Potassium and Metal Release Related to Glaucony Dissolution in Soils

Plant nutrients such as potassium (K) may be limited in soil systems and additions (i.e., fertilizer) are commonly required. Glaucony is a widely distributed and abundant marine-derived clay mineral present in soils worldwide which may serve as a source of potassium. The South Island of New Zealand contains numerous deposits of glaucony-rich rocks and related soils providing an opportunity to explore how glaucony might be a beneficial source of potassium. Here, the geochemistry of glaucony and its suitability as a mineral source of soil K from four deposits in New Zealand was examined using spatially resolved chemical analyses and dissolution experiments. Geochemical and morphological analyses revealed that glaucony from all deposits were K-enriched and were of the evolved (6%–8% K2O) to highly evolved type (>8% K2O). Glaucony derived from growth inside pellets contain elevated K and Fe concentrations compared to bioclast-hosted glaucony. Solubility analysis showed that K was released from glaucony at rates higher than any other metal present in the mineral. Additionally, decreasing the pH and introducing an oxidizing agent (i.e., birnessite which is ubiquitous in soil environments) appeared to accelerate K release. Trace metals including Cr, Zn, Cu, and Ni were present in the solid phase analysis; however, further investigation with a focus on Cr revealed that these elements were released into solution at low concentrations and may present a source of soil micronutrients. These results suggest that glaucony may offer a source of slow releasing K into soils, and so could be used as a locally sourced environmentally sustainable K resource for agriculture, whether in New Zealand or worldwide


Introduction
Potassium (K) is an essential macronutrient for plant growth and is involved in essential biochemical processes in plants [1][2][3]. In soils, a majority of K is present in unavailable/nonexchangeable mineral forms such as feldspars and micas. Potassium that is bioavailable may be present as exchangeable K adsorbed onto soil colloids or in soil solutions. Soil solutions contain the most readily available source of K for plant uptake; however, K may be leached from the system and is dependent on a dynamic equilibrium with exchangeable/adsorbed potassium. Vegetation is largely dependent on the supply of exchangeable K which is directly related to the rate of transformation of K-containing minerals (i.e., feldspars and micas) [4][5][6][7]. In undisturbed soils, K availability and supply is generally not a concern, especially when the soils contain abundant mineral sources of potassium. When soils are utilized for agriculture, the relationships between nonexchangeable, exchangeable, soil solution K, plant uptake, and harvesting may result in an imbalance where K is being removed faster than it can

Sample Preparation
Weakly indurated samples were disaggregated by hand in warm water. Loose sediments were then wet sieved into 1000-500 µm, 500-212 µm, and 212-63 µm grain size fractions before being dried at 45 • C for 24 h. Glauconitic minerals were then separated from the bulk sample using a Frantz isodynamic magnetic separator. The separator operated at 1 A voltage, 14 • sideways tilt and 20 • longitudinal tilt, followed by a run using the same orientation of the unit and 0.5 A. Separated samples were then rinsed in dilute HCl (10%) to remove any traces of carbonate components aggregated to the surface of the glaucony grains, followed by rinsing 5 times with distilled water. A short ultrasonic bath (3 s) was the final stage of purification before the sediment was again oven dried at 45 • C for 24 h. Grains were hand-picked using a fine paint brush under the binocular microscope. One hundred hand-picked grains were mounted on epoxy resin stubs. From the non-indurated rocks 11 samples were chosen to pick grains from. Three were from the Waipara River section and 8 were from various outcrops around the Oamaru region. Where possible, 3 grains were picked from each grain size for each sample, resulting in 9 grains representing a sample. Grain mounts were polished using gradually finer polishing agents, beginning with 120 fixed grit silicon carbide paper, followed by 3 µm, 1 µm, and eventually 0.5 µm diamond paste on 450 Lampan cloth. These grain mounts were used for analysis using laser ablation induction coupled plasma mass spectroscopy (LA-ICP-MS) and scanning electron microscope with energy dispersive X-ray spectroscopy (SEM + EDS).
Only well indurated samples were present at the Oparara Quarry. These grains could not be disaggregated by hand and mechanical crushing of the rock would have pulverized the glaucony making further analysis impossible. Instead the rock was cut into thin sections which were polished using the same method as described for the epoxy mounts above. These sections were used for LA-ICP-MS and SEM + EDS.

