Monocot‐dicot variability in response to an iron oxide‐metallic iron source

Abstract

Iron from a mixture of Fe oxide and metallic Fe was more available to corn (Zea mays L.) than it was to soybeans when the plants wore grown in calcareous soil or in nutrient solution. All this Fe, however, was DTPA (diethylene triamine pentaacetic acid) extractable. In solution culture the Fe was available to the soybean (Glycine max L.) plants unless CaCO₃ was included in the nutrient solution.     

Iron chlorosis in macadamia as affected by phosphate‐iron interactions 1

Iron (Fe) chlorosis induced by heavy phosphate (P) fertilizations is a serious problem for macadamia (Macadamia integrifolia) in Hawaii. To address this problem, a study was conducted to quantify the effects of P‐Fe interaction on macadamia leaf composition and chlorosis. The soil used was a limed Oxisol (Tropeptic Eutrustox, Wahiawa Series), pH 5.5. Phosphate was added as treble superphosphate at 0, 150 and 500 mg P/kg. The 150 mg P/kg rate was designed to yield approximately 0.04 mg P/L in the soil solution, a level considered adequate for macadamia growth. The 500 mg P/kg rate was intended to produce approximately 0.2 mg P/L, a level required by many horticultural crops but considered excessive for macadamia. Iron was added as Fe‐DTPA at 0, 5 and 10 mg Fe/kg soil, and factorially imposed on the P treatments. Color Index, a numerical rating based on hue, value and chroma from a Munsell Color Chart for Plant Tissues, was correlated with leaf chlorophyll concentration and used as an indicator of chlorosis.

Phosphate concentrations in leaves increased with increasing P application rates as expected, but decreased remarkably with increasing Fe rates (at a constant P rate). Plant Fe unexpectedly remained unchanged with increasing Fe rates but decreased with increasing P rates. The results suggest that (1) soil‐solution Fe was not a limiting factor to macadamia growth as is often incorrectly assumed for high P‐fertilized soils, (2) Fe uptake was restricted not because soil‐solution Fe was low but because plant P was excessively high, and (3) Fe translocation from roots to leaves may have been hampered by high P in the plants. Consequently, Fe chlorosis was intensified primarily by P fertilization (actually, by high plant P concentrations) and secondarily by P‐Fe interactions. Chlorosis, as measured by Color Index, can be diagnosed by a leaf Fe/P ratio < 0.06, and predicted by a soil‐solution ³√Fe/P ratio < 15.     

Manganese‐iron interactions in the plant‐soil system

Dicotyledonous plants had significantly higher Mn and Fe intake rates on a near neutral soil, had a significantly higher Mn intake rate on a slightly calcareous soil, but had lower Mn and Fe intake rates on a calcareous soil, than monocotyledonous plants. This dependency on soil reaction suggests that dicots utilize primarily a chemical reduction mechanism, whereas monocots utilize some less pH‐dependent mechanism (possibly Mn(III)‐, Fe(III)‐organic complexation) to mobilize soil Mn and Fe. Soluble soil Mn and Fe fractions in the rhizosphere were consistently positively correlated with each other, as were Mn and Fe intake rates. These results suggest that for soil‐grown plants, Mn and Fe uptake was positively interrelated because both Mn and Fe were mobilized by similar root processes.     

Loss‐on‐ignition as an estimator of soil organic carbon in a‐horizon forestry soils

Abstract

The determination of soil organic matter by wet digestion techniques is a slow and laborious analysis. Loss‐on‐ignition (LOI) provides a simple alternative technique for the estimation of soil organic carbon in non‐calcareous A horizon soils of the Natal midlands and Zululand forestry regions. Using multiple regressional techniques, the relationships between loss‐on‐ignition, Walkley organic carbon and soil texture for 55 soils were determined over a range of ignition temperatures. The relationships hold best for soil samples with relatively low organic carbon contents (< 5%). The optimum temperature for ignition was found to occur at 450°C and resulted in the relationship: Soil organic carbon = 0.284*LOI percent. No advantage is gained through ignition at higher temperatures due to the loss of clay mineral structural water, even if the soil texture is accurately known.     

