Mehlich‐1‐ and DTPA‐extractable lead in soils in relation to soil properties

Abstract

Mehlich‐1 and DTPA extractants are frequently used to predict metal availability in soils. Metal extractability by the acid or chelate extractant reflects the metal characteristics and metal‐soil interactions. In this study, samples of eight topsoils from the southeastern United States were incubated with added lead (Pb) at the rate of 40 mg#lbkg^‐1. After five months in the greenhouse, Mehlich‐1 and DTPA extractants were employed to extract Pb in both metal‐amended and natural soils. For the natural soils, Pb concentration in the DTPA extractant was always higher than that in the Mehlich‐1 extractant. This indicates that the DTPA chelate extractant is able to dissolve some Pb in soils which is not solubilized by protons. The negative correlation found between Mehlich‐1‐extractable Pb and soil clay content might result from two mechanisms: i) strong association between Pb and soil surfaces, or ii) readsorption of Pb during extraction. None of the correlations between DTPA‐extractable Pb and soil properties was significant, suggesting that the DTPA‐extractable Pb is not heavily dependent on soil properties. The DTPA extractant showed a high ability to solubilize Pb in the natural soils possibly due to a high affinity of Pb for soil organic matter.     

Aluminum and iron estimated by Mehlich‐3 extractant in mine soils in Galicia,northwest Spain

Abstract

The efficiency of Mehlich‐3 reagent as an extractant for aluminum (Al) and iron (Fe) was studied in Galician coal mine soils, in the process of reclamation. Mehlich‐3 Al and Fe values were compared to those from other Al and Fe tests and with phosphorus (P) sorption. The soils are very heterogeneous, consisting mainly of carbonaceous and non‐carbonaceous clays and shales, which are often rich in pyrite. Some of them have been amended with topsoil or fly ash. One hundred forty samples, from 0 to 15 or 15 to 30 cm, were analyzed. The pH values ranged from 2.7 to 8.4; oxalate and pyrophosphate Al from 0 to 1.17%, and 0.02 to 0.58%, respectively; and oxalate and pyrophosphate Fe from 0.02 to 2.25% and 0 to 1.28%, respectively; PSI (P sorption index) values varied between 0 and 55.1. The Mehlich‐3 Al values ranged from 24 to 2600 mg kg^‐1. A close relationship was observed between Mehlich‐3 and oxalate Al values (r=0.77), although the regression line tended to be curvilinear. Mehlich‐3 Al was better correlated than oxalate Al to pyrophosphate Al (r=0.66 vs. r=0.59) and also to pH‐NaF (r=0.89 vs. r=0.74). The Mehlich‐3 Al is almost as good as oxalate Al in estimating non‐crystalline Al, the correlation coefficients between log PSI and log (Mehlich‐3 Al) or log (oxalate Al) being 0.51 and 0.57, respectively. The Mehlich‐3 Fe correlated to available (r=0.63), exchangeable (r=0.65) and soluble Fe (r=0.66), but not to non‐crystalline Fe.     

Effect of High‐Molybdenum Compost on Soils and the Growth of Lettuce and Barley

A pot experiment evaluated the growth of lettuce (Lactuca sativa L.) and barley (Hordeum vulgar) and accumulation of molybdenum (Mo) in plants and soils following amendments of Mo compost (1.0 g kg^?1) to a Truro sandy loam. The treatments consisted of 0 (control), 12.5, 25, and 50% Mo compost by volume. The Mo compost did not affect dry‐matter yield (DMY) up to 25% compost, but DMY decreased at the 50% compost treatment. The 50% compost treatments increased the soil pH an average of 0.5 units and increased the nitric acid (HNO₃)–extractable Mo to 150 mg kg^?1 and diethylenetriaminepentaacetic acid (DTPA)–extractable Mo to 100 mg kg^?1 in the growth medium; the same treatment increased tissue Mo concentration to 569 and 478 mg kg^?1 in the lettuce and barley, respectively. Plants grown in the 25% compost produced about 55 mg kg^?1 of total Mo in the growth medium; this resulted in tissue Mo concentration of 348 mg kg^?1 in lettuce and 274 mg kg^?1 in barley without any phytotoxicity. Our results suggested that 55 mg Mo kg^?1 soil would be an appropriate limit for Mo loading of soil developed from compost additions, a value which is presently greater than the Canadian Council for Ministers of the Environment (CCME) Guidelines for the use of type B compost in Canada.     