LA-ICP-MS
Sixty grains from the polished mounts were chosen to be analyzed for trace and rare-earth elements (REEs) using LA-ICP-MS facilities in the Otago Community Trust Centre for Trace Element Analysis at the University of Otago. Laser ablation was conducted with a Resonetics RESOlution M-50-LR laser ablation system incorporating a Coherent CompexPro 102 193 nm ArF excimer laser and Laurin Technic two-volume sample cell. The laser was operated at a constant energy of 100 mJ and 12.5% transmission for an on-sample fluence of 4 J/cm 2 . Ablated material was carried by He gas (650-750 mL/min) from a two-volume sample cell, mixed with Ar (650-750 mL/min) and N 2 (2-6 mL/min), and input into an Agilent 7500cs ICP-MS. Data for 52 mass peaks were collected in time resolved mode with one point per peak. Integration times were 10 ms for 3 Li, 4 Be, 23 Na, 24 Mg, 27 Al, 29 Si, 31 P, 39  Background (laser off) data were acquired for 20 s followed by 40 s with the laser using spot diameters of 75 µm, a 5 Hz repetition rate, a 200 µm long track at 5 µm/s, and approximately 20 µm deep giving about 100 mass scans. Each set of 9-12 samples was bracketed by analysis of standard glasses NIST 610 and 612 [27].
Raw mass peak count rates were background subtracted, corrected for mass bias drift and converted to concentrations using an offline spreadsheet. After triggering, several seconds are required for a steady signal to be reached, and these initial data were excluded from the calculations. Trace element concentrations were obtained by normalizing count rates for each element to those for Si in the sample and standard using known SiO 2 and trace element concentrations in National Institute of Standards and Technology (NIST) 610 [27] and the SiO 2 in the sample estimated by summing the major element oxides to 100 wt%.

SEM with EDS
Major element concentrations were measured using a JEOL JSM 6100 SEM with an Oxford Aztec EDS analysis system at the University of Canterbury. Polished epoxy grain mounts and a polished thin section were carbon coated prior to analysis. Measurements were made at 800x magnification with a 4 s acquisition time. Two squares were drawn on each grain to select the area to be scanned, these were about 100 µm 2 and were drawn in the most homogenous looking part of the grain. Images were taken of each grain prior to chemical analysis. Chemical data for 87 grains (77 from stubs and 10 from the thin section) were collected. Total Fe was calculated as Fe 2 O 3 and mineral formulas were calculated as being anhydrous as no loss on ignition data was obtained.

Solubility Analysis
Two separate experiments were conducted to assess elemental release from glaucony into solution over time.

Experiment 1
Glaucony solubility was measured in H 2 O (acidified to pH 3.7 with dilute HCl) over a 16 day period. 0.37 g of glaucony from the 212-500 µm grain size fraction of WP were left in 40 mL of ultrapure water. Tubes were lying horizontally on a table shaker (IKA model KS260) operating at 50 revolutions per minute. Extractions of solution were made at 25,73,190, and 382 h. Ten mL of solution was extracted from 3 tubes resulting in a triplicate analysis for each time period. The extractions were filtered using 2 µm filters to remove solid particulates. Samples were kept in a room at 22 • C throughout the experiment. Two samples of ultrapure water were taken for analyses, one at 25 h and one at 382 h. Following each extraction, pH was measured using a Mettler Toledo Seven GoDuo Pro SG78 pH/conductivity probe.
Elemental proportions were measured using induction coupled plasma mass spectroscopy (ICP-MS) in the Chemistry Department at the University of Canterbury. Solutions were acidified using trace metal grade nitric acid to pH 2 before measurements of Na, Mg, Al, P, K, Ca, V, Cr, Mn, Co, Fe, Ni, Cu, Zn, As, Sr, Zr, Ru, Cd, Sn, Sb, Cs, Ce, Ti, 206 Pb, 207 Pb, 208 Pb, Th, and U were made using an Agilent 7500cx ICP-MS with 2-260 atomic mass units quadrupole mass analyzer. Only K, Na, Mg, Fe, Ca, and Al measurements are presented here, but the complete dataset is available upon request.

