Time of Day Effect on Foliar Nutrient Concentrations in Corn and Soybean

Foliar nutrient concentrations vary during the day. Field research was conducted to quantify and better understand this variation in corn (Zea mays L.) and soybean (Glycine max L. Merr.) with foliar sampling during the vegetative and reproductive stages. Time of day effects occurred inconsistently across nutrients. Nitrogen (N), manganese (Mn), iron (Fe), and zinc (Zn) foliar concentrations were generally high early in the day. Phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) foliar concentrations varied inconsistently with time of day, while concentrations of boron (B) in both crops and copper (Cu) in corn were not affected. Limiting foliar sampling to after 10:00 AM reduced the variation for soybean but not for corn. Interpretation by Diagnosis and Recommendation Integrated System (DRIS) did not reduce the time of day effect. The variation caused by time of day, along with other causes, affects confidence in interpretation of foliar results suggesting use of the information with either additional foliar sampling or soil testing in making nutrient management decisions.     

Interactive Effect of Phosphorus and Nitrogen on Leaf Anthocyanins,Tissue Nutrient Concentrations,and Dry‐Matter Yield of Floralta Limpograss during Short Day Length

Abstract

A field trial was conducted during the short‐day period of 2004–2005 at Ona, Fl., to study the factorial effect of nitrogen (67, 90, and 134 kg N ha^?1) and phosphorus (0, 5, 10, 20, and 40 kg P ha^?1) rates on forage dry‐matter yield, quality, nutrient uptake, and leaf pigment concentration of limpograss (Hemarthria altissima). The N and P fertilizers were applied 45 days before each of two harvests. There was no interaction between N and P rates on any of the measured variables. Cool‐season forage yield increased curvilinearly from 137 to 350 kg ha^?1 in winter and 237 to 1389 kg ha^?1 in early spring, whereas crude protein (CP) concentration increased from 145 to 158 g kg^?1, as P was increased from 0 to 40 kg ha^?1, but yield and CP were not affected by N rate. There was a decreasing linear relationship between leaf concentration of anthocyanins and P rate of application such that forage obtained with 0 kg P ha^?1 had 61% more leaf anthocyanins and purple pigmentation than with 40 kg P ha^?1. There was no effect of N on anthocyanins content. It was concluded that increased level of leaf anthocyanins was due to the cumulative stress from cool weather and lower plant‐tissue P levels, which resulted in reduced growth and yield of limpograss. In cool weather, P played a critical role in controlling leaf purple pigmentation and forage yield.     

Influence of Gyttja on Shoot Growth and Shoot Concentrations of Zinc and Boron of Wheat Cultivars Grown on Zinc‐Deficient and Boron‐Toxic Soil

Abstract

Greenhouse experiments were carried out to study the influence of gyttja, a sedimentary peat, on the shoot dry weight and shoot concentrations of zinc (Zn) and boron (B) in one bread wheat (Triticum aestivum L., cv. Bezostaja) and one durum wheat (Triticum durum L., cv. Kiziltan) cultivar. Plants were grown in a Zn‐deficient (DTPA‐Zn: 0.09 mg kg^?1 soil) and B‐toxic soil (CaCl₂/mannitol‐extractable B: 10.5 mg kg^?1 soil) with (+Zn = 5 mg Zn kg^?1 soil) and without (?Zn = 0) Zn supply for 55 days. Gyttja containing 545 g kg^?1 organic matter was applied to the soil at the rates of 0, 1, 2.5, 5, and 10% (w/w). When Zn and gyttja were not added, plants showed leaf symptoms of Zn deficiency and B toxicity, and had a reduced growth. With increased rates of gyttja application, shoot growth of both cultivars was significantly enhanced under Zn deficiency, but not at sufficient supply of Zn. The adverse effects of Zn deficiency and B toxicity on shoot dry matter production became very minimal at the highest rate of gyttja application. Increases in gyttja application significantly enhanced shoot concentrations of Zn in plants grown without addition of inorganic Zn. In Zn‐sufficient plants, the gyttja application up to 5% (w/w) did not affect Zn concentration in shoots, but at the highest rate of gyttja application there was a clear decrease in shoot Zn concentration. Irrespective of Zn supply, the gyttja application strongly decreased shoot concentration of B in plants, particularly in durum wheat. For example, in Zn‐deficient Kiziltan shoot concentration of B was reduced from 385 mg kg^?1 to 214 mg kg^?1 with an increased gyttja application. The results obtained indicate that gyttja is a useful organic material improving Zn nutrition of plants in Zn‐deficient soils and alleviating adverse effects of B toxicity on plant growth. The beneficial effects of gyttja on plant growth in the Zn‐deficient and B‐toxic soil were discussed in terms of increases in plant available concentration of Zn in soil and reduction of B uptake due to formation of tightly bound complexes of B with gyttja.     

