Experimental investigation of plant uptake of caesium from soils amended with clinoptilolite and calcium carbonate

Radioactive caesium (137Cs), from the Chernobyl accident of 1986, has unexpectedly remained in a bioavailable form in upland, sheep-grazing soils of Great Britain. As a potential remedial measure, clinoptilolite was tested in a greenhouse pot experiment for its effectiveness in selectively taking up Cs from two British soils; a lowland loam (Denbigh 1 brown earth) and an upland peat (Crowdy 2 series)Rye-grass, (Lolium perenne, Arno variety) that was grown on 10% (by weight) clinoptilolite-treated soils containing up to 40 mg kg-1 added Cs, resulted in grass leaf tissue Cs concentrations below 30 mg Cs kg-1 grass in all cases. Where no clinoptilolite had been added Cs in grass leaf-tissue reached 1860 mg kg-1 (grown on peat) and 150 mg kg-1 (grown on loam) at added Cs levels of 40 mg kg-1 soil. The addition of calcium carbonate (as ground limestone) to the Cs-treated, clinoptilolite-free peat soil enhanced the grass concentration of Cs by approximately 5 times, but this effect was not observed with the concentration of Cs in grass grown from loam soils with the same treatments. Compositions of clinoptilolite grains subsequently separated from the experimental soils ranged from 7.5 to 9.0 mg Cs kg-1 for the control soils (no added Cs). Soils treated with 40 mg Cs kg-1 gave Cs concentrations in clinoptilolite of 155-170 mg kg-1 (separated from loam) and 347-354 mg kg-1 (separated from peat).     

The influence of organic soil amendments on sulfate adsorption and sulfur availability in a Brazilian Oxisol

Soil management practices that involve additions of organic materials may influence plant sulfur availability in highly-weathered, acid soils. This study evaluated the effects of organic additions on sulfate adsorption and sulfur availability in a limed (3,4 t ha^-1) and unlimed Typic Haplustox soil of the Cerrado Region of Brazil. In unlimed soil, the proportion of applied sulfate (600 kg S ha^-1 as gypsum) that was adsorbed temporarily decreased over two cropping seasons by incorporation of 10 t dry matter ha^-1 crop^-1 of guinea grass (Panicum maximum Jacq.) but not when a similar quantity of a tropical legume, feijâo de porco (Canavalia ensiformis L.), was added. Liming reduced sulfate adsorption and resulted in sulfate leaching to a depth of 30 to 45 cm. Both plant materials temporarily reduced sulfate adsorption in laboratory studies when added to an unlimed soil at a rate equivalent to 40 t ha^-1. Analysis of soil properties affected by organic additions and liming showed significant correlations between sulfate adsorption and soil pH, extractable aluminum, calcium and magnesium, and surface charge. Maize dry matter yields increased by 1.3 to 3.5 t ha^-1 with addition of both organic materials. However, only the feijâo de porco treatment resulted in increases in sulfur uptake for the years in which organic materials were applied. Determining the effects of organic material additions on plant sulfur availability is complicated by the combined effects of sulfur mineralization, sulfate adsorption, and the plant's ability to utilize adsorbed subsoil sulfate.Joint contribution of Cornell University and CPAC-EM- BRAPA. This research was supported by USAID through the Title XII CRSP subgrant SM-CRSP-10 from North Carolina State University     

Extraction of caesium from mosses and incorporation into an oxalate crystalline lattice

The ratios of stable and radioactive isotopes in the environment are not constant and their effects on living organisms are not the same. Whilst radionuclides can be monitored by measuring their radioactivity levels, it is also important to determine the stable isotopes. The method described in this paper represents a simple and cheap process by which to extract caesium from moss, without significant destruction of the plant, and to incorporate the isolated caesium into a water-soluble crystal. Ammonium oxalate and phosphoric acid are water-soluble substances at 5% concentration but, when the solutions are combined, crystallisation occurs. Crystals were estimated to contain 68.30% O, 20.98% C, 4.68% H, 6.04% N and less than 0.01% P, corresponding to NH4HC2O4 x H2C2O4 x 2H2O (C4H11NO10). The crystal mass did not vary with the species of moss, although the effectiveness of caesium extraction did. Typically, 73.0 +/- 6.1% of the solubilised caesium was entrapped within the crystals.     