Comparison of Mehlich 3‐ and bray 1‐extractable phosphorus levels in a Starr clay loam amended with poultry litter

Abstract

Two soil tests commonly used to characterize the availability of soil phosphorus (P) are Bray P1 (B1) and Mehlich 3 (M3) extradants. The objective of this investigation was to compare M3‐ and B1‐extractable P levels in a Starr clay loam (fine‐loamy mixed thermic Fluventic Dystrochrepts) amended with relatively low to high surface applications of poultry litter (PL). The following eight treatments were applied to the soil for pasture renovation in 1991 and 1992: 1) a control, 2) P application as monocalcium phosphate [Ca(H₂PO₄)₂‐H₂O], 3) P application as Ca(H₂PO₄)₂‐H₂O and nitrogen (N) application as ammonium nitrate (NH₄NO₃) and urea, and 4) five levels of poultry litter (PL) based on N content. The five P levels as PL were 56.6, 113.2, 169.8, 226.4, and 283.0 kg ha^‐1 in 1991, and 49.2, 98.5, 147.7, 196.9, and 246.1 kg ha^‐1 in 1992, respectively. Soil samples were obtained from the 0‐ to 5‐cm layer in the spring of 1992 and 1993 for extractable P determination. Levels of P extracted from all treatments by the M3 and B1 extractants were linearly correlated both years (r²=0.96 in 1991 and r²=0.99 in 1992). However, M3 extracted more P from the PL treatments, whereas B1 extracted more P from the control, P application, and N and P application treatments. Curvilinear relationships were obtained between P uptake and levels of either M3‐ or B1‐extractable P for the PL treatments (r²=0.713 for M3 and 0.663 for B1 in 1991 and r²=0.925 for M3 and 0.933 for B1 in 1992). These close relationships in 1992 between extractable P and uptake of P for the PL‐ treated soils indicate that both the B1 and M3 extractants could be used to evaluate excess P in PL‐amended soils.     

Molybdenum and Copper Uptake by Forage Grasses and Legumes Grown on a Metal‐Contaminated Sludge Site

Abstract

Wastes applied to agricultural land can contain significant concentrations of bioavailable molybdenum (Mo). Because Mo uptake by forage crops could lead to hypocuprosis in ruminants, more knowledge is needed about which crops are most efficient in accumulating Mo. At an old sewage sludge‐amended site, the concentrations of Mo, copper (Cu), and several other trace metals were measured in various grass species. Generally, the grasses grown on the sludge site contained higher Mo concentrations than the same species grown on a nearby control site. However, because Cu concentrations were also higher in the sludge‐grown grasses, Cu:Mo ratios in the grasses were frequently higher on the sludge site. In contrast, all legumes tested (alfalfa, birdsfoot trefoil, red clover, pea), as well as canola and beets, had lower Cu:Mo ratios when grown on the sludge site. Sulfur concentrations in the two crops analyzed for this element (canola and pea) were higher on the sludge site than the control. It is concluded that Mo, Cu, and sulfur (S) bioavailability remains elevated in the soil several decades after sewage sludge application.     

