Clay mineral dissolution following intensive cultivation in a tropical sandy soil

Dur, J.C. ^{1, 2}; W. Wiriyakitnateekul ³; G. Lesturgez ^{2, 4}; F. Elsass ^{1, 5}; M. Pernes ²;
C. Hartmann ⁴ and D. Tessier ²

Keyword: clay mineral, acidification, analytical transmission electron microscopy, neoformation smetites

Introduction

Sandy soils are widespread in the tropics and constitute an important economic resource for agriculture despite their inherent low fertility (FAO, 1975). Such soils occupy a large area of the Northeast Thailand plateau (Ragland and Boonpuckdee, 1987). These soils are often characterised as being of a light sandy texture, acidic to depth (pH around 4.0 in CaCl₂) with very low exchange properties (CEC <2 cmo_cl kg^-1) and therefore a low nutrient supplying capacity (Imsamut and Boonsompoppan, 1999). Soil acid­ification is a major concern in tropical sandy soils since climate and high leaching rates are prevailing conditions for the development of this phenomenon. Intensive cultivation results often in accelerated acidification which has brought into question the long-term sustainability of agronomic production systems (Helyar, 1976).

In Northeast Thailand, degradation of acid sandy soils by further acidification under permanent agriculture has been clearly highlighted (Noble et al., 2000). However accelerated acidification occurs sometime without pH drop in these acid sandy soils (Lesturgez et al., 2005). Regarding the low organic matter content and the absence of other weatherable minerals, the authors suggested the role of clay minerals in buffering the soil pH.

In order to ascertain the role of clay minerals in buffering capacity, a mineralogical study has been conducted in Northeast Thailand with the objective to identifying changes in nature and properties of clay minerals possibly induced by 50 years of intensive cultivation. The study has been conducted on a forest soil, an adjacent cultivated area and the parental material of these soils in order to highlight (i) general clay properties of the soils, (ii) evolution following pedogenesis and (iii) changes induced by 50years of intensive cultivation.

Materials and methods

Site and sampling

The study was conducted on a representative upland sandy soil of Northeast Thailand. The site selection was based on previous studies (Lesturgez, 2005) and a paired site approach was conducted to quantify differences between Dipterocarp forest (undeveloped) and agricultural (developed) areas. The soil at the studied sites belongs to Warin soil series (N 16º16′; E 102º47′): Dipterocarp forest and adjacent cultivated land that was deforested 50years ago and used for cassava and sugarcane cultivation to date. The same soil type was observed in both areas with little topographical differences (i.e. slope) between these two areas. The undisturbed Dipterocarp forest, close to agricultural field, had a well-defined boundary to separate the two land use systems. Selected characteristics of the soil at the studied sites are presented in Table1. Samples were collected at 0-10cm depth both in the soil under forest (FS) and in adjacent cultivated area (CS) and at 3.5 m depth for parent material (PM). Samples were air-dried and passed through 2mm sieve.

X-ray diffraction (XRD)

40g sub-samples of air -dried material were dispersed in water overnight by shaking. Clay fractions (<2 µm) were obtained by sedimentation from the initial samples after organic matter oxidation (H₂O₂-treated clay). Mineralogical analysis was performed by X-ray diffraction on oriented samples obtained by sedimentation of the <2 µm fractions on glass slides and air-dried. Samples were run in a Siemens D5000 system with CoKα radiation. The diffraction patterns were decomposed into elementary curves using the program DECOMPXR (Lanson, 1997) in order to quantify the proportion of different clay minerals. The decomposition was done on the 1^st order diffraction peaks for kaolinite, illite and smectite (or vermiculite).

Table 1. Selected properties of the studied soils

Laser granulometry (LG)

Particle size distributions (PSD) were measured on <2 µm fractions dispersed in distilled water (ultrasound ~15 min). The quantity of material was adjusted in order to obtain adequate particle concentration (obscuration = 45%). PSD was produced using a LS-230 Beckman-Coulter laser grain-size analyser, with a range of particle size from 0.04 to 2000 µm, divided into 116 fractions. PSD was performed under agitation of clay suspension in the measuring cell. Three repeated measurements were undertaken in each sample.