Spectroscopic properties of bulk and dichromate oxidation resistant soil organic matter from an anthroposequence in a Mediterranean environment

The shallow soils of Pianosa—a small calcareous island in the Mediterranean Sea, facing Tuscany, central Italy—developed under the same pedogenic factors with the exception of vegetation and, in the last two centuries, human impact. Originally, they were covered by “macchia,” a tangled mix of bushy species, or, in a few more fertile sites, by Quercus ilex L. Currently there are six land uses on the island: (1) pristine macchia, (2) groves of Olea europaea L. totally reinvaded by macchia, (3) degraded residual thickets of Q. ilex, (4) cropland, (5) recently abandoned pastures, and (6) stands of Pinus halepensis Mill. The related soils were analysed by solid-state cross polarisation/magic angle spinning ¹³C-nuclear magnetic resonance (NMR) to compare the spectroscopic properties of their organic pool. The prevalence of O-alkyl C over other C forms is evident in all cases, while there is large variability with respect to the alkyl C to aromatic C ratio between both the different soils, and the horizons of a same soil. On the whole, the results show that on Pianosa Island, human impact has profoundly altered not only the amount but also the quality of soil organic matter (SOM). The SOM residual of the dichromate oxidation in the Walkley–Black method—a widely used method to measure soil organic C—was also analysed by NMR. It comprised almost exclusively alkyl C and aromatic C, as a result of the removal of carbonyl and O-alkyl C. Most of the aromatic C seems to belong to charcoal.     

Iron oxide solubilization by organic matter and its effect on iron availability

The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.The most important influence that organic matter has on the solubilization of Fe is through reduction. Respiration of organic matter creates reduction microsites in soil where Fe²⁺ concentrations increase above those of the Fe(III) hydrolysis species. Fluctuating redox conditions in these microsites are conducive to the formation of a mixed valency ferrosic hydroxide. This metastable precipitate maintains an elevated level of soluble inorganic Fe for prolonged periods and increases Fe availability to plants. The release of reducing agents and acids next to roots, as well as the production of siderophores by microorganisms within the rhizosphere, contribute to the solubilization and increased availability of Fe to plants.     

Decomposition of cellulose,soil organic matter and plant litter in a temperate grassland soil

The formation of mor humus in an experimental grassland plot, which has been acidified by long-term fertiliser treatment, has been studied by comparing the rates of cellulose, soil organic matter and plant litter decay with those in an adjacent plot with near-neutral pH and mull humus. The decomposition of cellulose filter paper in litter bags of 5 mm, 1-mm and 45-μm mesh size buried at 3 to 4 cm depth the plots was followed by measuring the weight loss and changes in glucose content over a 6 month period. Soil pH was either 5.3 or 4.3. Decomposition of native soil organic matter and plant litter in soil from the same plots were followed using CO₂ evolution in laboratory microcosms. Cellulose weight loss at pH 5.3 was greatest from the 5-mm mesh bags and least from the 45-um mesh bags. At pH 4.3 there was little weight loss from bags and no significant differences in weight loss between bags with different sized mesh. There was, however, a reduction in the glucose content of the hydrolysed and derivatised filter paper with time. The decomposition rate of native soil organic matter in the low pH soil was increased to that observed in the less acid soil when the pH of the former was increased from 4.3 to 5.3. The increase in decomposition rate of added plant litter in the more acid soil as a result of CA(OH)₂ addition was only 60% of that observed in the soil with pH 5.3. These data support the hypothesis that the absence of soil animals and the restricted microbial decomposition in the acidic soil was responsible for mor humus formation.     