Experiment 2
The second series of solubility experiments tested the influence of pH and the presence of birnessite (a high valent Mn oxide known to be a strong oxidizing agent in soil environments) on elemental release from glaucony. These experiments are similar to those conducted by Oze et al. [28] where birnessite was synthesized following the same methods. Please note that the synthetic birnessite was cleaned multiple times in ultrapure water and diluted HCl in order to ensure the removal of any easily releasable elements such as potassium. The experimental parameters are shown in Table 2. This experiment was run over 103 h at 25 • C using 0.25 g of magnetically separated glaucony from WP04 and EQ01. Samples were left in 10 mL of solution on a table shaker operating at 80 rotations per minute. The solutions were a pH 4.6 acetate buffered solution and a pH 8 NaOH buffered solution. All solution pHs were monitored and maintained throughout the duration of the experiments. Birnessite (0.015 g) was added to one of two samples at both acidic and basic pH, resulting in the analysis of glaucony solubility with and without birnessite at both high and low pH. All experiments were completed in triplicate. Elemental concentrations were measured using ICP-MS using a Thermo ICP-MS X Series II spectrometer at the Environmental Measurements Facility at Stanford University. The elements selected for analysis were Mg, Al, K, Ca, Cr, Mn, and Fe.  2 display examples of the variation in the grain morphology of glaucony using bionocular microscopy and scanning electron imaging. Table 1 summarizes the proportions of glaucony and the dominant host grains for sampled outcrops.     2 display examples of the variation in the grain morphology of glaucony using bionocular microscopy and scanning electron imaging. Table 1 summarizes the proportions of glaucony and the dominant host grains for sampled outcrops.

Major Elements
The average stoichiometric formula for all 88 grains of glaucony analyzed by SEM-EDS is K 0.74 (Fe 1.27 Al 0.30 Mg 0.39 ) 1.96 (Si 3.77 Al 0.23 ) 4 (O 10 )(OH) 2 . Table 3 shows concentration averages, minima, maxima, and standard deviations for K 2 O, SiO 2 , MgO, and Fe 2 O 3 for all grains analyzed using SEM-EDS. Overall, K concentrations range from 6.75 to 9.84 wt%, with an average of 8.58 wt% and standard deviation of 0.81 (N = 88). The average K concentration is highest in grains from Campbell's Beach (9.40 wt%), and decreases in the following order: Ross Farm, Earthquakes, Waipara, Gee's Beach and Oparara with values of 8.95, 8.72, 8.66, 7.95, and 7.66 wt%, respectively. Standard deviations for K range from 0.87 at Gee's Beach (N = 12) to 0.32 at Campbell's Beach (N = 11).      Table 4 shows trace element concentrations and the standard deviations for grains analyzed using LA-ICP-MS. Titanium, V, Cr, Zn, and Rb are present in significantly higher concentrations than all other trace elements in all samples, with average values across all outcrops of 616, 212, 268, 172, and 335 mg kg −1 , respectively. Nickel is the next highest concentrated element with an average of 41 ppm across all outcrops. Titanium shows significant variation with 272 ppm at Campbell's Beach and 915 ppm at Waipara. Average V concentration ranges from 74 ppm (Campbell's Beach) to 418 ppm (Oparara). Chromium is present in relatively consistent concentrations in all outcrops, ranging from 174 ppm at Gee's Beach to 334 at Waipara. Zinc concentration shows only minor variation with Earthquakes and Campbell's Beach both measuring 130 ppm and Gee's Beach containing the maximum at 200 ppm. Rubidium is enriched at Oparara with 580 ppm. In outcrops from Oamaru, Rb ranges from 250 to 280 ppm and the Waipara greensand contains 335 ppm on average. The potassium concentration released from Waipara (WP) glaucony during lexiviation testing is shown in Figure 4. The first measured time interval at 25 h contained 700 ppb K. After 73 h the concentration of K increased to 800 ppb and after 190 h K concentration reached 1400 ppb. Over the total 400 h that grains were left to dissolve in acidified H 2 O, 1640 ppb K was released into solution. The release of K was almost linear over the 400 h with a release rate of 2.7 ppb K h −1 . Figure 4 shows the concentrations and rates of Na, Mg, Fe, Ca, and Al release measured by ICP-MS. Sodium had the second highest release rate of 0.4 ppb Na h −1 . Chromium from these experiments was undetectable in most analyses and had a maximum concentration of 0.1 ppb.