Changes in distribution of inorganic soil phosphorus forms with phosphate desorption by iron oxide‐impregnated paper strips

Abstract

The release of soil phosphorus (P) to solution has been described by extraction of soil with iron (Fe)‐oxide coated paper strips. Little information is available, however, on where this P is coming from. The effect of removal of reversibly adsorbed soil P on the distribution of inorganic P forms was investigated for 12 Italian soils. Phosphate was removed from these soils by Fe‐oxide strips after incubation with P (0 and 100 mg P kg^‐1) for 90 days. With no applied P, 3 to 17% of the total soil active P [saloid‐P, aluminum‐phosphate (Al‐P), iron‐phosphate (Fe‐P), and calcium‐phosphate (Ca‐P) was removed by the Fe‐oxide strips. The change in strip‐P following P addition (100 mg kg^‐1 soil), ranged from 12.9 to 53.5 mg P kg^‐1, with P coming almost entirely from the active P fractions. A close relationship between the changes in desorbed strip‐P after P equilibration and soil P sorption index (SI) was found for the studied soils (r²=0.96). Thus, the release of soil P for plant uptake or transport in runoff was a function of the amount of “actively”; sorbed P and an estimate of P sorption.     

Pattern of iron distribution in the soil‐plant system and its possible relation to iron‐chlorosis

Abstract

The frequent concentration‐ranges of various nutrient elements in soils and in plants are compared. Iron is different from almost all other nutrient elements in the fact that its optimal concentration range in plants is much lower than its frequent concentration range in soils. It is suggested that this observation is related to a chemical‐physiological mechanism of control on the uptake of iron by plants which in turn may explain the situations in which iron deficiency conditions in plants arise.     

Iron‐stress response of inoculated and non‐inoculated roots of an iron inefficient soybean cultivar in a split‐root system

The objective of this study was to establish whether the iron‐stress responses observed in T203 soybean (Fe‐inefficient) with active nodules are products of the nodules or of the entire root system. A split‐root system was used in which half the roots of each plant were inoculated and actively fixing nitrogen and the other half were not. Soybean cultivar T203 is normally Fe‐inefficient and does not exhibit the Fe‐stress responses, however an iron‐stress response did occur during active N₂ fixation in earlier studies. This implies that the active nodules influenced the plant's ability to respond to Fe‐deficiency stress. In this study, the Fe‐stress response (H⁺ and reductant release) observed in T203 soybean was limited to the inoculated side of the split‐root system. The severe Fe chlorosis which developed in these plants was overcome in a manner similar to Fe‐efficient cultivars undergoing normal Fe‐stress response and the T203 plants completely regreened. Exudation of H⁺ ions was similar in both the presence and absence of Fe, and was generally limited to inoculated roots. Reductant release was nearly nonexistent from the non‐inoculated roots and was greater for the Fe‐stressed (‐Fe) plants than for non‐stressed (+Fe) plants. Thus, the response observed, which alleviated Fe chlorosis, appeared to be associated with N₂ fixation of the active nodules.     

Phosphorus immobilization in wood waste‐based potting media

Abstract

Microbially induced nitrogen (N) immobilisation in potting media is accompanied by immobilisation of soluble phosphorus (P), with the P/N ratio of the immobilised elements being about 0.15. Fertiliser N applied to counter N immobilisation should be accompanied by this amount of soluble P if plants are not to suffer from P deficiency. Essentially none of the immobilised P in potting media that contained aged pine bark or eucalypt sawdust was available for short‐term growth of ‘Giant Butterfly’ pansies or Hakea francisiana subsequently grown in them. One implication of these results is that the N drawdown test for potting media will underestimate N requirements if P is not included in the charging solution.     

Ability of an iron‐efficient and an iron‐inefficient corn inbred to take up 59Fe and FeSO4 added to Yolo loam soil

Abstract

The addition of CaCO₃ and MgCO₃ to Yolo loam soil (pH 6) resulted in lower Fe concentrations in shoots of the Fe‐inefficient Ys₁/Ys₁ corn inbred (Zea mays L.) and higher levels in shoots of the Fe‐efficient WF9 inbred than in controls. When ⁵⁹Fe with and without carrier FeSO₄ was blended with the soil, the specific activity was similar for the two inbreds in nonamended soils, but was increased in the Ys₁/Ys₁ for the lime amendments. Sulfur acidification of soil decreased the specific activity of ⁵⁹ Fe in shoots by increasing the pool of available Fe. From 5 to 33% of the Fe in plants came from the FeSO₄ source. It was greatest in Ys₁/Ys₁ with lime‐amended soils and least in S‐acidified soil.     