Phosphorus sorption and extractability in Andic soil incubated with plant residues of variable P content

Sodium acetate extractable P and P sorption were measured in a high P-sorbing Andept soil (Mission series) following incubation with Winter wheat (Triticum aestivum (L.)Nugaines) and red clover (Trifolium pratense (L.)Florex) plant residue. The effects of plant residue P content, moisture regime, and quantity added were evaluated over a twelve week incubation period. Increased extractable P and decreased P sorption occured when the P content of residue exceeded 0.1% P for both winter wheat and red clover. The extractable P decreased and the P sorbed increased for all treatments and the check as the incubation time progressed. Extractable P increased and P sorption decreased more under air dry moisture conditions than at –0.03 MPa or saturation. Extractable P levels increased and P sorption decreased with increasing amounts of added residue. The effects of the residue additions decreased with increasing incubation time.     

Relationship between availability indices and plant uptake of nitrogen and phosphorus from organic products

A better knowledge of the plant-availability of nitrogen (N) and phosphorus (P) in organic products may help to improve the efficient use of these products as fertilizers. In the present study, availability indices for N and P of nine widely differing organic products obtained by different fractionation methods were compared with the plant uptake of N and P from these products. The fractionation methods included CaCl₂ extraction, thermal fractionation (heating of organic products), and pepsin extraction, for N, and extraction with diluted sulphuric acid, P-Bray-I, P-Olsen, and extraction using an iron oxide coated filter paper, for P. The results of pot experiments with ryegrass using a double-pot technique (Janssen, 1990) over 62 (N experiment) and 93 days (P experiment) were used as reference for plant-availabe N and P. The 0.01 M CaCl₂ extractable inorganic N reasonably predicted plant-available N only in organic products with a high inorganic N fraction. Thermal fractionation and pepsin extraction provided a reasonable index for mineralizable N in organic products having a high fraction of mineralizable N. Of the P fractionation methods, the extraction using iron oxide coated filter paper was the best indicator of plant-available P in the products.     

Effects of substrate availability on the temperature sensitivity of soil organic matter decomposition

Soil carbon is a major component in the global carbon cycle. Understanding the relationship between environmental changes and rates of soil respiration is critical for projecting changes in soil carbon fluxes in a changing climate. Although significant attention has been focused on the temperature sensitivity of soil organic matter decomposition, the factors that affect this temperature sensitivity are still debated. In this study, we examined the effects of substrate availability on the temperature sensitivity of soil respiration in several different kinds of soils. We found that increased substrate availability had a significant positive effect on temperature sensitivity, as measured by soil Q ₁₀ values, and that this effect was inversely proportional to original substrate availability. This observation can be explained if decomposition follows Michaelis–Menten kinetics. The simple Q ₁₀ model was most appropriate in soils with high substrate availability.     

Concentration and composition of dissolved organic carbon in streams in relation to catchment soil properties

Two adjacent catchments in the Otway Ranges of Victoria, Australia (Redwater and Clearwater) produce water with markedly different concentrations of dissolved organic carbon (DOC) during summer. Water from Redwater Creek had a DOC concentration of 32 mg L^–1, while water from Clearwater Creek had a DOC concentration of 3.8 mg L^–1. Examination of the catchments revealed that while climate, topography, vegetation and land use were similar, the soils were different. The objective of this study was to examine the relationship between the concentration and chemical composition of DOC in stream waters and the nature of soils in the two catchments. Soil mapping determined that clayey soils formed on Cretaceous sediments (Cretaceous soils) occurred throughout both catchments, but that Redwater Catchment also contained a large area (39%) of sandy soils formed on Tertiary sediments (Tertiary soils). The concentration of DOC in forest floor leachate was high in both the Tertiary and Cretaceous areas; however, the concentration of DOC in water draining areas dominated by Tertiary soils was greater than that in water draining areas dominated by Cretaceous soils. Laboratory experiments showed that the Cretaceous soils had higher adsorption capacities for forest floor leachate DOC than the Tertiary soils. The difference in DOC concentrations of the streams was therefore attributed to the difference in adsorption capacity of catchment soils for DOC. Adsorption capacities of the soils were found to be a function of their clay contents and specific surface areas.Solid-state³C nuclear magnetic resonance spectroscopy and pyrolysis-mass spectrometry were used to determine the chemical structure of DOC found in streams and forest floor leachate samples and that remaining in solution after interaction with soil. Chemistry of DOC in forest floor leachate was similar before and after interaction with soil, indicating no preferential adsorption of a particular type of carbon. Thus, differences between the chemical structure of stream DOC and forest floor leachate DOC could be attributed to microbial modifications during its movement through soils and into the streams, rather than losses by adsorption.     