Effects of calcium lignosulphonate on Mehlich‐III extractable calcium,magnesium, and potassium quantity‐intensity relationship in the soil with or without potassium dihydrogen phosphate

Abstract

Calcium lignosulphonate (CaLS), a waste product from the pulp and paper industry, is expected to affect reaction of K fertilizer in the soil, thus influences their availability to crops. A clay soil (Typic Humaquept) was incubated with various amounts of CaLS (0 to 150 g kg^‐1 soil) and potassium dihydrogen phosphate (0–25.64 mmol kg^‐1 soil) for 240 h under moist conditions at 5 and 15°C. Subsamples were extracted with deionized water and the Mehlich‐III (M3) solution for the analyses of pH, and P, Ca, K and Mg concentrations and K adsorption (K_ad). Higher temperature reduced M3 extractable K (K_M3). CaLS and K additions increased M3 extractable Mg (Mg_M3) with Ca contributed more than did K as indicated by the standardized estimates. Additions of CaLS increased K_M3. Potassium adsorption decreased with the increases in CaLS addition rates. Significant positive linear relationships were observed between K_ad and the concentration ratio of [K⁺]/[Ca²⁺]^½, suggesting that the potassium buffering capacity of the soil was reduced by the additions of CaLS, with the desugared CaLS being more effective than the non‐desugared CaLS. The increased slope values with the increases in CaLS additions of the linear relationships between K_M3 and [K]/[Ca²⁺]^½ indicated that CaLS improved the quantity and intensity relationships and increased the power of the soil supplying plants with K.     

Metal‐Associated Forms and Speciation in Biosolid‐Amended Oxisols

Abstract

The objective of this study was to determine the effects of pH and ionic strength on the distribution and speciation of zinc (Zn), copper (Cu), and cadmium (Cd) in surface soil samples from two Brazilian Oxisols amended with biosolids. Soils and biosolids were equilibrated in an experimental dual‐chamber diffusion apparatus that permits the soils and biosolids to react through a solution phase via diffusion across a membrane. After equilibrium was reached, soil and biosolids samples were sequentially fractionated to identify various solid forms of Zn, Cu, and Cd. Metal concentrations in the solution phase were determined and mass balance calculated. Equilibrating pH had no major effect on Cu solubility from biosolids and, at pH range from 4 to 7, most Cu remained in the biosolids. Soluble Zn and Cd concentration increased with decreasing pH because of the increased solubility of the biosolids. Copper and Zn were primarily associated with the residual fraction and Fe oxides in one soil, but were primarily associated with chemically unstable fractions, or adsorbed to the surface of oxides, in the other soil. In both soils, Cd was primarily associated with readily bioavailable fractions. The effect of pH on the metal distribution was more evident than the ionic strength effect. Free ions were the predominant metal species in solution, especially at lower pH values.     

Seasonal growth and composition and accumulation of N‐P‐K in dryland and irrigated pumpkins

Growth and N‐P‐K uptake in pumpkin (Curcubita moschata Poir.) cv ‘Libby‐Select’ were studied in dryland and irrigated culture. In both moisture regimes, maximum rates of dry matter accumulation occurred between the early and mid‐fruiting developmental stages. Higher total dry matter production with irrigated than dryland culture was primarily associated with increased shoot growth. Concentrations of N, P, and K in foliage generally decreased as pumpkin age increased. Irrigated pumpkins in conjunction with higher total vegetative dry matter accumulated more N, P, and K than dryland pumpkins. Up through early fruit development, N, P, and K accumulation was primarily in leaves and vines and by the late growth stages was almost entirely in the fruit. Total N, P, and K uptake at late fruiting was estimated at 219, 32, and 228 kg/ha in irrigated pumpkins and 180, 21, and 177 kg/ha in dryland pumpkins. Approximately 58% of the N, 52% of the K, and 68% of the P accumulated by late‐fruiting was absorbed by the plant after the early‐fruiting stage in both moisture regimes. Potassium redistribution from vegetative tissues during late fruit development decreased foliar K contents 32% in dryland pumpkins and 21% in irrigated pumpkins.     