Analytical transmission electron microscopy (TEM-EDS)

Dispersed clay suspensions were deposited on collodion-coated Cu grids and air-dried. Images were produced in a Philips 420 STEM transmission electron microscope operated at 120kV and equipped with a Megaview II CDD camera. Magnification at 10500x covered the particle-size ranged from 0.5-2.0 µm and magnification at 31000x ranged from 0.05 µm-1.00 µm.

Microanalyses were performed using an Oxford INCA energy dispersive spectrometer (EDS) with an ultrathin windowed Si (Li) detector connected to the microscope. In order to limit irradiation damages, analyses were done using a probe spot 100nm in diameter (Romero et al., 1992). The chemical compositions (Al, Si, K, Ca, Mg, Fe, Na and Ti) were determined on individual particles selected regarding dispersion. A minimum of 100 particles was analysed for each sample in order to get a representative set of data. Minerals were identified by their chemical compositions, on the basis of calculated structural formula expressed in number of constituting cations. Quantitative discriminations were used to sort particle analyses into mineralogical classes (Figure1).

Figure 1. Procedure for identifying clay minerals using analytical data from TEM-EDS. Numbers of atoms in structural formulae are calculated on the basis of 11 and 7 oxygen atoms per half cell for 2:1 and 1:1 clay minerals, respectively. Si, Al, Fe, K, Ca: number of atoms in the calculated structural formula. IC: Interlayer Charge Value (Ca²⁺ + Na²⁺⁺ K⁺) of the particle

Figure 2. X-ray diffractograms (Co kα) of clay fractions (<2 µm) for cultivated soil, forest soil, and parent material

Results

Mineralogical determination of clay fraction by XRD    

XRD patterns indicated the presence of quartz (4.3 and 3.3Å), kaolinite (7.3 and 3.6 Å) and 2:1 clay minerals (illite at 10.0Å and smectite at 14.3Å) in all three samples (Figure2). The intensity of quartz peaks (4.3 and 3.3Å) was higher in soils (CS and FS) than in parent material (PM). According to peak intensity, kaolinite was the main crystalline phase with 78%, 88% and 88% in CS, FS and PM, respectively (Table2). On the other hand, the proportion of smectite was only of 20%, 6% and 12% in CS, FS and PM respectively. The diffraction intensity of illite was significant only in FS (6%). As the peaks of kaolinite were very large compared to a reference kaolinite (Brindley and Brown, 1980), they have been decomposed into three elementary peaks modelling crystallinity, each peak position corresponded to a structural order: highly disordered (7.37Å), slightly disordered (7.22Å) and well ordered (7.15Å). The proportion of the three types of peaks within the kaolinite fraction has been recalculated and expressed in percentage of total peak surface (Table3). The proportion of highly disordered kaolinite was equivalent for CS and FS (40%) but lower in PM (34%). The slightly disordered kaolinite was correlatively more important in PM (66%) while the well ordered kaolinite was present only in CS (3%).

Table 2. Selected characteristics of the clay fractions (<2 µm) for parent material, forest and cultivated soil

Particle size distribution by laser granulometry (PSD)

Particle size distribution (PSD) was expressed in particle surface area (SA) as a function of the particle equivalent diameter. Average and standard deviation of the three replicates are presented in Figure3. Particle size distributions were bimodal shape in all samples. For PM, the main particle size mode was around 0.1 µm (0.04-0.30 µm) and covered 86% of total SA (Table2). PSD for CS and FS had two strong modes around 0.1 µm and 1.0 µm, thus appearing very different from PM. The SA of small particles (mode 0.1 µm) strongly decreased from PM to FS and CS. When comparing FS and CS, the SA of small particles decreased from FS to CS.

Figure 3. Particle size distributions of clay fractions (<2 µm) for cultivated soil, forest soil, and parent material by laser granulometry

Morphology and chemical composition of particles by TEM-EDS

Examples of TEM images are shown for FS (Figure4). The shape and size of particles were similar in CS and FS. The size of particles observed on TEM images allowed discrimination between quartz grains and clay minerals. Most of quartz grains were the large particles while clay minerals were much smaller (Figure4-A). A significant proportion of small particles was organised into aggregates of 0.5-2.0 µm (Figure4-B). Dispersed kaolinite particles appeared to be eroded both in CS and FS and constituted the majority of PM ranging between 40 and 120nm. A great diversity of particle shapes was observed in Figure4-C.