Water availability controls microbial temperature responses in frozen soil CO2 production

Soil processes in high-latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO₂ production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO₂ production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO₂ temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO₂ production at low temperatures is crucial for a thorough understanding of low-temperature soil processes and for accurate predictions of C-balances in northern terrestrial ecosystems.     

Long-term tillage effects on soil organic carbon and dissolved organic carbon in a purple paddy soil of Southwest China

The impact of conservation tillage practices on soil carbon has been of great interest in recent years. Conservation tillage might have the potential to enhance soil carbon accumulation and alter the depth distribution of soil carbon compared to conventional tillage based systems. Changes in the soil organic carbon (SOC) as influenced by tillage, are more noticeable under long-term rather than short-term tillage practices. The objective of this study was to determine the impacts of long-term tillage on SOC and dissolved organic carbon (DOC) status after 19 years of four tillage treatments in a Hydragric Anthrosol. In this experiment four tillage systems included conventional tillage with rotation of rice and winter fallow system (CTF), conventional tillage with rotation of rice and rape system (CTR), no-till and ridge culture with rotation of rice and rape system (NT) and tillage and ridge culture with rotation of rice and rape system (TR). Soils were sampled in the spring of 2009 and sectioned into 0–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm depth, respectively.Tillage effect on SOC was observed, and SOC concentrations were much larger under NT than the other three tillage methods in all soil depths from 0 to 60 cm. The mean SOC concentration at 0–60 cm soil depth followed the sequence: NT (22.74 g kg^?1) > CTF (14.57 g kg^?1) > TR (13.10 g kg^?1) > CTR (11.92 g kg^?1). SOC concentrations under NT were significantly higher than TR and CTR (P < 0.01), and higher than CTF treatment (P < 0.05). The SOC storage was calculated on equivalent soil mass basis. Results showed that the highest SOC storage at 0–60 cm depth presented in NT, which was 158.52 Mg C ha^?1, followed by CTF (106.74 Mg C ha^?1), TR (93.11 Mg C ha^?1) and CTR (88.60 Mg C ha^?1). Compared with conventional tillage (CTF), the total SOC storage in NT increased by 48.51%, but decreased by 16.99% and 12.77% under CTR and TR treatments, respectively. The effect of tillage on DOC was significant at 0–10 cm soil layer, and DOC concentration was much higher under CTF than the other three treatments (P < 0.01). Throughout 0–60 cm soil depth, DOC concentrations were 32.92, 32.63, 26.79 and 22.10 mg kg^?1 under NT, CTF, CTR and TR, and the differences among the four treatments were not significant (P > 0.05). In conclusion, NT increased SOC concentration and storage compared to conventional tillage operation but not for DOC.     

Responses of soil biota and nitrogen availability to an invasive plant under aboveground herbivory

Plant and Soil - Recently, much attention has been paid to the plant-mediated effects of aboveground herbivory on soil ecosystems. However, studies about the herbivore-induced effects of invasive...     

Implications of soil organic carbon and the biogeochemistry of iron and aluminum on soil phosphorus distribution in flooded forests of the lower Orinoco River,Venezuela