Effects of Several Metals on Both Fe(III)‐ and Cu(II)‐Reduction by Roots of Fe‐Deficient Cucumber Plants

Abstract

The ferric‐chelate reductase induced by Fe deficiency is also able to reduce other ions such as Cu²⁺. This Cu(II)‐reduction has been less studied and it has been suggested that Cu²⁺ ion rather than Cu²⁺‐chelate serves as the substrate. Ferric‐chelate reductase activity is inhibited by some metals, but the mechanisms implicated are not known. In the present work we use Fe‐deficient cucumber seedlings to study the interactions of Cu²⁺, Ni²⁺, Mn⁴⁺, and Fe³⁺ on both Fe(III)‐reduction and Cu(II)‐reduction activities. The response of Cu(II)‐reduction activity to Cu concentration, in the presence or absence of citrate, was also studied. Results showed that inhibition of the ferric‐chelate reductase activity by Cu²⁺ or Ni²⁺ could be partially reversed by increasing the concentration of Fe‐EDTA. The Cu(II)‐reduction activity was even stimulated by Fe‐EDTA or Ni²⁺; it was inhibited by a high concentration of Cu²⁺ itself; and it was not affected by the absence of citrate. Mn⁴⁺ caused a moderate inhibition of both Fe(III)‐reduction and Cu(II)‐reduction activities. Results agree with the hypothesis that free Cu²⁺ ion is the substrate for Cu(II)‐reduction and suggest that the mechanisms involved in Fe(III)‐reduction and Cu(II)‐reduction could have some differences and be affected by metals in different ways. The mode of action of metals on the reductase activity are discussed, but they are still not well known.     

Soil‐Applied Selenium Effects on Tissue Selenium Concentrations in Cultivated and Adventitious Grassland and Pasture Plant Species

Abstract

According to international nutritional standards, plant selenium (Se) concentrations in Belgium are too low. To correct this situation, adding Se in fertilizers for pastures and grasslands is suggested, similar to activities in Finland. However, there is a lack of data on meadow plant species' ability to absorb Se. Therefore, a pot experiment was initiated using 24 meadow plant species cultivated on a Belgian cambisol receiving standard fertilizer treatment, with or without the addition of 9 g Se ha^?1 yr^?1 as sodium selenate. Soil Se analysis confirmed the low Se status of the native soil. Mean foliar Se concentration in the control group was 0.05 mg kg^?1. Because plant deficiency may occur at levels less than 0.10 mg Se kg^?1, data provided further evidence for Se deficiency in Belgium plant production. When grown with Se, plant species showed wide variations for Se concentration, ranging from 0.08 to 0.49 mg Se kg^?1. All values were less than 2 mg Se kg^?1, the suggested threshold toxicity level for dairy cattle. There were two different types of plants in terms of response to Se fertilization. Most of the tested plants were known as nonaccumulators. There were also two probable secondary accumulators: Sinapis arvensis and Melilotus albus. Finally, one has to question the reliability of plant Se enhancement using this method when floristic composition is poorly controlled.     