Analytical transmission electron microscopy (TEM-EDS) revealed that kaolinite contained iron. The 2:1 clay minerals were smectite and mixed-layer illite/smectite in all samples, vermiculite was in FS and illite was only in CS. Smectite was associated with kaolinite in aggregates. Particles of titanium and iron were also detected. The main 2:1 clay mineral was smectite with 54, 29 and 10% in CS, FS and PM, respectively (Table2). Mixed-layer illite/smectite was low in proportion with 3, 14 and 5% in CS, FS and PM, respectively. Al-vermiculite developed mainly in FS (12%). The repartition of clay minerals was determined by EDS following the marked trends between samples, but their proportions were markedly different from XRD determinations.

Figure 4. TEM images of clay fractions (<2 µm) for cultivated soil, forest soil, and parent material at magnification ×10500 (A), ×31000 (B) and ×105000 (C)

Discussion

Methodological aspects

When comparing results obtained by XRD and EDS analyses, the discrepancy between quantitative data relates to two essential methodological aspects:

• XRD detected the diffraction ability of minerals, which depended on their crystallinity and chemical composition. It is worthy of note that kaolinite contains iron and the presence of this element strongly enhances the structure factor, i.e. the diffraction intensity. Furthermore, XRD technique is not suitable for poorly crystallised phases and not able to produce diffraction peaks for amorphous material or loose monolayers such as smectite, and even for loose bilayers (Dudek et al., 2002).
• EDS analyses were sorted using strict rules applied to calculated structural formulae, discarding from the kaolinite group, all particles containing Mg and having a high interlayer-charge (K and Ca). Such particles are thus identified as 2:1 clay minerals.

XRD detected well crystallised phases of clay minerals in parent material but it was underestimated for the fine neoformed phases as well as dissolution products in soils. The discrepancy between XRD and EDS is not a trivial issue, but suggests a transition from 1:1 to 2:1 clay minerals in a significant part of the clay fraction.

Mineralogical properties of soils in Northeast Thailand

The studied soils contained a very small quantity of clay minerals. The clay fraction consisted of a mixture of 1:1 (kaolinite) and 2:1 clay minerals (illite and smectite), and contained a significant proportion of small quartz grains as noted in other soils of this region (Bruand et al., 2004). Kaolinite showed crystal disorder (XRD) and appeared to be dissolved (TEM) in soils as well as in parent material. Some iron was detected in kaolinite as a lattice substitution for Al. XRD patterns and EDS analysis confirmed the presence of expandable 2:1 clay minerals especially smectite, but EDS also identified mixed-layer illite-smectite and Al-vermiculite in small proportion. Kaolinite appeared to be disordered in these soils as well as in parent material since the XRD peak around 7A was very lar ge.

Evolution through pedogenesis

To investigate in more detail changes in crystallinity between the different samples, the “kaolinite” XRD peaks have been decomposed. The proportion of highly disordered kaolinite is equivalent in CS and FS but more important in soils than in PM, and slightly disordered is correlatively more important in PM (Table3). Crystal disorder is probably related to kaolinite dissolution. This may be consistent with the hypothesis that kaolinite in parent material was poorly crystallised and this phenomenon has been accentuated by weathering through pedogenesis.

Table 3. Normalised proportions of disordered kaolinite species in kaolinite fraction

Particle size distributions of clay fractions showed the loss of finer particles (mode 0.1 µm) in soils when compared to those in PM. The value corresponding to the loss was 50% for CS and 37% for FS. On the other hand, EDS result indicated a higher proportion of kaolinite in PM (82%) than in the soils (34% in CS and 42% in FS). TEM images showed that the fine particles (mode 0.1 µm) corresponded to fine grained kaolinite. These results strongly suggested that a disappearance (completely dissolved) of the small particles of kaolinite may be due to their dissolution when buffering acid addition and/or their leaching to depth. On the other hand, EDS detected a much higher proportion of 2:1 clay minerals in soils than in parent material, suggesting that the clay neoformation may possibly occur on the basis of degraded kaolinite.

