A DTPA soil test was developed to identify near‐neutral and calcareous soils with insufficient available Zn, Fe, Mn, or Cu for maximum yields of crops. The extractant consists of 0.005M DTPA (diethylenetriaminepentaacetic acid), 0.1M triethanolamine, and 0.01M CaCl2, with a pH of 7.3. The soil test consists of shaking 10 g of air‐dry soil with 20 ml of extractant for 2 hours. The leachate is filtered, and Zn, Fe, Mn, and Cu are measured in the filtrate by atomic absorption spectrophotometry.The soil test successfully separated 77 Colorado soils on the basis of crop response to Zn, Fe, and Mn fertilizers. Critical nutrient levels must be determined separately for each crop using standardized procedures for soil preparation, grinding, and extraction. The critical levels for corn using the procedures reported herein were: 0.8 ppm for Zn, 4.5 ppm for Fe, and tentatively 1.0 ppm for Mn, and 0.2 ppm for Cu.Development of the soil test was based, in part, on theoretical considerations. The extractant is buffered at pH 7.30 and contains CaCl2 so that equilibrium with CaCO3 is established at a CO2 level about 10 times that of the atmosphere. Thus, the extractant precludes dissolution of CaCO3 and the release of occluded nutrients which are normally not available to plants. DTPA was selected as the chelating agent because it can effectively extract all four micronutrient metals. Factors such as pH, concentration of chelating agent, time of shaking, and temperature of extraction affect the amount of micronutrients extracted and were adjusted for maximum overall effectiveness.

We investigated the capability of the plant-growth-promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22 (T-22) to solubilize in vitro some insoluble or sparingly soluble minerals via three possible mechanisms: acidification of the medium, production of chelating metabolites, and redox activity. T-22 was able to solubilize MnO2, metallic zinc, and rock phosphate (mostly calcium phosphate) in a liquid sucrose-yeast extract medium, as determined by inductively coupled plasma emission spectroscopy. Acidification was not the major mechanism of solubilization since the pH of cultures never fell below 5.0 and in cultures containing MnO2 the pH rose from 6.8 to 7.4. Organic acids were not detected by high-performance thin-layer chromatography in the culture filtrates. Fe2O3, MnO2, Zn, and rock phosphate were also solubilized by cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on measurement of the equilibrium concentration of the chrome azurol S complex in the presence of other chelating substances. A size exclusion chromatographic separation of the components of the culture filtrates indicated the presence of a complexed form of Fe but no chelation of Mn. In liquid culture, T. harzianum T-22 also produced diffusible metabolites capable of reducing Fe(III) and Cu(II), as determined by the formation of Fe(II)-Na2-bathophenanthrolinedisulfonic acid and Cu(I)-Na2-2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid complexes. This is the first report of the ability of aTrichoderma strain to solubilize insoluble or sparingly soluble minerals. This activity may explain, at least partially, the ability of T-22 to increase plant growth. Solubilization of metal oxides by Trichoderma involves both chelation and reduction. Both of these mechanisms also play a role in biocontrol of plant pathogens, and they may be part of a multiple-component action exerted by T-22 to achieve effective biocontrol under a variety of environmental conditions.

High Cd content in durum wheat (Triticum turgidum L. var durum) grain grown in the United States and Canada presents potential health and economic problems for consumers and growers. In an effort to understand the biological processes that result in excess Cd accumulation, root Cd uptake and xylem translocation to shoots in seedlings of bread wheat (Triticum aestivum L.) and durum wheat cultivars were studied. Whole-plant Cd accumulation was somewhat greater in the bread wheat cultivar, but this was probably because of increased apoplastic Cd binding. Concentrationdependent 109 Cd 2؉ -influx kinetics in both cultivars were characterized by smooth, nonsaturating curves that could be dissected into linear and saturable components. The saturable component likely represented carrier-mediated Cd influx across root-cell plasma membranes (Michaelis constant, 20-40 nM; maximum initial velocity, 26-29 nmol g ؊1 fresh weight h ؊1 ), whereas linear Cd uptake represented cell wall binding of 109 Cd. Cd translocation to shoots was greater in the bread wheat cultivar than in the durum cultivar because a larger proportion of root-absorbed Cd moved to shoots. Our results indicate that excess Cd accumulation in durum wheat grain is not correlated with seedling-root influx rates or root-toshoot translocation, but may be related to phloem-mediated Cd transport to the grain.