Sodium bicarbonate‐DTPA test for macro‐and micronutrient elements in soils

Abstract

Individual soil tests are used to assess plant nutrient element needs. Separate soil tests, however, are time consuming and costly. Our objective was to develop a 0.5M sodium bicarbonate (NaHCO₃) soil phosphorus (P) test in combination with 0.005M diethylenetriaminepentaacetic acid (DTPA) so macronutrient dements: ammonium‐nitrogen (NH₄‐N), nitrate‐nitrogen (NO₃‐N), P, potassium (K), calcium (Ca), and magnesium (Mg); and micronutrients: iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) could be quantified in one extraction. The NaHCO₃‐DTPA extracting solution is a combination of 0.5M NaHCO₃ and 0.005M DTPA and has a pH of 7.60±0.05. Sodium in the solution enhances the NH₄, K, Ca, and Mg extraction; bicarbonate (HCO₃) is for P extraction; DTPA chelates Ca, Mg, and micronutrients; and the water is for NO₃ extraction. Soil samples (0–15 cm depth) came from two sources. The first set was from 12 N x P dryland proso millet (Panicum miliaceum L.) experiments, conducted from 1985 through 1987 in eastern Colorado. These soils were extracted with potassium chloride (KCl), NaHCO₃, ammonium acetate (CH₃‐COONH₄), DTPA, ammonium bicarbonate DTPA (AB‐DTPA), and with the NaHCO₃‐DTPA solutions. The second set included 25 soils from Alabama, Georgia, North Carolina, and South Carolina and were analyzed only for available P with the NaHCO₃ and NaHCO₃‐DTPA methods. Simple linear correlations for macronutrient elements and micronutrients were highly significant. Critical levels for the macronutrient elements: NO₃‐N, P, and K were 27, 11, and 144 mg kg^‐1, respectively; and the critical levels for the micronutrients: Fe, Mn, Zn, and Cu were 3.9, 0.35, 0.97, and 0.24 mg kg^‐1, respectively.     

Comparing tests for soil fertility. III. Evaluation of phosphate availability in Alfisols by Olsen,Mehlich 3, electro‐ultrafiltration,and the paper‐strip methodology procedures

Abstract

The suitability of the “iron‐impregnated paper‐strip”; (Pi) methodology for the assessment of phosphate (P) availability in soils has been tested on the basis of a comparison with the classical Olsen method as well as with the new‐generation electro‐ultrafiltration (EUF) and Mehlich No. 3 (M3) extraction procedure. The EUF extractions were performed by a first 30 min run at low (L) energy conditions (20°C, max 15 mA, and 200V), followed by a 10 min run at high (H) energy conditions (80°C, max 150 mA, and 400V). The total EUF‐P extracted (T‐EUF‐P), given by the sum L‐EUF‐P + H‐EUF‐P, was also considered. The investigation was carried out on twenty samples of representative Alfisols ("Terra Rossa") from the pedoclimatic environments of the Mediterranean area. The soil samples were characterized by a large variability of their available‐P values, whose ranges were, as mg‐kg^‐1 soil, 0.1–34.0 for L‐EUF‐P, 0.2–39.3 for HEUF‐P, 0.3–73.3 for T‐EUF‐P, 0.8–113.6 for Olsen‐P, 1.0–122.4 for Pi‐P, 1.1–224.6 for M3‐P. The respective mean values were 4.6, 5.1, 9.6, 17.2, 20.1, and 29.7 mg P/kg soil, with ratios L‐EUF‐P:H‐EUF‐P:T‐EUF‐P:Olsen‐P:Pi‐P:M3‐P increasing as 0.27:0.29:0.56: 1.00:1.17:1.73. A good compliance among all methods was verified; the most significant correlations were determined for Pi‐P versus M3‐P (R²=0.993***) and for Pi‐P versus Olsen‐P (R²=0.988***), clearly indicating the liability of the Pi procedure for the assessment of the P fertility in Alfisols. A multiple linear regression analysis revealed that all the available P values, afar from the individual procedure, were significantly (P<0.01) dependent on M3 exchangeable iron (Fe) (positively) and aluminum (Al) (negatively). This could lead one to argue that the prevailing available soil P source in the investigated Alfisol samples was that connected with the labile Fe‐P pool. Withal, one could infer that the A1‐M3 and Fe‐M3 parameters should also be considered for the making of P fertilizer recommendations. From this standpoint, the M3 is a well‐proved multielement extractant suitable for the simultaneous determination of available P and exchangeable Fe and Al as well.     