Relationships among soil phosphorus distribution, soil organic carbon and biogeochemistry of iron and aluminum were studied along a flooded forest gradient of the Mapire river, Venezuela. Soil samples were collected during the dry season in three zones subjected to different flooding intensity: MAX inundated 8 months per year, MED inundated 5 months per year, and MIN inundated 2 months per year. Total labile phosphorus (resin + bicarbonate extractable fractions) was significantly higher in MIN than in MAX. The longer non-flooding period in MIN probably allowed a higher accumulation of microbial biomass in soils of this zone and consequently a greater release of the bicarbonate organic fraction. The moderately labile phosphorus fraction associated with the chemisorbed phosphorus on amorphous and some crystalline aluminum and iron was significantly lower in MAX than in MIN following the same tendency observed for crystalline iron oxides. This result allowed us to hypothesize that the combined effect of a long flood period and a high soil organic carbon content in the MAX, could be appropriate conditions for microbial reduction of stable forms of iron. The ratio of soil organic carbon to total organic phosphorus decreased from MAX to MIN, indicating higher mineralization of organic phosphorus in MIN. Our results suggests two distinct flood-dependent mechanisms operating for phosphorus release along the gradient. In MAX mineralization process appears to be limited, while microbial mineral dissolution appears to be an important source of phosphorus. In MIN supply of phosphorus is associated with the stability of soil organic matter.     

光同化产物在裙带菜（Undaria pinnatifida）中的运转

本文报告了用１４Ｃ标记的光同化产物在裙带菜［Ｕｎｄａｒｉａｐｉｎｎａｔｉｆｉｄａ（Ｈａｒｙ．）Ｓｕｒ］孢子体中的运转现象。观察到光同化产物约需２０ｍｉｎ才能从叶片表皮进入中肋的髓部。在自然条件下,光同化产物主要自叶片梢部经中肋向生长部运转,在叶片梢部和生长部之间存在明显的源一库关系。运转速度约为４２～４８ｃｍ／ｈ。用组织放射自显影的方法证实了髓部的喇叭丝是主要的运转组织。向下运转的溶于酒精的光同化产物中,甘露醇占５０％以上。还在光同化产物中观察到游离的谷氨酸、天门冬氨酸和丙氨酸。     

Physiological response of Epipremnum aureum for cobalt-60 and cesium-137 translocation and rhizofiltration

In this study, three separate experiments were carried out to explore the potential of Epipremnum plants for Co-60 and Cs-137 uptake and rhizofiltration from solutions. Experiment I was carried out to screen the effects of different concentration (0-10 mM) of stable Co and Cs salts on some physiological components of the Epipremnum over 20 d. Data from the experiment showed that 1 mM of either Co or Cs had no adverse effects on the chlorophyll, soluble sugar, and protein of Epipremnum. Over 20 d at a high concentration (10 mM), some effects of Co or Cs were detectable but were not inhibitory. Experiment H showed an increase in the concentration ratios (CRs) of carrier-free Co-60 and Cs-137 relative to their concentration in the solution. Transport indexes (TIs) of Co-60 only increased with increasing its concentration in the solution. CRs increased with time and more than 50% of Co-60 and Cs-137 was detected at 5 d. Based on the data obtained from experiments I and II, both stable Co and Cs were used as carriers for Co-60 and Cs-137 to study their uptake and translocation in Experiment III. It was found that the Epipremnum plant had bioconcentration factors (BCFs) of 10.69 and 2.26 for Co and Cs, respectively. However, TI was 13.8 for Co and 35.6 for Cs. The accumulation of Co-60 and Cs-137 in the roots of Epipremnum might offer a method for Co-60 and Cs-137 rhizofiltration.     

Metabolic rates of tsetse flies in the field as measured by the excretion of injected caesium

ABSTRACT Teneral tsetse flies, Glossina morsitans morsitans Westw., were injected with labelled caesium (¹³⁷Cs) <18 h after emergence and released in the Zambezi Valley of Zimbabwe between May 1983 and June 1984, and again in February 1985. Radioactivity in flies recaptured time t days after injection indicated a three-stage exponential loss of caesium, identical for both sexes. For t≫4 the estimated rate constant (-0.119 per day) was significantly lower than for 4≫t≫12 (–0.252 per day). By day 15 about 97% of the isotope had been excreted; thereafter the loss rate fell by an order of magnitude. The data for t>4 days were well fitted by the sum of two exponentials but no smooth function was found to fit all three phases. The loss rate from the rapidly metabolized pool increased exponentially with temperature at the same rate as for male tsetse kept in the dark in the laboratory. However, the loss rate in the field was lower at every temperature, suggesting that these flies live at 2–6^oC lower than the average Stevenson screen temperature. Published estimates of hunger cycle and daily flight durations, made on the basis of measured rates of caesium excretion, are invalid because they use the assumption that flies are living in the field at screen temperatures. The data suggest that both sexes have the same metabolic rate up to the age of about 15 days, which implies that the females (being larger and having to nourish a larva in the latter stages of this period) must be less active and/or live at even lower temperatures than the males.     