Effect of Zinc × Boron Interaction on Plant Growth and Tissue Nutrient Concentration of Corn

ABSTRACT

A pot experiment was conducted in a greenhouse on a calcareous soil (fine, mixed, mesic, Fluventic Haploxerepts) to study the interaction of zinc (Zn) and boron (B) on the growth and nutrient concentration of corn (Zea mays L.). Treatments consisted of a factorial arrangement of seven levels of B (0, 2.5, 5, 10, 20, 40, and 80 mg kg^{? 1}as boric acid), two sources of Zn [zinc sulfate (ZnSO₄ · 7H₂O) and zinc oxide (ZnO)], and three levels of Zn (0, 5, and 10 mg kg^{? 1}) in a completely randomized design with three replications. Plants were grown for 70 d in 1.6 L plastic containers. Applied Zn significantly increased plant height and dry matter yield (DMY) of corn. Source of Zn did not significantly affect growth or nutrient concentration. High levels of B decreased plant height and DMY. There was a significant B × Zn interaction on plant growth and tissue nutrient concentration which were rate dependent. In general, the effect of B × Zn interaction was antagonistic on nutrient concentration and synergistic on growth. It is recommended that the plants be supplied with adequate Zn when corn is grown in high B soils, especially when availability of Zn is low.     

Nutrient charge,composition of media and nitrogen‐form affect growth of vinca

Growth of vinca [Catharanthus roseus (L.) G. Don ‘Grape Cooler'] was compared under several cultural conditions. Conditions investigated included two types of media (a peat‐lite mix and a mix containing 25% pine bark) and five types of nutrient charges in the peat‐lite media (sulfated micros, chelated micros, sulfated or chelated micros with pH adjustment to 5.5, and no charge). Nitrogen (N) source effect on growth was also investigated. Plants were grown at five different ratios of nitrate‐N to ammonium‐N. Greatest growth as measured by shoot length and shoot dry weight occurred in the peat‐lite media at either the sulfated micro or chelated micros adjusted to pH 5.5 and at the highest ratios of nitrate‐N to ammonium‐N. Root dry weight and growth were negatively affected by high levels of ammonium‐N in the fertilizer solution.     

Determination of Optimum Rate and Growth Stage for Foliar‐Applied Phosphorus in Corn

Abstract

Foliar applications of fertilizer phosphorus (P) could improve use efficiency by minimizing soil applications. Nine experiments were conducted in 2002 and 2003 to determine foliar P rates and appropriate growth stages for application. Treatments comprised of 10 factorial combinations of three foliar P application timings and four rates of foliar P. Foliar application times were V4 (collar of fourth leaf visible), V8 (collar of eighth leaf visible), and VT (last branch of the tassel completely visible but silks not yet emerged) corn growth stages. Foliar P rates were 0, 2, 4, and 8 kg ha^?1. Foliar P applied at the VT growth stage improved grain and forage P concentration, which was reflected in increased grain yield in some of the experiments. A foliar P rate of 8 kg ha^?1 improved yield to some extent and forage and grain P concentration more than the smaller rates. The results suggest that foliar P could be used as an efficient P‐management tool in corn when applied at the appropriate growth stage and rate.     

Seasonal Changes in Nutrient Concentrations of ‘Newhall’ and ‘Skagg's Bonanza’ Navel Oranges

Seasonal changes in nutrient concentrations of leaf and fruit structural parts (rind and pulp) from ‘Newhall’ (Citrus. sinensis Osbeck) and ‘Skagg's Bonanza’ (C. sinensis Osbeck) navel oranges were investigated during fruit development in two successive years. Leaf calcium (Ca), manganese (Mn), and potassium (K) concentrations were relatively constant throughout the whole season with the exception of an increase of K at stage 1, the period of fruitlet growth [before 80 days after full bloom (DAFB)], whereas the magnesium (Mg), boron (B), iron (Fe), and zinc (Zn) concentrations declined distinctly during stage 2 (80–180 DAFB), the period of fruit rapid enlargement. In rind, Ca, B, Fe, and Mn concentrations reached the greatest levels at stage 2, different from K and Mg, which increased at stage 1 and decreased thereafter. In pulp, concentrations of Ca, K, Mg, and Mn declined gradually with time, whereas a small rise in B toward the end of sampling and a clear increase of Fe at stage 2 were observed. It was suggested that ‘Newhall’ required greater B inherently in fruits as the cultivar had greater B concentrations in fruit parts and had greater rind/leaf B concentration ratios than ‘Skagg's Bonanza.’ ‘Newhall’ had relatively greater rind Ca content and exhibited Ca distribution more uniformly within its fruit parts, which probably enhanced the crack resistance.     