Evolution of mineralogy due to intensive cultivation

Particle size distributions of <2 µm fractions revealed a decrease of finer particles (mode 0.1 µm) in CS compared to FS. On the other hand, EDS analysis revealed a discrepancy in the content of mixed-layer illite/smectite and Al-vermiculite between CS (4%) and FS (26%) and a correlative discrepancy in the content of smectite between CS (54%) and FS (29%). These results pointed out the difference between these two soils: kaolinite dissolution was associated with a neoformation of smectite which appeared to be accentuated in CS.

Conclusion

The aim of this study was to evaluate the evolution of clay minerals, especially kaolinite and 2:1 clay minerals, in acid sandy soils of Northeast Thailand subjected to intensive cultivation. Our results have shown that the main clay mineral is kaolinite. Quartz is also present in all clay fractions but its grain size is coarser compared to clay minerals (TEM images). The presence of expandable 2:1 clay minerals has been highlighted, especially smectite as well as mixed-layer illite/smectite and vermiculite, though in smaller proportions. With respect to kaolinite, the great diversity of morphological shapes, very small particle size and the presence of iron in crystal structure were consistent with a dissolution process due to soil acidification. Kaolinite is thus characterised by fragility under acidic conditions.

Results also indicate, by using three different analytical methods (especially TEM-EDS analysis), that fine kaolinite particles become scarcer with increasing pedogenesis whereas 2:1 clay minerals tend to be neoformed. The first phenomenon may be related to dissolution of fine grained kaolinite particles and/or their leaching to depth. The second phenomenon can be linked to the properties of 2:1 clay minerals which are less readily eluviated.

The neoformation of smectite containing Ca as exchangeable cation appeared to dominate in cultivated soil, whereas in forest soil it appeared to be associated with neoformation of other 2:1 clay minerals such as Al-vermiculite and mixed-layer illite/smectite. Further studies, focusing on the clay evolution in soil profiles, are needed to validate these hypotheses and to investigate the role of smectite in soil pH buffering.

Acknowledgments

This work formed part of a research program funded by the Department of Technical and Economic Co-operation (DTEC), the Land Development Department (LDD), the Institute of Research for Development (IRD) and the National Institute of Agronomic Research (INRA) and this work received a financial support from the French Ministry of Foreign Affairs through the French Embassy in Thailand.

References

Brindley, G.W. and Brown, G. (Editors), 1980. Crystal structures of clay minerals and their X-ray identification, Monograph No. 5. Mineralogical Society, London, England, 495 pp.

Bruand, A., Hartmann, C, Ratana-Anupap, S., Sindhusen, P., Poss, R. and Hardy, M., 2004. Composition, fabric, and porosity of an Arenic Haplustalf in Northeast Thailand: Relation to penetration resistance. Soil Science Society of America Journal, 68, 185-193.

Dudek, T., Srodon, J., Eberl, D.D., Elsass, F. and Uhlik, P., 2002. Thickness distribution of illite crystals in shales. I: X-ray diffraction vs. High-resolution transmission electron microscopy measurements. Clays and Clay Minerals, 5, 562-577.

FAO (Editor), 1975. Sandy Soils (Report of the FAO/UNDP seminar on reclamation and management of sandy soils in the Neareast and North Africa, held in Nicosia, 3-8 December 1973). FAO Soils Bulletin 25. FAO, Rome, Italy, 245 pp.

Helyar, K.R., 1976. Nitrogen cycling and soil acidification. Journal of the Australian Institute of Agricultural Science, 42, 217-221.

Imsamut, S. and Boonsompoppan, B., 1999. Established soil series in the Northeast of Thailand. Reclassified according soil taxonomy 1998. Department of Land Development, Bangkok, Thailand, 154 pp.

Lanson, B., 1997. Decomposition of experimental X-ray diffraction patterns (profile fitting): a convenient way to study clay minerals. Clays and Clay Minerals, 45, 132-146.

Lesturgez, G., 2005. Densification des sols sableux sous culture mécanisée. Cas du Nord-Est Thallandais. Thèse, Université Henri-Poincaré, Nancy, France, 160 pp.