Experiment 1-No Birnessite at pH 3.7
The potassium concentration released from W aipara (W P) glaucony during lexiviation testing is shown in Figure 4. The first measured time interval at 25 h contained 700 ppb K. After 73 h the concentration of K increased to 800 ppb and after 190 h K concentration reached 1400 ppb. Over the total 400 h that grains were left to dissolve in acidified H2O, 1640 ppb K was released into solution. The release of K was almost linear over the 400 h with a release rate of 2.7 ppb K h −1 . Figure 4 shows the concentrations and rates of Na, Mg, Fe, Ca, and Al release measured by ICP-MS. Sodium had the second highest release rate of 0.4 ppb Na h −1 . Chromium from these experiments was undetectable in most analyses and had a maximum concentration of 0.1 ppb.  of K was released from a solution in one experiment containing birnessite at pH 4.6 over 103 h (i.e., 0.6 µg h −1 ). This was 9.3 times more K than the same sample at pH 4.6 without birnessite and 13 times more than the same sample dissolving at pH 8 with no birnessite. This relationship is true for both WP and EQ samples, which measure 8.66 and 8.72 wt% K 2 O, respectively. Soil Syst. 2018, 2, 70 11 of 17 K was released from a solution in one experiment containing birnessite at pH 4.6 over 103 h (i.e., 0.6 μg h −1 ). This was 9.3 times more K than the same sample at pH 4.6 without birnessite and 13 times more than the same sample dissolving at pH 8 with no birnessite. This relationship is true for both W P and EQ samples, which measure 8.66 and 8.72 wt% K2O, respectively. Birnessite decelerates Ca release in both samples at either pH. Glaucony released an average of 1.14 times more Ca without birnessite in the system from EQ (either pH) and W P (pH 4.6) samples and 8.3 times more Ca from W P at pH 8. Birnessite also increased the rate of Cr released from glaucony. No Cr was present in samples that contained no birnessite and 1.1 to 1. Birnessite decelerates Ca release in both samples at either pH. Glaucony released an average of 1.14 times more Ca without birnessite in the system from EQ (either pH) and WP (pH 4.6) samples and 8.3 times more Ca from WP at pH 8. Birnessite also increased the rate of Cr released from glaucony. No Cr was present in samples that contained no birnessite and 1.1 to 1.3 ppm was released from samples containing birnessite. Manganese concentrations were also higher in samples containing birnessite. Birnessite does not appear to affect the release of Al into solution.
Acidic conditions accelerated the release of K, Mg and Ca. In solutions of pH 4.6 with and without birnessite, Ca concentrations averaged 29,589 ppm, whereas, in pH 8 solutions Ca averaged 4968 ppm. Potassium concentrations were 1.6 times higher in the acidic solution compared to the basic solution when birnessite was present, and 1.5 times higher in acidic conditions when birnessite was absent. Magnesium concentration increased eight-fold in the acidic solution compared with the basic solution in the presence of birnessite, and 4.7 times in the absence of birnessite.
Basic conditions accelerated Fe release. In the pH 4.6 solution, no Fe was detected, despite it being the second most abundant element in the solid phase (averaging 25.38 and 23.77 wt% in WP and EQ, respectively) ( Table 3). In the basic solutions with and without birnessite, Fe concentration averaged 198 ppm. Neither basic nor acidic conditions had an effect on the release of Al into solution. Chromium release was only detected when birnessite was present.