Effect of Zinc Humate on Growth of Soybean and Wheat in Zinc‐Deficient Calcareous Soil

Abstract

Humic acids have many benefits for plant growth and development, and these effects may be maximized if these materials are combined with micronutrient applications. In the present study, pot experiments were conducted to evaluate the effects of zinc (Zn) humate and ZnSO₄ on growth of wheat and soybean in a severely Zn‐deficient calcareous soil (DTPA‐Zn: 0.10 mg kg^?1 soil). Plants were grown for 24 (wheat) and 28 days (soybean) with 0 or 5 mg kg^?1 of Zn as either ZnSO₄ or Zn humate. Zinc humate used in the experiments was obtained from Humintech GmbH, Germany, and contained 5% of Zn. When Zn was not supplied, plants rapidly developed visible symptoms of Zn deficiency (e.g., chlorosis and brown patches on young leaves in soybean and necrotic patches on middle‐aged leaves in wheat). Adding Zn humate eliminated Zn‐deficiency symptoms and enhanced dry matter production by 50% in soybean and 120% in wheat. Zinc‐humate and ZnSO₄ were similarly effective in increasing dry matter production in wheat; but Zn humate increased soybean dry matter more than ZnSO₄. When Zn was not supplied, Zn concentrations were 6 mg kg^?1 for wheat and 8 mg kg^?1 for soybean. Application of Zn humate and ZnSO₄ increased shoot Zn concentration of plants to 36 and 34 mg kg^?1 in wheat and to 13 and 18 mg kg^?1 in soybean, respectively. The results indicate that soybean and wheat plants can efficiently utilize Zn chelated to humic acid in calcareous soils, and this utilization is comparable to the utilization of Zn from ZnSO₄. Under Zn‐deficient soil conditions, plant growth and yield can be maximized by the combined positive effects of Zn and humic acids.     

Evaluation of Nutrient Release in Salt‐Saturated Soils

Abstract

The effects of irrigating with saline water on native soil fertility and nutrient relationships are not well understood. In a laboratory experiment, we determined the extent of indigenous nutrient [calcium (Ca), magnesium (Mg), potassium (K), manganese (Mn), and zinc (Zn)] release in salt-saturated soils. Soils were saturated with 0, 75, and 150 mmolc L^?1 sodium chloride (NaCl) solution and incubated for 1, 5, 10, and 15 days. The saturation extracts were analyzed for pH, ECe, and water‐soluble Ca, Mg, K, Mn, and Zn, and the remainder soil samples were analyzed for exchangeable forms of these elements. In a subexperiment, three soil types (masa, red‐yellow, and andosol) were saturated individually either with 100 mmolc L^?1 of NaCl, sodium nitrate (NaNO₃), or sodium sulfate (Na₂SO₄) salt. These salts were also compared for nutrient release. Soils treated with NaCl released higher amounts of water‐soluble than exchangeable nutrients. Except for Zn, the average concentrations of these nutrients in the soil solution increased significantly with time of incubation, but concentrations of the exchangeable forms varied inversely with time of incubation. The masa soil exhibited the highest concentrations of Ca and Mg, whereas K was highest in andosol. The extract from soils treated with NaCl contained greater amounts of soluble cations, whereas soils treated with Na₂SO₄ produced the lowest concentration of these elements irrespective of the type of soil used.     

Seasonal Changes and Nutrient Concentrations of “Cristalina” Cactus Pear Cladodes in Response to NPK Fertilization

There are scarce and conflicting reports on fruit yield (FY) and cladode nutrient concentrations for cactus pears (Opuntia spp.). The objective of this study was to examine the macro- and micronutrient cladode concentrations, the association between FY nutrient concentrations, and seasonal changes in macro- and micronutrients in fruiting cladodes of “Cristalina” cactus pear plants exposed to a nitrogen (N), phosphorus (P), and potassium (K) fertilization matrix over three growing seasons. The experiment was conducted in a randomized complete block design under field conditions. Seven samples of cladode were collected at different phenological stages for the measurement of N, P, K, calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and boron (B). The analysis suggested that the most important nutrients for FY (20.5 t ha^?1) were N and P at 90 and 30 kg ha^?1, respectively. The seasonal pattern for macro- and micronutrients is shown and discussed. The latest topic deserves further research for understanding the cactus pear ionome and suggesting mineral fertilization programs to the cactus pear growers.     