Assay of urease enzyme activity in an ammonium‐fixing soil

Abstract

Rates of substrate disappearance and product formation were compared as measures of urease enzyme activity in an NH₄‐fixing and in a non‐fixing soil under tris‐, borate‐ or non‐buffered assay conditions over 4h at 37°C. Tris‐buffered urease activity of the NH₄‐fixing soil was 119 μg urea‐N hydrol./g/h or 116 μg (KCl‐extractable) NH₄‐N/g/h indicating prevention of NH₄ fixation by the buffer; without tris, NH₄ production rates amounted to only 35% of coresponding urea hydrolysis rates. Equal rates of urea disappear‐ ance and NH₄ formation occurred in the non‐fixing soil irrespective of buffer amendment.

Tris‐inhibition of NH₄ fixation during 4h incubation at 37°C, however, depended on NH₄ Cl rate and buffer strength. 0.025–0.10 M tris (pH 9.0) reduced NH₄ fixation to negligible amounts at < 0.03 M NH₄C1 whereas, at 0.06–0.24 M NH₄Cl, substantial NH₄ fixation occurred in the presence of 0.05 M tris; NH₄ fixation in unbuffered soil, however, always exceeded that in tris‐buffered soil. Borate buffer (0.06M, pH 10) did not influence the extent of NH₄ fixation.

Tris significantly enhanced urea hydrolysis in the slightly acid, non‐fixing soil but not in the moderately alkaline NH₄ ‐fixing soil indicating an effect of soil type on pH optima of urease enzyme activity. The urease activities of both soils in borate were considerably lower than in tris, possibly because of the combined effects of excess alkalinity and high substrate concentration.     

Soybean response to iron‐deficiency stress as related to iron supply in the growth medium

The Fe‐inefficient T203 and the Fe‐efficient A7 and Pioneer 1082 (P1082) soybeans (Glycine max (L.) Merr.) were grown hydroponically with no (0 mg Fe L^‐1 ; ‐Fe) and a minute level (0.025 mg Fe L^‐1 ; +Fe) of Fe to (a) compare their responses to Fe‐deficiency stress and (b) relate Fe‐efficiency in soybeans to their ability to initiate the Fe‐stress‐response mechanism at low levels of Fe. With no Fe in solution, P1082 released similar levels of H⁺ ions, but released less reductant from their roots and there was less reduction of Fe³⁺ to Fe²⁺ by their roots than by A7 roots. These responses were also one day later and occurred after a more severe chlorosis and a lower leaf Fe had developed in P1082 than in A7. With 0.025 mg L^‐1 of solution Fe, it was not necessary for the Fe‐stress response mechanism to be fully activated to make Fe available in A7 soybean, whereas a strongly enhanced Fe stress response was observed in P1082. Increased Fe uptake and regreening of leaves immediately succeeded initiation of the Fe stress response in both cultivars and at both levels of Fe. Thus, P1082 was slightly less efficient than A7 soybean, but would be classed more efficient than the previously studied soybean cultivars A2, Hawkeye, Bragg, Pride, Anoka, and T203. These results support the hypothesis that the most efficient soybeans are those which can initiate the Fe‐stress response mechanism with little or no Fe in the growth medium. The near simultaneous occurrence of the factors in the Fe‐stress response mechanism (H ion and reductant release, reduction of Fe to Fe by roots), and the immediate increase in leaf Fe and chorophyll contents following that response suggest that all these factors act in concert, not independently, to aid in the absorption and transport of Fe to plant tops.     

Distribution pattern of root‐supplied 59iron in iron‐sufficient and iron‐deficient bean plants

In order to study the iron (Fe) distribution pattern in bean plants with different Fe nutritional status, french bean (Phaseolus vulgaris L.) seedlings were precultured in a complete nutrient solution with 8x10^‐5 M FeEDTA for five days. Thereafter, plants were further supplied with 8x10^‐5 M FeEDTA (Fe‐sufficient) or with only 2x10^‐6 M FeEDTA (Fe‐deficient) for another eight days. At this stage, the Fe‐deficient plants had much lower chlorophyll contents and lower dry weight of the leaves but higher reducing capacity of the roots compared with the Fe‐sufficient plants. For studies on short‐term distribution of Fe, the Fe‐sufficient plants were supplied 8x10^‐5 M ⁵⁹FeEDTA (specific activity 9.9 GBq/mol) and the Fe‐deficient plants 1x10⁶ M ⁵⁹FeEDTA (specific activity 98.8 GBq/mol). The plants were harvested after 4 and 24 hours. Despite a much lower supply of ⁵⁹FeEDTA/(factor 80), the Fe‐deficient plants took up significantly more ⁵⁹Fe but translocated less to the shoots (14.6% after 24 h) compared with the Fe‐sufficient plants (29.4% after 24 h). However, regardless of the Fe nutritional status of the plants, the majority of ⁵⁹Fe was translocated in the primary leaves. Our results demonstrate a similar distribution patterns of root‐derived ⁵⁹Fe in the shoots of Fe‐sufficient and Fe‐deficient plants, and thus, no preferential direct translocation of Fe to the shoot apex in the Fe‐deficient plants.     

