects and advantages:
The object of the invention is to use chelating agents to release essentially
insoluble Ca and Mg from sodic soil, thereby ameliorating problems caused
by high sodicity (SAR). A concomitant object and advantage of the
invention is that release of Ca and Mg cations in chelated form leads to
much smaller increases in soil salinity (EC) than traditional methods which
instead involve adding amendments that generate dissociated Ca or Mg
cations.
Further objects and advantages of the invention will become apparent from
consideration of the following findings:
(a) Adding calcium chloride to a loam soil increased soluble Ca in the
filtrate from a saturated paste by 900 milligrams per liter (mg/L) causing
filtrate EC to increase by more than 5 dS/m; whereas, adding the chelating
agent ethylenediamine tetra-acetic acid (EDTA) released 1,000 mg/L of Ca
causing an EC increase of less than 2 dS/m.
(b) A naturally sodic soil, originally yielding a saturated paste filtrate
with
an SAR value of 11.0 and an EC of 0.7 dS/m, had an SAR value of 4.7 and
an EC of 1.5 dS/m after treatment with the chelating agent citric acid at the
rate of 2 g acid per L of soil.
(c) A saturated paste of the above mixture of sodic soil and citric acid when
filtered under vacuum yielded approximately 1 mL of filtrate per minute,
CA 02551943 2007-06-13
whereas a saturated paste of said sodic soil untreated with citric acid
yielded
less than 0.1 mL per minute. This order of improvement in filtration rate of
saturated pastes was typical of all soils following treatment with a chelating
agent.
(d) Chelating agents were applied at the rate of 1 kg per square meter to the
surface of small plots on a sodic area (SAR = 8 approx.) in a cultivated field
at the Parkland Conservation Farm (www.parklandconservationfarm.com)
near Mundare, Alberta in Oct. 2005, then left over winter. Replicated water
infiltration tests done in May 2006 indicated that untreated soil was in the
"slow" class with a mean water infiltration rate of 0.25 cm/h, statistically
significantly lower than the rate for adjacent plots given a chelating agent
in
the form of malic acid (0.95 cm/h, "moderately slow") or EDTA (4.91 cm/h,
"moderate"). Improved water infiltration can be expected to improve crop
yield.
(e) A mixture of sodic waste and soil (with SAR = 7.5 and EC = 0.8 dS/m)
when treated with EDTA at the rate of 0.5 g per L of waste had an SAR =
3.7 and EC = 1.7 dS/m. Treatment with sulfuric acid at an equivalent rate
was less effective in terms of both improving sodicity and curbing salinity,
resulting in an SAR = 5.1 and EC = 2.7 dS/m.
(f) In a pilot-scale test in June 2006 on a drilling lease near Edson, Alberta
24 kg of the chelating agent EDTA was mixed by backhoe into a previously
made blend of approximately five cubic meters of lease subsoil (SAR = 0.2)
with one cubic meter of a sodic, alkaline drilling waste material (SAR = 19
and pH = 10). Several hundred cubic meters of this waste was generated as a
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result of drilling for natural gas at the site. The EDTA-treated 5:1 blend had
an SAR of 1.5 whereas a control, untreated blend had an SAR of 2.6 which,
though not very high, nevertheless exceeded the current provincial guideline
for increase in SAR following drilling waste disposal.
(g) In a parallel pilot-scale test, the same quantity of EDTA was mixed by
backhoe with one cubic meter of the same, raw drilling waste. An hour was
deliberately allowed to elapse before blending the treated waste with 5 cubic
meters of lease subsoil. The result obtained (SAR = 1.2) on analyzing the
blend thus treated indicated that EDTA had not been inactivated by the one
hour of contact with the alkaline waste material.