Glauconite Abundance and Geochemistry in Context to Paleoenvironment
All of the glaucony analyzed in this research falls into the evolved to highly evolved maturity parameters set by Odin and Matter [15]. Iron, Al, Mg, Si, and K are all present in concentrations comparable to published chemical data and the minerals analyzed in this research all fall into the true field of glaucony [15].
The concentrations of major elements generally show only minor variations between outcrops. The exceptions are the Stoney Creek limestone (Oparara Quarry) which is enriched in Al and depleted in Fe and K, and Gee's Beach which has a higher average Fe content. The sample from Oparara contained bioclastic-hosted glaucony. According to Banerjee et al. [11], the maturation of glaucony is hindered in bioclast-hosted grains as local alkaline conditions inhibit the dissolution of the calcite test, restricting the creation of new pore space and the percolation of seawater into the semi-confined micro-environment where maturation and further precipitation occurs [15]. These grains had less substitution of Fe for Al and the calcitic tests took longer to dissolve and lacked porosity and permeability, resulting in inhibited maturation and lower K content than pelletal-hosted glaucony which was prevalent in other outcrops.
The Gee Greensand, as sampled at Gee's Beach and Campbell's Beach, contains 30%-40% phosphorite in the bulk rock and was heavily bioturbated, indicating deposition in an organic-rich environment [29]. The high organic content would have created a relatively oxygen depleted seafloor sediment pile which increased the mobility of Fe, allowing diffusion through the seafloor sediments and creating increased Fe concentrations in the zone of glauconitization. The Waipara greensand and Campbell's Beach contain the next highest Fe concentrations and this is likely also due to the mobilization of Fe in sub-oxic conditions. Trace elements, including heavy metals such as As and Cr, have been identified in glauconitic minerals and can contaminate soil. Elevated concentrations of As (7-31 ppm), Be (5-18 ppm), Cd (<0.2-1.2 ppm) and Cr (130-1000 ppm) were identified in greensands that were used as fertilizers on New Jersey pastoral land [10]. During the 1880's, 1 Mt of greensand was applied to New Jersey farmland per year, sometimes at rates up to 100 t per acre [30]. Arsenic concentrations up to 5.95 µg L −1 in New Jersey stream waters exceeded the state standard and were chemically fingerprinted to have derived from glauconitic sediments present in the groundwater environment [31]. For these reasons, the type and abundance of trace metals in glaucony needs to be addressed before using this mineral as a soil additive. Several trace elements of concern for soil contamination were present in the New Zealand glaucony including Cr, Zn, Ni, Cu, and U. From these elements, it was found that only Cr was present in elevated concentrations (averaging 268 ppm from all grains). For this reason, Cr was selected as an element for further assessment. As the oxidized form of Cr (i.e., hexavalent Cr or Cr (VI)) is toxic, birnessite, a known soil oxidizer, was combined with the dissolution experiments in order to determine if the New Zealand glaucony could potentially be a source of non-anthropogenic hexavalent chromium.