Nutrient‐Release Rates of Controlled‐Release Fertilizers in Forest Soil

Abstract

Nutrient‐release rates of controlled‐release fertilizer (CRF) with four different labeled release periods were evaluated. Samples (30 g) sealed with nylon mesh were buried at a clearcut forest site (Price soil series) in western Oregon, USA, in February 2000 and excavated every 7 weeks for 14 months to determine residual weight and composition. Cool, dry soil conditions apparently prolonged nutrient release beyond labeled rates; the fertilizer with the shortest release period (3–4 months) released approximately 72% of the fertilizer (by weight), whereas that with the longest release period (8–9 months) released 48%. Release varied among individual nutrients [nitrate (NO₃)>ammonia (NH₃)>potassium (K)>sulfur (S)>magnesium (Mg)>phosphorus (P)]. Minimal changes in micronutrient [iron (Fe), manganese (Mn), zinc (Zn), and molybdenum (Mo)] contents were attributed to the formation of insoluble compounds with P. Variable release among individual nutrients demonstrates a limitation toward delivering a full range of nutrients and suggests that further refinement of CRF technology is needed to optimize nutrient availability under realistic field conditions.     

Productivity and Chemical Composition of Tomato and Cucumber Plants Growing in Nickel‐Polluted Soils Fertilized with Biona‐312

Abstract: The effect of ion‐exchange substrate Biona‐312 additions to nickel (Ni)–polluted soil on yield and mineral composition of cherry tomato and cucumber was evaluated. The plants were grown on the following media: untreated soil (the control series) and soil with added Ni (40 and 100 mg of Ni kg^?1, respectively) as well as Biona‐312 (2 and 5% mass additions, respectively). In the presence of 40 mg of Ni kg^?1 of soil, the plant yield did not change significantly, whereas in conditions of 100 of mg Ni kg^?1, it decreased significantly. Biona‐312 application on Ni‐polluted soil increased the productivity of both species. The content of macronutrients in the plant biomass varied depending on the growth stage, Ni level, and Biona‐312 dose. For both species, higher Ni content was observed in the aboveground organs than in roots, but tomato contained more Ni than cucumber. Biona application reduced the Ni content in plant biomass of both species after being introduced into soil with a higher Ni level.     

Use of Agro‐industrial Organic Compound: Yield and Nutrient Absorption in Wheat

Abstract

This study, focusing on the cultivation of wheat, consisted of applying an organic compound to the soil for a period of 2 years. The organic compound was made from a mixture of organic residues from a jelly industry (bovine chips and sharings) and other residues from a sugar‐alcohol industry. The mixture was applied at random into the intergrowth lines in a top‐dressing process, without incorporation, and in cumulative increasing doses. In the first year, the applications ranged from 2 to 25 t ha; in the second year, the applications ranged from 8 to 75, in wheat plants. Comparing the results of the chemical treatment to the organic application, it was observed that the organic compound increased the yield of wheat during the 2 years. Furthermore, the organic compound did not contaminate the plants, such as in the absorption of nutrients by the grains, once they have presented normal patterns of nutrient concentrations.     

Effect of Nutrient Solution PH on the Vegetative and Reproductive Growth and Physiological Characteristics of Rose Cv. ‘Grand Gala’ in Hydroponic System

The rose plants were cultivated in the pots containing 75% cocopeat+ 25% perlite and were nourished with ½ Hoagland nutrient solutions with pH of 4.5, 5.5, 6.5, and 8. The results revealed that plant fresh and dry weight, flower bud, and stem diameter increased with the increase in pH from 4.5 to 6.5, but then started to decrease with further increase in pH up to 8. The highest number of buds, chlorophyll content, F_v/F_m, performance index (PI), and leaf phosphorus, iron, manganese, and copper concentrations were obtained at pH 6.5 and the lowest one at pH 8. The highest and lowest zinc concentration was observed at pH 6.5 and 4.5, respectively. According to the results, pH 6.5 was evaluated as optimum for hydroponic culture of rose, which resulted in the higher uptake of elements and better vegetative and reproductive growth and higher quality of rose plants.     