Lesturgez, G., Poss, R., Noble, A., Grilnberger, O., Chintachao, W. and Tessier, D., 2005. Soil acidification without pH drop under intensive cropping systems in Northeast Thailand. Agriculture, Ecosystems & Environment, (Accepted).

Noble, A.D., Gillman, G.P and Ruaysoongnern, S., 2000. A cation exchange index for assessing degradation of acid soil by further acidification under permanent agriculture in the tropics. European Journal of Soil Science, 51, 233-243.

Ragland, J. and Boonpuckdee, L., 1987. Fertilizer responses in Northeast Thailand: 1. Literature review and rationale. Thai Journal of Soils and Fertilizers, 9, 65-79.

Romero, R., Robert, M., Elsass, P. and Garcia, C, 1992. Evidence by transmission electron spectroscopy of weathering microsystems in soil developed from crystalline rocks. Clay Minerals, 27, 21-33.

¹ Unité de Phytopharmacie
² Unité de Science du Sol, INRA, Centre de Recherche de Versailles-Grignon, Route de St-Cyr, 78026 Versailles, France, [email protected]
³ Office of Science for Land Development, Land Development Department, Chatuchak, Bangkok 10900, Thailand.
⁴ Institut de Recherche pour le Développement, UR176 SOLUTIONS, Chatuchak, Bangkok 10900, Thailand.
⁵ Present address: Centre de Géochimie de la Surface, UMR 7517, 1 Rue de Blessig, 67084 Strasbourg, France.

Hydrotalcite: leaching-retarded fertilizers for sandy soils

Gillman, G.P.¹ 

Keywords: exchangeable fertilizers, nitrate, anionic calys

Introduction

To meet ever increasing worldwide demand for food and fibre on decreasing areas of land on which to produce them, farmers have turned to high-yielding crop varieties that generally require large amounts of soil nutrients for maximum production. This has led to a dramatic rise in the use of synthetic high-analysis fertilizers, whereby nutrients can be applied in a soluble and therefore plant-available form in a highly efficient manner. An unfortunate down-side of applying soluble fertilizer is the ease with which the chemical species can escape the producers’ fields and pollute off-site environments.

The electrostatic retention of positively charged cationic species on negatively charged particle surfaces in soil is the basis of a well recognized phenomenon, referred to as soil Cation Exchange Capacity (CEC). The subject is exhaustively dealt with in the literature but is perhaps best summarized in a succinct entry in the Encyclopaedia of Soil Science by Bache (2002). In short, the negative charge on the surfaces of soil organic matter and soil clay particles is balanced by accumulation of cations such as Ca²⁺ and K⁺ close to these surfaces, affording the cations some protection from leaching, while still being available for uptake by plant roots.

Nature does not provide a similar mechanism for the retention of negatively charged anions in soil, except in the relatively rare occurrence of a small amount of pH dependent positive surface charge in the deeper layers of oxidic soils (Schofield, 1949; Gillman, 1974). Instead, plant nutrients such as N, P, and S are normally retained in the compounds of soil organic matter, and are only released as anions (NO₃^-, H₂PO₄^-, and SO4⁼) following organic matter mineralization. Limited amounts of P and S can enter soil solution by desorption from clay surfaces or from dissolution of primary minerals such as apatite and gypsum. The rate of release of N, P, and S from these sources is usually insufficient to satisfy plant demand in intensive agriculture, and synthetic fertilizers containing these anions are therefore applied. Being relatively or highly soluble, they are prone to leaching loss especially in light-textured soils (Brady and Weil, 1999).

A class of clay minerals known as layered double hydroxides (LDH) has in recent years been receiving a good deal of attention because of the wide range of uses to which they can be applied. Uses include catalysts for organic molecule conversion, fire retardants, anion scavenging, and medical and pharmaceutical preparations (Carlino, 1997). A distinguishing feature of LDH clays is their high structural positive charge density brought about by co-precipitation of di- and tri-valent cation hydroxides in proportions that allow the tri-valent cation to occupy some di-valent cation sites. Hydrotalcite (HT) the best known example, forms when 25-30% of the cation sites in the brucite (Mg(OH)₂) structure are occupied by Al, leading to a net positive charge that can theoretically reach about 400 cmol_c/kg. The similarity between bentonite and HT in the way structural charge arises, leading to intercalation of ionic species with opposite charge, is depicted in Figure 1.