(h) A mixture of sodic waste and soil, treated with EDTA at the rate of 30 g
per L of waste, was found to be completely non-toxic using the MicrotoxTM
bioassay, the standard bioassay test in drilling waste disposal in Alberta,
Canada (AEUB 1996). This finding is consistent with literature on toxicity
of EDTA, e.g. http://ptcl.chem.ox.ac.uk/-hmc/hsci/chemicals/EDTA.html
(i) EDTA was added to 2 L portions of alkaline drilling cuttings at the rate
of
8 g/L. One portion of EDTA-treated cuttings received no further treatment,
while a replicate portion was subjected to a "hot-roll" test at 80 degrees
Celsius for 16 h, to simulate downhole drilling conditions. Sub-samples (75
mL) of untreated cuttings and of the regular and the hot-rolled EDTA-treated
cuttings were blended with 150 mL of silty soil (SAR = 0.5). A saturated
paste of each blend was then made and the filtrate analyzed for SAR and
residual EDTA. The blend of soil and untreated cuttings had an SAR = 4.8,
whereas the SAR was 2.4 using regular EDTA-treated fluid and 2.2 with
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hot-rolled EDTA-treated fluid. The latter two filtrates both had 300 mg/L
approximately of EDTA.
The above results indicate that EDTA was not degraded during the hot-roll
process, suggesting that it could be used as an additive in drilling fluids to
reduce the sodicity of the drilling waste thus produced. This pre-treatment
with EDTA might avoid problems with SAR during waste disposal.
(j) To test the stability of mixtures of sodic soil and chelating agents,
citric
acid (1.5 g) or EDTA (1.5 g) was added to 450 g portions of a moist blend of
a sodic drilling waste and a silty soil, having SAR = 6.0 and EC 1.7 dS/m.
(The rate of addition corresponded to 20 kg of chelating agent per cubic
meter of drilling waste.) The mixtures were stored at room temperature in
glass jars, keeping the screw-cap one quarter turn loose. De-ionized water
was added from time to time to maintain the required weight and moisture
content. Sub-samples (50 g) were removed at intervals and saturated pastes
made, filtered and analyzed.
The mixture treated with citric acid had an SAR = 4.3 and EC = 2.3 dS/m
initially but after 10 days' storage had SAR = 5.3 and EC = 1.8 dS/m, then
after 2 months was indistinguishable from the initial, untreated material with
SAR = 6.0 and EC 1.7 dS/m, as already stated.
In contrast, the mix given EDTA had an initial SAR of 2.5 and EC of 3.2
dS/m, values that were maintained in five subsequent samplings within
experimental uncertainty; and which had still not changed at the time of
submitting this application, after more than ten months' storage.
Since the rate of a biological process like degradation typically doubles with
a 10 degree Celsius rise in temperature, the result suggests that the SAR and
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EC of EDTA-treated material would not alter for many years in cold subsoil.
Such long-term stability is often mentioned in the literature on EDTA.
(k) A loamy sand soil containing no measurable natural lime was mixed with
sodic waste material, giving a mixture with SAR = 27.2 and EC = 2.8 dS/m.
After adding EDTA at the rate of 12 g per L of waste material, the SAR fell
to 6.2 and EC was 3.0 dS/m. In a companion test, adding calcium carbonate
to the soil-waste mixture as well as EDTA did not affect results, indicating
that additional Ca was unnecessary even in the absence of lime in the soil.
Lime in sodic waste materials may provide sufficient Ca in such cases.
(1) The rate of application of a chelating agent that will produce a required
SAR result can be estimated by assuming that Ca is released from a soil in
an amount proportional to the rate of agent applied. For example, EDTA and
Ca form a 1:1 stoichiometric complex such that 292 g of EDTA (one mole)
can release 40 g of calcium when added to soil. Some magnesium is also
released, whose amount can be estimated reasonably well by assuming that
the ratio of Mg to Ca in a saturated paste of the untreated soil would be
maintained. Fair agreement was observed between actual SAR values and
those estimated as outlined above; the agreement improved when empirical
allowance was made for displacement of exchangeable Na by the extra Ca
and Mg.
(m) Preferably, bench-scale tests can be conducted, so as to arrive at
suitable
rates of chelating agent application that would cause a desirable SAR result
while ensuring that EC is kept at a desirable, low level. Such tests can also
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be used to determine whether supplementing the chelating agent with a Ca
or Mg compound would be beneficial.
SUMMMARY:
In accordance with the present invention, a method for improving soils or
soil-waste mixtures adversely affected by sodium consists of blending them
with a chelating agent, at a rate chosen to release sufficient calcium and
magnesium in chelated form so as to reduce sodicity to a desirable level,
while at the same time thus curbing salinity increase.