Tissue‐to‐solvent ratio and other factors affecting determination of soluble polyphenols in tropical leaves

Abstract

Soluble polyphenol concentrations of tropical leaves are believed to affect nitrogen (N) mineralization potential during decomposition. However, polyphenol concentrations reported for the same species have varied several‐fold among studies. Testing potential explanations for these discrepancies, we found high tissue‐to‐solvent ratios, as specified in one of two popular extraction procedures, led to reduced estimates. Use of aqueous methanol rather than water increased recovered polyphenols somewhat, but was not adequate to explain differences among studies. Elevated drying temperatures decreased recovery of soluble polyphenols. Folin‐Ciocalteu reagent produced less precipitate and was more reproducible than Folin‐Denis reagent.     

Decomposition and Nutrient Release from Litter Fall in the Semi‐arid Tropics of Sudan

In Sudan, tree plantations remain the first choice and are widely used in protecting arable lands from sand movement. Decomposition and nutrient changes from leaves of some agroforestry trees (Eucalyptus microtheca, Ficus spp., and Leucaena leucocephala) and litter fall from guava (Psidium guajava) and mango (Magnifera indica) were monitored (in a 12‐week litter‐bag experiment). Rate of dry‐matter weight loss from guava (0.098 wk^?1) was significantly (P < 0.01) faster than from mango residues (0.04 wk^?1). Corresponding values for Leucaena, Eucalyptus, and Ficus were 0.0533, 0.0524, and 0.0438 wk^?1, respectively. In general, micronutrients tend to accummulate during a decomposition period. Potassium (K) was the only element found to be consistently lost by leaching very rapidly from all litters. Nitrogen (N) was released at a significantly (P < 0.03) higher rate from Leucaena (0.0558 wk^?1) compared to Ficus (0.0399 wk^?1) and Eucalyptus (0.0301 wk^?1). Mobility of nutrients from the litters was in the order of K > phosphorus (P) = N > calcium (Ca) > magnesium (Mg). It is concluded that ficus and mango leaves are suitable for improving quality of arid soils through buildup of soil organic matter and supplying easily released organic sulfur (S) (environmentally sound management practice) whereas litter from guava is suitable for temporary nutrient correction. Mixing of guava and mango residues may slow fast decomposition of the former.     

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.     

Nutrient‐solution techniques for evaluation of iron efficiency of soybean 1

Abstract

The iron (Fe) efficiency of soybean [Glycine max (L.) Merr.] genotypes generally has been evaluated in the field on calcareous soil. A nutrient‐solution system has been developed to permit evaluation of Fe efficiency throughout the year. The objectives of this study were to assess the effectiveness of nutrient‐solution tests for evaluating the Fe efficiency of soybean genotypes and to evaluate alternative nutrient‐solution techniques that could minimize the cost of labor and chemicals. Five bicarbonate (HCO₃ ^‐) concentrations and three solution‐change schedules were evaluated in a factorial arrangement. Eight soybean genotypes with a wide range of Fe efficiency were evaluated in each treatment and in replicated field tests on calcereous soil during 3 years. Rank correlation coefficients between mean chlorosis scores of genotypes in nutrient solution and field tests ranged from 0.81 to 0.91 for the three solution‐change schedules and from 0.85 to 0.89 for the five HCO₃ ^‐ concentrations. Replacing the solution every 4 d was not superior to replacing it only at each stage of plant development or not changing the solution throughout the test. A stepwise increase in HCO₃ ^‐ level at each stage of plant development was not superior to utilizing a constant level of HCO₃ ^‐ throughout the test. The most economical evaluation of the Fe efficiency of soybean genotypes in nutrient solution can be achieved with no change in the solution and one or more HCO₃ ^‐ levels that are held constant throughout the test.