Use of paper cups as an extraction vessel for soil nutrient‐element analysis

Abstract

To compare the wax coated paper cups with Erlenmeyer flasks as nutrients extraction vessels and to compare their cost effectiveness, 45 soil samples with a wide range in properties were selected. Samples 1–25 were analyzed for extractable P, K, Ca, and Mg and 26–45 for Fe, Zn, Mn, and Cu first time using wax coated paper cups, and a second time using Erlenmeyer flasks as the extraction vessel. The levels of each nutrient extracted in Erlenmeyer flasks and those in paper cups were almost identical. The coefficient of correlation between the levels of each nutrient (P, K, Ca, Mg, Fe, Zn, Mn, and Cu) extracted in paper cups and those extracted in Erlenmeyer flasks were highly significant. Paper cups appeared to be more cost effective than Erlenmeyer flasks.     

The determination of phosphate‐extractable sulphate in soil with an anion‐exchange membrane

Abstract

A method for extracting sulphate from soils using strips of a phosphated anion‐exchange membrane is described. The results obtained by this method are in good agreement with those obtained by extraction with Ca(H₂PO₄)₂solutions and the method has a number of practical advantages over the use of phosphate solutions. No charcoal treatment, centrifuging or filtering is required and the strips are reuseable. No organic interferences are encountered during the turbidimetric measurement of the extracted sulphate.     

Phosphorous levels on the ability of an iron‐inefficient and an iron‐efficient corn inbred to take up iron from nutrient solution

Abstract

The Fe‐inefficient corn (Zea mays L.) inbred Ys₁/YS₁ was compared with the Fe‐efflcient WF9 inbred for ability to take up Fe from solution culture at different P levels. A high level of P depressed Fe uptake more in the Ys₁/Ys₁ than in the WF9 inbred. The Ys₁/Ys₁ roots were high in P.     

Reduction of structural iron in selected iron‐bearing minerals by soybean root exudates grown in an in vitro geoponic system

Research on the reduction of iron (Fe) by plant‐root exudates has been conducted using hydroponic solutions containing Fe salts or chelates. These solutions, however, fail to reflect the true soil environment because plants derive their majority requirement from the solid Fe(III) sources. An in vitro Geoponic system (IVGS) is developed to study the reduction of Fe‐bearing clay minerals, i.e., Upton and SWa‐1 (smectite), and Si‐containing amorphous Fe oxide by soybean‐root exudates. Surface sterilized soybean seeds, [Glycine max (L.) Men.] cv. Williams (marginally susceptible to Fe chlorosis), were germinated in presterilized glass culture tubes containing semi‐solid agar media (sucrose free) and Fe minerals. These tubes were placed in an incubator programmed for a white‐fluorescent light cycle for 16 h and temperature setting of 25±2°C. After 15 d of plant growth, the system was analyzed for Fe²⁺ and total Fe. The amount of structural Fe reduction was 0.012, 0.095 and 0.182 mmol/g for Upton, SWa‐1, and Si‐containing amorphous Fe oxide samples, respectively. The reduction of structural Fe in the Fe containing minerals was likely caused by phenolic root exudates which oxidized to diquinones.     

Evaluating iron‐impregnated paper strips for assessing available soil phosphorus

Abstract

Iron (Fe)‐impregnated filter paper strips (Pi) have been proposed as a method for measuring available soil phosphorus (P). A well‐defined Pi method has not yet been developed and Pi strips are often prepared with different filter papers and procedures. A study aimed at arriving at a consistent Pi method is thus needed. Four types of Pi strips, prepared with the two most widely used papers, Whatman No. 50 and 541, following a procedure that incorporates improvements both proposed in the literature and made in our laboratory, were evaluated for P extraction capacity and error. Two of the best strips, which are significantly different in P extraction capacity, along with the Mehlich 1 (0.05M HCl and 0.0125M H₂SO₄) and the Olsen method (0.5M NaHCO₃, pH 8.5) were further evaluated in a greenhouse experiment involving eight soils planted with corn (Zea mays L.). Results indicated that strips prepared with both Whatman No. 50 and 541 were appropriate for P extractions as long as strips were washed with deionized water after treatment with ammonium hydroxide (NH₄OH). At room temperatures the strips probably contain both hydrous Fe hydroxides and oxides in both crystalline and amorphous forms. Pi P was well correlated with Olsen P and P uptake in all soils, indicating that Pi is generally applicable in diverse soils. No obvious advantage was found for the Pi with respect to the Olsen method. Both the Pi and the Olsen method were better extractants with respect to the Mehlich 1, which was ineffective for extracting P in calcareous soils. Extractable P by Mehlich 1, Olsen, and Pi all correlated highly with accumulated plant available P estimated by eight sequential crops in the greenhouse. However, none of the methods could account for all the variation in plant P removal.     