Figure 1. Illustrating the similarities between bentonite and hydrotalcite in the mechanism of retention of cations and anions on their surfaces

The HT used in the above-mentioned applications is usually in carbonated saturated form (CO₃-HT) and being of high purity, is expensive. However, technical grade HT, such as Cl-HT or NO₃-HT, useful for environmental applications discussed in this paper can be produced less expensively from magnesium sources such as magnesite (MgCO₃) or concentrated seawater, and aluminium sources such as bauxite (Al(OH)₃) or scrap aluminium (Gillman and Noble, 2005).

This paper discusses the use of HT as a platform for nutrient delivery to soils, more particularly to soils prone to nutrient leaching owing to their light texture. The results of laboratory experiments assessing the resistance of nitrate-saturated HT (NO₃-HT) to leaching and to denitrification are presented, along with comparison in greenhouse trials of NO₃-HT and urea as a nitrogen fertilizer in sandy soils. Finally, the concept of using NO₃-HT to stabilize soluble phosphate in feedlot wastes is canvassed as a means of closing the nutrient cycle in the animal-waste-fertilizer-soil-plant-animal cycle.

Designer Fertilizers Using Hydrotalcite

Three of the macronutrients required by plants viz. N, P, and S can be incorporated into the HT structure as nitrate, phosphate, and sulfate respectively, thereby allowing their addition to soil in exchangeable and therefore slow-release form. Micronutrients that can exist in anionic form (borate, silicate, molybdate etc.) could also be introduced in this manner. Furthermore, by manufacturing a range of HT products, with total saturation of each with only one anion type, complete flexibility is allowed for the design of fertilizer blends that match the requirements of soils and the crops to be grown on them. In fact, by combining this principle with another involving negatively charged substrates (e.g. bentonite) loaded with individual cationic nutrients, a totally flexible Designer Fertilizer concept is achieved (Gillman and Noble, 2005).

Column Leaching Experiment

A 5 g sample of aged NO₃-HT was mixed with 500 g of sand, and the mixture placed in a 70 mm diameter Perspex column with a filter paper in the bottom supported by a silk screen. A filter paper was also placed on top to evenly distribute distilled water added at about 2.5 ml/minute, a rate that maintained unsaturated flow conditions. Leachate was collected in 25 ml aliquots for the first 20 pore volumes, and thereafter in 250 ml aliquots until 70-75 pore volumes had been collected. Besides analysing each aliquot for nitrate-N, Mg was also determined to allow estimation of the rate of HT breakdown, as was NH₄⁺, to estimate the amount of residual nitrate associated with NH₄NO₃ which is a by-product in HT synthesis.

There was a flush of nitrate from the column in the first 4-5 pore volumes, representing about 40% of the nitrate added as HT product (Figure 2). The uppermost curves show the cumulative total amount of added nitrate recovered, while the curve marked ‘Breakdown’ refers to the nitrate fraction associated with HT dissolution, and was calculated from the Mg determination (1 mole of nitrate would be released from HT for every 2 moles of Mg). The ‘Soluble’ curve, prepared from the NH₄⁺ analyses, represents the nitrate associated with residual NH₄NO₃ not washed from the HT product after synthesis. Finally, the difference between the total amount of nitrate recovered and the sum of breakdown and soluble nitrate, is considered to be the nitrate removed from exchange sites of the relatively stable fraction of the HT.

The relatively high initial nitrate removal rate from the HT (50% at 15-20 pore volumes) is accounted as 22% from HT exchange sites, 20% from residual ammonium nitrate, and about 8% from the dissolution of HT particles. It would appear from these results that NO₃-HT added to soil would act as both a ready source of N at the early stage of plant development, and as a reserve source of N at later stages of the growth cycle.