DRAWINGS: Not applicable
DETAILED DESCRIPTION:
First Embodiment
One embodiment of the invention concerns the amelioration of cropped soils
whose high sodicity has been found to hamper cultivation, water infiltration
and drainage, with resulting adverse effects on crop yield.
Operation - first embodiment
A soluble chelating agent such as malic acid can be added as an aqueous
solution, said solution being applied to the surface by existing techniques
known in the art, such as infiltration and irrigation. These methods would be
suited to treating large areas or whole fields.
Alternatively, for fields where high sodicity was present only in relatively
small patches, said agent could be applied in solid form to the surface of an
affected area then washed into the soil, either by natural precipitation or by
CA 02551943 2007-06-13
applying irrigation water. This patchwork method of application would of
course be more economical than treating an entire field.
The objective in either case would be a gradual improvement of ease of soil
cultivation and rate of water infiltration into the soil, as a result of
several
applications of the chelating agent, with the additional object of improving
crop yield.
Naturally-occurring multidentate carboxylic acid chelating agents are suited
to this embodiment of the invention because of their tendency to biodegrade
in soil. Thus, there would be no residue of such agents within a few weeks or
months of treatment. This lack of persistence will be an advantage in cases
where the intention is to improve the rooting zone of a crop.
Second embodiment
Another embodiment of the invention concerns the treatment of sodic
material such as drilling waste or other by-product which must be disposed
of, for example, either to a landfill or by application to local soil, and
which
in accordance with published guidelines has to meet certain SAR and EC
thresholds in order to be so disposed of.
Operation - second embodiment
This use of the invention concerns treatment of sodic by-product materials
before their disposal for example by means of mixing and encapsulation in
subsoil (currently permitted under disposal guidelines in western Canada).
In such cases a chelating agent such as EDTA, with a persistent effect on the
SAR of the treated material, would be preferred in order to provide a long
term reduction in SAR after disposal.
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Application of a sparingly soluble powder like EDTA is more conveniently
done by applying it to the waste material before blending said waste with a
receiving soil. It is advantageous to add the powder in this way rather than
apply it to the receiving soil first, or to a mixture of the waste and soil,
since
drilling by-products often have a slurry-like texture which becomes dry on
being mixed with soil, thus making blending with a powder more difficult.
FIGURES Not applicable
Conclusion, ramifications and scope
While the above descriptions contain certain specificities, they should be
construed as examples of the preferred embodiments of the invention rather
than limitations on its scope. Other ramifications and variations are possible
within the teachings of the invention.
For example, a chelating agent could be applied either in solid form,
aqueous solution, or as a slurry so as to aid admixture, depending on the
properties of said agent especially its water solubility.
Other plant-growing media such as composts or garden soil could be
conveniently treated in batches with a chelating agent so as to combat any
sodicity.
Sodic subsoil inaccessible from the ground surface could be treated with a
chelating agent via tubes, pipes, drilled holes, trenches or the like leading
into the subsoil.
Chelating agents could be used either individually or in combination with
another such agent, either natural or synthetic, or together with a
traditional
ameliorant salt containing calcium or magnesium.
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Accordingly, the scope of the invention should be determined by the
following claims and their legal equivalents, rather than by the examples
given above.
Literature References
AEUB 1996. Drilling Waste Management. Guide G 50. Alberta Energy and
Utilities Board, Calgary, Alberta, Canada.
Ashworth, J. et al. 1999. A comparison of methods for gypsum requirement
of brine-contaminated soils. Canadian Journal of Soil Science 79: 449-455.
Ashworth, J. and Webster, J. 2004. The gypsum requirement of drilling
wastes applied to soil. Proceedings of the Alberta Soil Science Workshop
41: 61-67.
Ayres, G.H. 1968. Quantitative Chemical Analysis. 2"d Edition. Harper and
Row, New York.
Chaberek, S. and Martell, A.E. 1959. "Organic sequestering agents."
Wiley & Sons, New York.
Naidu, R. et al. 1993. Sodicity in South Australia - a review. Australian
Journal of Soil Research 31: 911-929.
Richards, L.A. 1954. (editor) Handbook 60 of the U.S. Dept of Agriculture:
Diagnosis and Improvement of Saline and Alkali Soils.
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