Factors of plant nutrient availability relevant to soil testing

Summary In most arable soils the nitrate availability depends mainly on the quantity of nitrate present in the rooting zone at the beginning of the growing season. Easily mineralizable organic N and the release of non-exchangeable NH₄ from clay minerals may in addition control the nitrogen availability during a season. In flooded soils, ammonium is the major form of nitrogen absorbed by plants. Ammonium dynamics in these soils is similar to that of potassium. The availability of both is controlled mainly by the intensity and buffering power for ammonium or potassium, respectively. Basically, intensity of the supply and buffering power for phosphate are the main factors determining the phosphate availability. The determination of the phosphate buffer power, especially in the root zone, however, remains to be difficult. Soil test methods should take into consideration the major factors and processes relevant to the availability of a particular plant nutrient.     

Assessment of methylammonium as an analog for ammonium in plant uptake from soil

We tested radioactive methylammonium (¹⁴CH₃NH _inf3 ^sup+ ) as a tracer for ammonium (NH₄ ⁺) in root uptake measurements from soil. Tomato (Lycopersicon esculentum Moll. cv T5) in 3 L pots filled with loamy sand soil received 40, 200, or 600 μmol ¹⁴CH₃NH₃ ⁺ or ¹⁵NH₄ ⁺. During a 4 h period, the plants absorbed ¹⁴CH₃NH₃ ⁺ at slower rates than ¹⁵NH₄ ⁺. Estimates of NH₄ ⁺ absorption based on ¹⁵NH₄ ⁺ absorption were 0.9–7.9 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹, whereas those based on ¹⁴CH₃NH₃ ⁺ absorption were 0.2–1.0 μmol NH₄ ⁺ g⁻¹ plant dry weight h⁻¹. After 4 h, approximately one-half of the applied ¹⁵N was not recovered in the plants or soil KCl extracts; apparently, this ¹⁵N was either immobilized or nitrified and denitrified by soil biota. By contrast, almost all the ¹⁴CH₃NH₃ ⁺ remained in the soil solution after 4 h, but after a 10 d incubation, approximately 20% had been released as ¹⁴CO₂. These differences in plant absorption rates and movement through soil pools indicate that CH₃NH₃ ⁺ cannot be used reliably as an NH₄ ⁺ analog in soil.     

Extractable sulphate and organic sulphur in soils and their availability to plants

Ten soils collected from the major arable areas in Britain were used to assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extractable inorganic SO₄-S and total soluble S(SO₄-S plus a fraction of organic S) were determined using ion chromatography (IC) or inductively-coupled plasma atomic emission spectrometry (ICP-AES), respectively. Water, 0.016 M KH₂PO₄, 0.01 M CaCl₂ and 0.01 M Ca(H₂PO₄)₂ extracted similar amounts of SO₄-S, as measured by IC, which were consistently smaller than the total extractable S as measured by ICP-AES. The amounts of organic S extracted varied widely between different extractants, with 0.5 M NaHCO₃ (pH 8.5) giving the largest amounts and 0.01 M CaCl₂ the least. Organic S accounted for approximately 30–60% of total S extracted with 0.016 M KH₂PO₄ and the organic C:S ratios in this extract varied typically between 50 and 70. The concentrations of this S fraction decreased in all soils without added S after two months growth of spring wheat, indicating a release of organic S through mineralisation. All methods tested except 0.5 M NaHCO₃-ICP-AES produced satisfactory results in the regression with plant dry matter response and S uptake in the pot experiment. In general, 0.016 M KH₂PO₄ appeared to be the best extractant and this extraction followed by ICP-AES determination was considered to be a good method to standardise on.