Figure 2. Cumulative amounts of nitrate (% of that applied) leached from a column of sand containing nitrate-saturated hydrotalcite

De-nitrification Studies

An aged HT slurry was dried to a point (dough consistency) where it could be rolled into balls of 4-5 mm diameter, that were then dried at 80ºC. Approximately 0.1 g of material was added to pre-weighed tubes for the accurate recording of particle mass. 50 g of sandy soil from a wheat field, 5 ml of 12 g/l sucrose solution, and 11 ml of water were then added, effecting an N concentration of approximately 100 kg N/kg soil and a C concentration of 500 mg C/kg soil. Forty tubes were prepared in this way to allow 10 tubes to be extracted at four time periods, i.e. after 3, 6, 11, and 14 days, for determination of nitrate remaining. A parallel set of tubes were prepared where 4 ml of 1 mg N/ml of KNO₃ solution was substituted for NO₃-HT and only 7 ml of water added to keep soil:water ratio constant, but with only 16 tubes to allow 4 tubes from this treatment to be extracted at each sampling occasion. Finally, 8 control tubes with no added N were included to allow 2 controls to be examined each time. The tubes were placed in a desiccator, the air evacuated, and stored at room temperature for the required intervals, at which times the tube contents were extracted with 2M KCl for nitrate analysis of the extracts.

The results of this de-nitrification experiment is summarized in Figure 3. It is clear that the location of nitrate in the HT inter-layers affords some protection, though the mechanism has not been investigated.

Figure 3. The recovery of nitrate when nitrate was added as nitrate-saturated HT( ), and as potassium nitrate (♦), to a saturated sandy soil with sucrose added as energy source, and stored under vacuum. Vertical bars are the standard error of the mean

The conditions of high vacuum, saturated soil, and adequate energy source were ideal for de-nitrification to occur, so that the rates of loss of nitrate might not be as rapid under some field conditions. The obvious retarding effect of HT warrants further investigation of the use of NO₃-HT for the growth of crops such as lowland rice, produced under de-nitrifying conditions. It should be remembered, however, that even in sandy soils, conditions can still exist for de-nitrification to occur when soil is temporarily saturated.

Greenhouse Trials with NO₃-HT

This trial compared the use of nitrate saturated HT and urea as sources of N for the growth of tropical maize, and consisted of 4 rates of N (25, 50, 75, 100 kg N/ha) and a control with zero N. The experiment was conducted in 5 kg freely draining plastic pots using a coarse-textured alluvial sand. The two nitrogen sources were applied as granules, drilled as a band across the diameter of the pot. After germination the maize plants were thinned to 4-5 per pot, and the pots were freely watered daily throughout the 9 week growth period. Macronutrients (except N) and micronutrients were applied as required. At harvest, the above ground biomass was dried and weighed to assess Dry Matter production.

The response of forage sorghum to nitrogen added as NO₃-HT and as urea growing in a sandy soil under freely-draining conditions is summarized in Figure 4. For N application rates greater than 40 kg/ha, significantly higher yield was achieved with the NO₃-HT application. Unfortunately, N content of the herbage was not determined but the plants that received nitrogen as NO₃-HT were greener and healthier in appearance than those fertilized with urea.

Figure 4. Response to nitrogen applied as urea and as nitrate-saturated HT of tropical maize growing in sandy soil placed in freely-draining pots. Vertical bar represents LSD between two treatment means

Stabilization of Feedlot Wastes

The rapid expansion of Confined Animal Feedlot Operations (CAFOs), more commonly known as Feedlots, has led to huge problems relating to disposal of wastes exiting these facilities. The organic matter and plant nutrient content of the materials make them prime candidates for use as fertilizers and soil ameliorants, but their generally high water content and/or intractable physical properties present major difficulties in respect of handling and storage, transport, and land application. Odour is often a problem if the facilities are located near residential areas. The above-mentioned problems result in concentration of applications of feedlot manure on relatively small areas close to the feedlot.

Nitrogen and phosphorus in the manures are of particular concern because applications of manure in excess of the plants’ capacity to adsorb these elements pose a threat of their escape to waterways, and the subsequent development of anoxic and/or eutrophic conditions. Since a plant’s P requirement is generally much less than that for N, regulatory authorities are devoting increasing attention to the rates of soluble P that can be applied to land.

Hydrotalcite has a high specificity for phosphate, and in this section, the possibility of stabilizing soluble P in feedlot wastes and using the treated material as slow-release fertilizer is canvassed.