Ion interactions in calcium‐efficient and calcium‐inefficient tomato lines 1

Two Ca‐efficient and 3 Ca‐inefficient tomato lines selected on the basis of dry matter production, Ca concentrations in tissues, and severity of Ca deficiency symptoms were grown in nutrient solutions containing 6 levels of total Ca ranging from 15 to 365 mg in 70 mg increments. All lines responded to increased Ca supply by increasing in dry weight and by accumulating Ca. The critical Ca concentrations in the shoots were 0.25% and 0.40% on a dry weight basis for the efficient and inefficient lines, respectively. Concentrations of Ca, K, Mg, P, and NO₃ ^‐ were lower in shoots and except for Mg were lower in roots of efficient plants than in the inefficient plants. For all lines as more Ca was available in the media and as Ca increased in the shoots and roots, the concentrations of the nutrients other than Ca declined. The declines in concentrations of K and Mg were not due to dilution by higher dry matter production in the efficient lines relative to the inefficient ones, although the total accumulation of Ca, P, and NO₃ ^‐ did not vary with Ca supplied. Antagonism among cations may account for differences in efficiency among lines of tomato.     

Re‐analysis of the Relationship between Organic Carbon and Loss‐on‐Ignition in Soil

The suitability of loss‐on‐ignition (LOI) as an alternative to direct measurement of organic carbon (OC) has been debated for decades without resolution. The literature contains an abundance of different linear regression models to describe the LOI–OC relationship, most based on untransformed values of LOI and OC. Such regression is suspect because the variables are unable to occupy Euclidean space. Logratio transformation—based on relative rather than absolute differences—eliminates this constraint. Re‐analysis of the relationship on new and 10 previously published datasets using logratio techniques reveals that the relationship is nonlinear and that the profusion of regression models is in part a function of the range of LOI. Although LOI may offer a crude estimate of OC at high LOI levels, OC/LOI ratios when LOI is less than about 25% are too variable for reliable OC estimation, and interstudy comparisons remain dubious. Direct measurement of OC is recommended.     

Nitrogen mineralization and nitrifier activity in limed and urea‐treated soils

Abstract

The effect of liming on mineralization and soil nitrifier activity (NA) was investigated with Brookston clay (pH 5.7) and Haldimand clay (pH 4.7). Liming increased the rate of mineralization in both soils but at a rate about 4‐times greater in Haldimand clay than Brookston clay. A significant increase in N mineralization due to liming occurred in both soils only when pH was raised above 6.0. The rate of mineralization was greater than nitrification in the Haldimand soil resulting in NH₄ ⁺ accumulation. Nitrifier activity increased with liming of Brookston clay, but decreased in Haldimand clay after 15 days of incubation. There was a significant increase in nitrifier activity due to liming from 15 to 60 days in Haldimand clay. After 60 days nitrifier activity in limed treatments increased by five times over the unlimed control.

The nitrification of urea powder (1000 mg N.kg^‐1) mixed into the soil was also studied in several soils incubated at 15°C for 28 days. There was evidence up to 14 days that nitrification of urea was correlated with initial nitrifier activity. Between 14 and 28 days, other factors such as soil pH and possible ammonia toxicity in coarser textured soils as well as nitrifier activity were important. Accumulation of nitrite occurred mainly in soils with a pH above 7.0 up to 28 days especially where nitrifier population enrichment was not done.     