Glaucony as a Source of Soil K and Fertilizer Resource
Solubility analysis of glaucony from the Waipara Greensand in water (pH~4) over 16 days showed that K was released into solution ( Figure 4). Over one year 2.53 g of K will be released from one kg of glaucony, equating to~6.94 mg of K per kg of glaucony per day. The results from this research are lower than concentrations measured by Karimi et al. [2], where 23 mg K per kg of glaucony per day was measured from a crushed glauconitic sandstone containing 2.24 wt% K 2 O dissolved in H 2 O, and Rao and Rao [32] who measured 13 to 20 mg K per kg of glaucony. Plants require 50-100 kg of K per hectare per year [2]; therefore,~20 t per ha of glaucony at 8.66 wt% K 2 O (the concentration of K 2 O in the Waipara greensand) would be required to satisfy the minimum plant requirement of K.
The specific gravity of glaucony ranges between 2.3-2.9 [17] and is most likely influenced by Fe content, where mature Fe-rich grains are denser than immature Fe depleted grains. Using the Waipara greensand as an example, the bulk rock contains 40%-65% glaucony which averages 25 wt% FeO. To produce 20 t of glaucony from an assay of 40% and a specific gravity of 2.70 g/cm 3 , 50 t or~18.50 m 3 of bulk rock would need to be excavated to satisfy the requirements outlined above, assuming a bulk rock density of 2700 kg/m 3 (the composition is dominantly quartz and glaucony). Magnetic separation would be required to concentrate the glauconitic fraction.
The average representative stoichiometric mineral formula for glaucony from the Waipara greensand was K 0.75 (Fe 1.29 Al 0.27 Mg 0.37 ) 1.94 (Si 3.82 Al 0.18 ) 4 . From this it was calculated that K comprised 47.66 mg of the 0.37 g of glaucony in the sample which equates to 130 g of K per 1 kg of glaucony. Thus, in 24 h the proportion of K released into solution was 0.006% of the total K in the sample. If dissolution rates remain constant, then~2% of the total K will dissolve from the mineral over one year. Rao and Rao [32] and Mazumder et al. [33] showed that the rate of K released into solution depends on the grain size and that finer grain sizes release more K than coarser grains. It would, therefore, be expected that over time the rate of K release would increase as the minerals undergo physical and chemical weathering and decrease in size and increase in surface area. Considering that the analysis conducted in this research used whole grains with no crushing, the K concentrations in solution reported above can be taken to represent minimum values. The release of K will be greater following crushing and subsequent increase of specific surface area where dissolution reactions occur.
When high valent Mn-oxides (birnessite) were present in the system, the mass of K released into solution increased. One explanation for this is that birnessite is aiding to oxidize a component of the glaucony structure, thereby, modifying the surface charge and fixing capacity of the clay structure. Another possibility is that birnessite may have contributed K, as KMnO 4 was involved its synthesis. Procedures were taken to ensure that the synthetic birnessite would not provide a source of K (i.e., electron microprobe, SEM-EDS, and XRD analyses for sample characterization and ensuring no detectable Mn release (detection limit at~100 ppb) into solution at pH 5 over a period of 500 h). If birnessite was a major contributor of K, it would have provided similar K concentrations at each pH when the background (i.e., the no birnessite experiment) was taken into account. This was not the case. Overall, our analyses support that birnessite is accelerating the release of K from glaucony; however, there is the possibility that the synthetic birnessite may have been a K contributor. Additionally, our analyses suggest that birnessite may be adsorbing calcium. More investigation to how birnessite may be accelerating K release or potentially adsorbing Ca requires further study.
Assuming that this rate increase remains constant over time, the amount of glaucony required to satisfy plant demands would decrease from 20 t to 1.5 t (~1.4 m 3 of bulk sediment) per ha per year if birnessite is present in the soil system. Furthermore, Tang et al. [34] showed that during the oxidation of Cr(III), Mn oxides were recycled in the presence of light and organic matter. In other words, after Mn(IV) has been reduced to Mn(III) it is regenerated by organic and/or oxygen radicals. The implication of this is that only a small amount of Mn oxides will facilitate enhanced K release from glaucony and this reaction pathway will be sustained as long as light and organic matter are present in the system. Therefore, given the ubiquitous presence of Mn oxides in soil environments it is likely that the release of K from glaucony will have a naturally occurring catalyst present before the product is even applied to the soil.
This is a simplified model of the K budget and adsorption onto soil colloids, and the excretion of plant enzymes for K release catalysis and additional sources/sinks of K will influence the total amount of K available. However, based on the solubility kinetics presented here, glaucony can provide a source of slow releasing K and one bulk application of mature glaucony may provide a base load which satisfies the minimum end of the plant requirements of K for up to fifty years. Additional inputs of imported potash would be used as a supplement according to soil characteristics and crop demands for potassium. It is recommended that applications are restricted to naturally acidic soils and soils that contain Mn-oxides as these factors will increase K release.
One concern with regards glaucony dissolution was that Na was released into solution very quickly compared to all other elements. Over one year it is predicted that 384 mg of Na will be dissolved from one kg of glaucony. This represents 20% of the total Na content present in one kg of glaucony. Given that the analysis included uncrushed grains with a low surface area, 20% is a significant portion of the total mass of an element to be released in one year. The dissolution analysis was conducted using a magnetically separated bulk sediment of concentrated glaucony and it is likely that there were salts (NaCl) and other minor accessory phases which may have contributed to the high Na concentrations. If soil salinization is a concern, a pre-application wash with a dilute acid could remove Na in accessory phases. Additionally, if electrolytes in soil solution are a concern, then we would recommend additional dissolution experiments to reflect the electrolyte and dissolved organic matter levels of the soil that glaucony will be added to.
In addition to providing a source of K to soil, glauconitic minerals may affect the cation exchange capacity (CEC) of a soil. Tedrow [30] found that the CEC of a glauconitic sandstone was between 19 and 31 milli-equivalents/100 g soil and that the highest values were measured in the finest grain sizes. McRae [17] reported ranges between 5-39 milli-equivalents/100 g and suggested that the CEC varied inversely with the concentration of the interlayer cation. This is to be expected as the increase in interlayer cations will neutralize the net negative charge on the surface of glaucony. Therefore, the more mature and K-rich the glaucony in question is, the less it will initially contribute to the overall CEC of the soil to which it is added. Over time and with weathering, an application of glaucony will be expected to increase a soil's CEC, potentially above that created by naturally occurring clay colloids and humic matter.
The slow release of K avoids the possibility of upsetting the nutrient budget in downstream ecosystems. Leaching through soil profiles into groundwater systems and the interconnected surface water bodies can unintentionally transfer nutrients from the intended place of deposition (i.e., a farm) to sensitive ecosystems. When high concentrations of nutrients are suddenly introduced into environments the naturally established ecological diversity may become disrupted. In New Zealand, P is usually the limiting nutrient and additions of P will often trigger algal blooms [1,35], and therefore, increases in K content are not likely to disrupt the established ecology. However, in areas where it is determined that K is a limiting nutrient the slow release of K from glaucony could make it a desirable product to use in terms of avoiding ecosystem disruption while providing plants with a minimum long-term baseload of potassium.