Swine lagoon sludge

One of the more common practices for treating the effluent from swine feedlots is to pond the effluent in dams to allow settling of solids for aerobic/anaerobic digestion, and evaporation of some water. The excess supernatant liquid can be re-cycled with additional fresh water to the facility, or it can be used for fertigation of nearby areas. The jelly-like sludge that slowly accumulates in the bottom of the dam has to be removed at intervals, but is difficult to handle owing to its gelatinous nature and its objectionable odour. Also, because of its relatively high soluble P content, there are grave environmental problems associated with applying it too liberally on surrounding land.

Swine lagoon sludge (1 kg) having 89% water content was mixed with 100 g of chloride-HT in a blender for 30 minutes to allow the HT to adsorb soluble P. An amount of powdered bentonite (350 g in this case) was then blended in until a workable ‘dough’ was produced, and this was forced through a perforated plate to form ‘noodles’ that were dried at 80ºC. This product could be easily broken down into granules (Figure 5). A foul-smelling intractable sludge had been converted into an odourless, easily-handled material that had a water soluble phosphate content of 0.2 ppm P, whereas the equivalent dry weight amount of original sludge contained 47.5 ppm P in soluble form.

Figure 5. Conversion of swine lagoon sludge to granular fertilizer by addition of hydrotalcite and bentonite, with reduction in water soluble phosphate content from 47.5 mg P kg^-1 to 0.2 mg P kg^-1

Chicken battery manure

The manure from a chicken battery usually contains far less water than effluent from other animal feedlots, and in fact can be retrieved in a relatively dry form from the battery floor under particular management conditions. The soluble phosphate content is generally high owing to the large amounts of phosphate fed to battery chickens.

To demonstrate the capacity of HT to stabilize this soluble phosphate, increasing amounts of chloride-HT, viz. 0, 0.2, 0.8, and 2.0 g were added to 4 g aliquots of dried chicken manure, and the resultant mixes shaken with 10 ml of distilled water for 18 hours. As evident in Figure 6, the HT was able to greatly reduce P concentration in the extracts, soluble P being converted to an exchangeable form that would be suitable as a slow-release fertilizer.

Figure 6. Reduction in water soluble phosphate content in chicken manure extract following incremental additions of hydrotalcite

Stabilization of phosphate in feedlot wastes is of particular benefit when the waste is to be applied to light-textured soils, where the benefits of addition of untreated material would be quickly lost via leaching. Also, production of dry granules allows economic transport over longer distances and easier land application. To achieve a more valuable product, however, other nutrient ingredients, either synthetic or natural, could be introduced at the blending stage to achieve higher and more balanced nutrient contents. In keeping with the overall theme of this paper, it would be preferable if all resultant nutrients were in slow-release form either as exchangeable or sparingly soluble species.

Conclusion

The use of technical grade hydrotalcite as a fertilizer platform, whether carefully designed to deliver specific amounts of anionic nutrients, or incorporated into nutrient-rich wastes to extend their effectiveness, offers great benefits with respect to economic efficiency and environmental protection, especially where agricultural enterprises are established on light-textured soil landscapes. In combination with a similar platform (e.g. bentonite) for delivering exchangeable cations, a complete system of leaching-retarded soil fertilization can be envisaged.

References

Bache, B.W. 2002. Ion Exchange. In: Lal, R., ed., Encyclopaedia of Soil Science, New York, Marcel Dekker, 726-729.

Brady, N.C. and Weil, R.R. 1999. The Nature and Properties of Soil. New Jersey, 12^th edn. Prentice-Hall Inc., 881 p.

Carlino, S. 1997. Chemistry between the sheets. Chemistry in Britain, 33, 59-62.

Gillman, G.P. 1974. The influence of net charge on water dispersible clay and sorbed sulfate. Australian. Journal of Soil Research, 12, 173-176.

Gillman, G.P., and Noble, A.D. in press. Environmentally manageable fertilizers: A new approach. Environmental Quality Management.

Schofield, R.K. 1949. Effect of pH and electric charges carried by soil particles. Journal of Soil Science, 1, 1-8.

¹ CSIRO Land and Water, Private Mail Bag PO, Aitkenvale. Qld. 4814, Australia

Assessment of salinity hazard by Time Domain Reflectometry in
flooded sandy paddy soils