Effect of moist‐incubation and microbial inhibition on changes in DTPA‐extractable Mn

Abstract

Effect of incubation and microbial inhibition at field capacity level on changes in DTPA‐extractable Mn in acidic, neutral, and alkaline soils was examined. Incubation decreased and microbial inhibition increased the level of Mn in soils with high pH (>5.7) and high microbial activity. The change was apparently partially associated with microbes since the level of Mn did not change in soils with low microbial activity and/or low pH.     

Nitrogen Dynamics and Losses in Direct‐Drilled Maize Systems

Abstract

The knowledge of nitrogen (N) losses in direct‐drilling agrosystems is essential to develop strategies to increase fertilizer efficiency and to minimize environmental damage. The objectives were i) to quantify the magnitude of N volatilization and leaching simultaneously as affected by different urea fertilization rates and ii) to evaluate the capacity of these specific plant–soil systems to act as a buffer to prevent nitrate leaching. Two experiments were conducted during 2001/02 and 2002/03 growing seasons in Alberti, Argentina. The crop was direct‐drilled maize and the soil a Typic Argiudoll. Ammonia losses, N uptake by crop at flowering and harvest, grain yield, N in previous crop residues, and soil nitrate content up to 2‐m depths were determined. Nitrogen availability, soil nitrate (NO₃)‐N up to 1 m plus fertilizer N, was linearly and highly associated with crop N uptake at flowering (R²=0.93, P<0.01) and at harvest (R²=0.852, P<0.01). Around 17.5% of fertilizer N was lost by volatilization in 10 days. The obtained values of residual nitrate N up to the 150‐cm depth were associated (R²=0.960, P<0.001) with those predicted by the nitrate leaching and economic analysis package (NLEAP) model. Maize in the direct‐drilling system was able to cycle N from the previous crop residues, N from soil organic matter, and N from fertilizers with few losses.     

Nutrient Accumulation and Utilization in Rice Under Film‐Mulched and Flooded Cultivation

Abstract

In recent years, plastic film mulched cultivation has been widely used in China as a strategy to reduce irrigation water consumption and increase water use efficiency (WUE). Experiments were conducted to study nitrogen (N), phosphorus (P), and potassium (K) accumulation and utilization in flooded rice comparing plastic film mulched (PFM) and conventionally flooded cultivation (CFC) in two successive years. In comparison with the plants under CFC, the plants under PFM had significantly higher N concentration at booting stage and lower K and P concentrations at heading and maturity stages and N concentrations after booting stage. More N accumulation was found in PMC‐treated plants than in CFC‐treated plants before booting, while thereafter CFC showed a significant increase in nutrient accumulation. N, P, and K accumulation ratios were significantly larger for PFM than for CFC at early stage (before booting), but the trend was just opposite after booting. The PFM‐treated plants substantially showed smaller nutrient accumulation rate through the whole growing process relative to the CFC‐treated plants. Although PFM‐treated plants had significantly higher physiological efficiency (PE) of all three nutrients than the CFC‐treated plants, its fertilizer recovery efficiency (RE) and agronomic nutrient utilization efficiency (AE) was markedly lower due to lower biomass and grain yield. In addition, the PFM‐treated plants had lower P/N and K/N in terms of nutrient accumulation, suggesting the importance to improve the P and K availability in rice film‐mulched cultivation.     

Zinc nutrition and levels of endogenous lndole‐3‐acetic acid in radish shoots

Radish (Raphanus sativus cv. Akamaru‐Hatsukadaikon) was grown for several experiments in a glasshouse with zinc (Zn) supply in the nutrient solution. Lack of Zn resulted in stunted growth and reduced leaf of radish shoots were observed. Two‐dimensional thin layer chromatography (TLC) and gas chromatography‐mass spectrometry (GC‐MS) analysis revealed the presence of endogenous indole‐3‐acetic acid (IAA) in Zn‐deficient radish shoots. An estimate has been made of alkali‐labile (1 and 7N NaOH) IAA in Zn‐deficient radish shoots with the use of gas chromatography (GC). The level of 7N NaOH‐labile IAA (peptidic + ester + free) and IN NaOH‐labile IAA (ester + free) in Zn‐deficient radish shoots was almost the same as that of control radish shoots. These results suggest that Zn nutrition did not affect the level of endogenous IAA in radish shoots.     