Influence of air drying and soil gas‐phase carbon dioxide on DTPA‐extractable iron in calcareous soils

Abstract

The extraction of a field‐moist soil with DTPA will result in a level of extractable iron (Fe) lower than that of the air‐dried soil. Soil gas‐phase carbon dioxide (CO₂) levels may be considerably higher than ambient atmospheric levels, especially in wet soils in the field. This study was undertaken to determine whether gas‐phase CO₂ level influences the quantity of Fe extracted by DTPA. Three moist calcareous soils were incubated for 21 days, each at three different partial pressures of CO₂, after which the moist soils were extracted with DTPA. A sample of each soil was also air dried, and was subsequently extracted with DTPA. In each case, DTPA‐extractable Fe from the moist sample was lower than that from the air‐dried sample; however, DTPA‐extractable Fe increased with increasing CO₂ partial pressure of in the moist soils. DTPA‐extractable Fe concentration for a given soil following air drying was not significantly influenced by the CO₂ partial pressure during incubation of the originally field‐moist soil. DTPA‐extract pH of the moist soils followed the same trend as soil‐solution pH (i.e., as CO₂ concentration of the soil gas‐phase increased, soil solution pH and DTPA extract pH both decreased); however, the slope of the pH versus log P_CO2 curve was less pronounced in the DTPA extract due to the buffering capacity of the triethanolamine. From this study, it is concluded that elevated soil gas‐phase CO₂ partial pressure does not contribute to the lower level of DTPA‐extractable Fe observed when the extraction is performed on a field‐moist versus an air‐dried soil; increased CO₂ partial pressure actually resulted in a slight increase in concentration of DTPA‐extractable Fe obtained from a field‐moist soil.     

Nitrogen source‐sink relationship in cotton

Although attempts have been made in developing soil and plant analysis procedures for cotton (Goseipium hirsutum) , N nutrition is not completely understood. To the present it has not been possible to predict when the maximum N utilization efficiency could be achieved. By exposing cotton plants to a ¹⁵N enriched solution for 5 periods of 30 days each in a temperature controled greenhouse, the accumulation and redistribution of N within the plant was followed in order to define the main N sources and sinks for each period. The main N source within the cotton plant after the first square stage was the leaves. Stems and burs acted as temporary storage organs remobilizing N to the seeds late in the growing season, while the roots were fairly neutral. Dry matter accumulation during the reproductive stage was not related to N redistribution in the plant, except for bolls. So at this stage, dry matter accumulation was not a important component in the N source‐sink relationship within the cotton plant.     

Critical soil phosphorus of a low‐P loess‐derived soil as affected by storage temperature

Abstract

Overwintering soil temperature may influence crop response to phosphorus (P) and indices of P availability in the humid, temperate, transitional climate of Tennessee. The effects of P fertilization and soil incubation temperature on sorghumsudangrass (Sorghum bicolor x S. Sudanese) grown on a Typic Hapludalf was investigated in a greenhouse study. In order to determine the effect of temperature on P availability, soils were incubated prior to cropping, at a constant temperature of 6°C or an average diurnal temperature of 24 and 36°C. Reagent grade Ca(H₂PO₄)₂.H₂O was used as the fertilizer source and applied at rates of 0, 10, 20, and 30 mg P kg^‐1 for the first test and 0, 20, 40, 60, and 80 mg P kg_‐ ¹ for the second test. Critical P concentration in the shoots for optimum yield was found to be 1.3 mg g‐¹, corresponding to soil solution and labile P concentrations of 5.5 μmol L^‐1 and 167 μg g^‐1, respectively. Optimum yield occurred for applications of >65 mg P kg^‐1 and was unaffected by soil incubation temperature. Applied P rates affected extractable P by five chemical extractants (Bray I, Bray II, Mehlich I, Mehlich III, and Mississippi), but soil incubation temperature had no affect. The extractants, however, were poorly correlated to plant P uptake and no one extractant appeared preferable to the others as an indicator of P availability.