We report the soil solution speciation of Cd in 64 fieldcollected contaminated soils containing between 0.1 and 38 mg Cd kg -1 . The soils were analyzed for pH (3.5-8.1), soil organic matter (8.0-108 g C kg -1 ), total dissolved Cd (0.03-182 µg Cd L -1 ), dissolved organic carbon (1.5-12 mg C L -1 ), and free Cd 2+ (10 -10 -2 × 10 -7 M). Free Cd 2+ activity in solution was determined using differential pulse anodic stripping voltammetry (DPASV), assuming DPASV is sensitive to easily dissociated inorganic ion-pairs and free Cd 2+ while excluding organic complexes. The solid/liquid partition coefficient (K d ) varied over a range from 10 to 100 000, and the fraction of the dissolved Cd present in solution as the estimated free Cd 2+ species varied between 0 and 60% but averaged about 20%. The dissolved concentrations of Cd and the free Cd 2+ activity in the soil solutions of contaminated soils of different origins can be predicted with reasonable accuracy using a simple competitive adsorption model dependent on pH and total metal loading.

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used 109Cd and 65Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. durum). Results showed that Cd2+ uptake was inhibited by Zn2+ and Zn2+ uptake was inhibited by Cd2+. Concentration-dependent uptake of both Cd2+ and Zn2+ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 micro m concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd2+ and Zn2+, confirming that Cd2+ and Zn2+ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.

Cadmium uptake by food crops needs to be understood in order to limit Cd accumulation in the food chain. Cadmium is a potentially toxic heavy metal with no known benefit to humans, and plant foods are the predominant sources of Cd in human diets. In this study, 124 paired samples of soil and grain were collected from a field of durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] cultivar Munich in northeastern North Dakota. This field on the Langdon Research Extension Center was selected for study because it provided a range in soil pH and salinity. Cadmium in the durum grain ranged widely from 0.025 to 0.359 mg kg−1 Accumulation of Cd in grain was strongly and positively associated with soil salinity as represented by soluble chloride, soluble sulfate, or extractable Na, and also with chelate‐extractable Cd. Relationships to salinity were curvilinear. Concentrations of Cd in grain were not closely related to soil pH. The relationship of Cd in grain to the logarithm of water‐extractable soil Cl− (Clw) was especially close. A predictive model based on chelate‐extractable Cd and logCl−w in soil accounted for 66% of the variability of Cd in grain. Based on these results, and published work for other crops, we believe that the accumulation of Cd in durum wheat grain is enhanced by Cl− in the soil. Although the mechanism is not clear, it is likely to involve increased solubility or availability of soil Cd resulting from the formation of chloro‐complexes in soil solution.

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Five chelating agents—DTPA, EDTA, EGTA, HEDTA, and NTA—were compared at pH 5.3 as extractants for Al, Fe, Mn, Zn, Cu, Cd, and Ni in 25 diverse soils. Extracting solutions contained 0.005M chelating agent, 0.01M CaCl2, and 0.1M acetic acid‐ammonium acetate buffer at pH 5.3. These extractants were compared to: a similar solution without added chelating agent, 0.1M HCl, and the DTPA soil testing solution (0.005M DTPA, 0.01M CaCl2, buffered at pH 7.3 by 0.1M triethanolamine). All extractants yielded clear filtrates that aspirated and burned cleanly during analysis by flame atomic absorption spectrometry. At the 5:1 extractant/soil ratio used, all buffered extractants maintained their pH satisfactorily. DTPA and HEDTA were among the most effective ligands for extracting all seven metals. EDTA extracted somewhat lower concentrations but was also a good extractant for all metals. NTA was among the best extractants for Al and Ni but among the least effective extractants for Fe, Mn, and Cu. EGTA was the least effective chelator of Fe, Zn, Ni, and especially Al. Despite its alkaline pH of 7.3, the DTPA soil‐testing solution was a moderately effective extractant for metals except Mn and ferrous Fe. Acid soils, reduced soils and sediments, and metal‐contaminated soils may release enough strongly complexed metal to reduce the effectiveness and the capacity of chelating extractants. Recently dredged lake sediment released particularly large concentrations of metals, primarily Mn2+ and Fe2+. Extracting metals effectively from metal‐rich soils may require increased concentration of chelating agents, higher extractant/soil ratios, or more than one extraction per sample.