Chromium in Glauconitic Minerals: Contaminant or Micronutrient?
Chromium exists in two stable oxidation states, trivalent chromium (i.e., Cr(III)) and hexavalent chromium (i.e., Cr(VI)). Of these, Cr(VI) is mobile and toxic, whereas, Cr(III) is less mobile and is a micronutrient [34,[36][37][38]. Chromium (VI) is reduced to Cr(III) in the presence of organic matter, Fe (II) and reduced sulfur species [39]. Once reduced, Cr(III) is insoluble and precipitates as Cr-Fe oxyhydroxides, adsorbs to the surfaces of Fe/Al oxides or becomes complexed with organic material before settling to the seafloor [36]. The oxidation of Cr(III) to Cr(VI) is mediated by Mn oxides and these are considered major oxidants capable of oxidizing Cr in natural soil [28,34,36]. It should be noted that an additional naturally occurring oxidant of Cr(III) present in terrestrial systems, not assessed here, is hydrogen peroxide (H 2 O 2 ) [40,41]. Chromium (VI) forms anionic species such as HCrO 4 − and CrO 4 2− which are mobile in marine environments and soil systems. The decomposition of labile organics in marine sediments remobilizes Cr resulting in bottom water enrichment. In the absence of MnO 2, Cr will be present as Cr(III) and will most likely be complexed with organic material or adsorbed onto Fe/Al oxides or hydroxyl groups of marine clays. In the presence