Nitrogen accumulation and partitioning in hail‐damaged soybeans 1

Hail damage to an experiment that was being used to investigate nitrogen (N) nutrition of soybeans [Glycine max (L.) Merr.] with ¹⁵N methodology provided a unique opportunity to study the effects of hail damage at the R3 stage of development on N uptake and partitioning through stage R5.8. Field plots were established on a silt loam soil (Typic Hapludol 1). Severely damaged (mean 72% leaf loss) and slightly damaged (mean 26% leaf loss) soybeans were compared for total reduced N and for ¹⁵N concentration in leaflets, petioles, stems, roots, pod walls, and seeds during the 28 days following the hailstrom. The concentration of total N and of ¹⁵N in all organs in both damage treatments declined significantly after the storm, but less in green leaflets (total N), and in green leaflets, green petioles, and pod walls (¹⁵N) of severely than of slightly damaged plants. Measurements on senesced leaflets and petioles showed that the concentration of ¹⁵N also decreased to a greater extent than that of the total N in these organs. This differential loss of ¹⁵N compared with total N suggests that the ¹⁵N was in a form that was less refractory than was the bulk tissue N, and provides evidence of separate mobile pools of N in the plant. Nitrogen budgets were calculated to compare the loss of N and ¹⁵N from abscising leaflets and petioles to the N accumulation of the damaged plants during podfill. These showed that loss from the leaflets and petioles contributed only 7% of the total N accumulated by the plants between R3 and R5.8. This study has exemplified the usefulness of ¹⁵N methodology in investigations of the nutrition and physiology of soybeans suffering leaf damage by hail.     

Extraction and reverse‐phase high‐pressure liquid chromatography of gibberellic acid in soil

Abstract

Several aqueous extractants buffered above pH 6.5 were evaluated for their ability to extract gibberellic acid from a Xerollic Calciorthid. More than 90% of the GA₃ added to soil was recovered with 0.01 M KH₂PO₄, pH 7.4. Gibberellic acid in soil extracts concentrated under vacuum was determined by reverse‐phase high‐pressure liquid chromatography (HPLC) by comparing the reproducible GA₃ peak areas with those of standards prepared in the same background solution.     

What Constitutes Plant-Available Molybdenum in Sandy Acidic Soils?

Molybdenum (Mo) is critical for the function of enzymes related to nitrogen cycling. Concentrations of Mo are very low in sandy, acidic soils, and biologically available Mo is only a small fraction of the total pool. While several methods have been proposed to measure plant-available Mo, there has not been a recent comprehensive analytical study that compares soil extraction methods as predictors of plant Mo uptake. A suite of five assays [total acid microwave digestion, ethylenediamenetetraaacetic acid (EDTA) extraction, Environmental Protection Agency (EPA) protocol 3050B, ammonium oxalate extraction, and pressurized hot water] was employed, followed by the determination of soil Mo concentrations via inductively coupled mass spectroscopy. The concentrations of soil Mo determined from these assays and their relationships as predictors of plant Mo concentration were compared. The assays yielded different concentrations of Mo: total digest > EPA > ammonium oxalate ≥ EDTA > pressurized hot water. Legume foliar Mo concentrations were most strongly correlated with ammonium oxalate–extractable Mo from soils, but an oak species showed no relationship with any soil Mo fraction and foliar Mo. Bulk fine roots in the 10- to 30-cm soil horizon were significantly correlated with the ammonium oxalate Mo fraction. There were significant correlations between ammonium oxalate Mo and the oxides of iron (Fe), manganese (Mn), and aluminum (Al). Results suggest that the ammonium oxalate extraction for soil Mo is the best predictor of plant-available Mo for species with high Mo requirements such as legumes and that plant-available Mo tracks strongly with other metal oxides in sandy, acidic soils.