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Use of micronutrients for agronomic and horticultural crops has increased markedly in recent years. Increased use is related to higher nutrient demands from more intensive cropping practices and also from farming marginal lands. Most of the fertilizers used to correct micronutrient deficiencies are water- soluble inorganic sources or soluble organic products such as synthetic chelates or natural organic complexes. These fertilizers may react with soil to decrease their availability to plants. The rates of such chemical reactions may differ considerably with each micronutrient fertilizer and soil environment. Recommended micronutrient rates have been based on results of numerous experiments, and these rates vary with crop, soil, and other factors. The usual application rates (on an elemental basis) range from 1 to 10 kg ha−1 for Cu, Fe, Mn and Zn; < 1 kg ha−1 for B; and < 100 g ha−1 for Mo. Because the metallic micronutrients (Cu, Fe, Mn, and Zn) generally are sorbed strongly by soil clays, they do not move significantly in soil. Hence, they are not leached readily from the zone of application to lower soil depths or into groundwaters. Mobility of these micronutrient cations is higher in sandy soils, especially with high leaching conditions. Therefore, their movement out of the root zone is possible under some situations. Significant residual effects of soluble Cu and Zn sources greatly reduce the need for controlled-availability Cu and Zn products. Controlled-availability Fe and Mn fertilizers have not been effective because the rapid oxidation of Fe and Mn and reactions with soil reduce their availability upon release. Because soluble B fertilizers form boric acid molecules in soil, they are mobile and subject to leaching conditions. While mobility of B is less than that of NO 3 - -N in soil, field results have demonstrated loss of applied B from the root zone in sandy soils. Slightly soluble B fertilizers, such as colemanite and ulexite, and fritted B products (powdered glass-like materials whose solubility is controlled by particle size) have been used in sandy soils for some crops. Molybdenum requirements are much lower than those of the other micronutrients. Deficiencies generally are corrected by liming the soil or by seed or foliar applications, so there is little need for controlled-availability Mo fertilizers. Little research has been conducted on controlled availability micronutrient fertilizers. While fritted products are considered in this category, they are difficult to handle and only may be of value in supplying B under specific conditions. Coating soluble granular micronutrient fertilizers also has been attempted, but there are few reported results of their relative effectiveness in comparison with conventional fertilizers and application methods. New micronutrient products may be needed for specific conditions such as greenhouse-culture or container-grown crops, but plant needs also may be met by multiple applications of soluble sources.

Many industrial by-products containing Zn are being processed and sold as Zn fertilizers. Some baghouse dusts and flue dusts which contain ZnO may be used as fertilizers or may be partially acidulated with H2SO4 to form Zn oxysulfates (various mixtures of ZnO and ZnSO4). Spent H2SO4 by-products of several industries, especially the galvanizing industry, also may be processed into ZnSO4 fertilizers. The level of water-soluble Zn (as a percentage of total Zn) in these fertilizers is mainly related to the relative contents of ZnSO4 (water soluble) and ZnO (water insoluble). Other published results have shown that availability to plants of most nutrients in a granular fertilizer is related to their water solubility. Response of corn (Zea mays L.) to fine and granular Zn fertilizers varying in level of water-soluble Zn was determined on a limed Zn-deficient Crowley silt loam (Typic Albaqualf) in three greenhouse pot experiments. Dry matter production and Zn uptake by corn were similar with several finely ground (

Commercial phosphate (P) fertilizers contain small amounts of heavy-metal contaminants which were minor constituents in phosphate rock (PR). Animal manures and sewage sludges (biosolids) are the main organic fertilizers and the latter also may contain heavy-metal contaminants. Heavy metals in biosolids may be found in the inorganic form or may be organically complexed, which could affect their chemical reactions in soil. These heavy metals may accumulate in soil with repeated fertilizer applications. Cadmium (Cd) is the heavy metal of most concern because it may affect human health. Other heavy metals of possible significance are arsenic (As), chromium (Cr), lead (Pb), mercury (Hg), nickel (Ni), and vanadium (V). Some countries have set tolerance limits on heavy-metal additions to soil because their long-term effects are unknown. These limits usually are set for the tillage layer (surface 20–30 cm) of soil where most root activity occurs. Controls on heavy-metal concentrations in sewage biosolids and their maximum total and annual loading rates to soil have been imposed in some countries. Regulations also have been proposed for phased-in limits on maximum heavy metal concentrations permitted in P fertilizers, or they are already in effect. Most of the fertilizer regulations relate Cd limits to P concentrations, so P application rates dictate Cd inputs to soil. Regulations affecting sewage biosolids include a number of heavy metals, while those concerning P fertilizers only include limits on Cd at this time.

Correction of Fe chlorosis is done mainly by foliar sprays because soil applications generally are ineffective, especially for annual crops. Inorganic Fe sources applied to soils react rapidly to forms which are not as available to plants; ferrous Fe is oxidized to the ferric form in well-aerated soils, especially as soil pH increases. Several synthetic chelates and organic complexes have been used with varying success, depending upon Fe source and rate, application method, plant species, and weather and soil conditions. Use of Fe-efficient cultivars is one method of counteracting Fe deficiencies in some species. Future prospects for improving control of Fe chlorosis lie more with development of Fe-efficient cultivars of Fe-sensitive species than with development of improved Fe fertilizers and methods of application. The techniques of molecular biology should be applicable to help solve this important plant nutrition problem, but priority has not been given to conduct this research at this time.

Response of corn (Zea mays L.) to fine and granular Mg fertilizers was determined on an acid, Mg-deficient Weston fine sand (Typic Ochraquults) in a greenhouse pot experiment. Corn forage yields were similar with finely ground (< 0.2 mm) MgSO4, MgO, MgCO3, and Mg-oxysulfate (a partly acidulated MgO product). Granular MgSO4 (100% water-soluble) was as effective as the finely ground products. Forage yields with granular MgO and Mg-oxysulfate were related to their granule size. Small (1.4 to 2.0 mm) granules were almost as effective as the finely ground fertilizers, but medium (2.4 to 3.4 mm) and coarse (3.4 to 4.8 mm) granules were increasingly less effective. Magnesium uptake by corn generally increased as forage yields increased.

Cation movement in soil under leaching conditions has been associated with N fertilization. Therefore, this study was conducted to determine whether the mobility of some heavy metals applied in the inorganic form or in sewage sludge is enhanced in the presence of various sources of N. Columns of heavy metal-amended soil in plastic well casings were cropped with tall fescue (Festuca arundinacea Schreb.) and leached three times with deionized H/sub 2/O. Heavy metal concentrations above check values were not detected in leachates from any column. Mobility of the heavy metals from the inorganic sources was slightly greater than that from the sewage sludge. Nitrogen fertilization did not affect the downward movement of Zn, Cd, Cr, Pb, or Ni in soil but enhanced uptake of these metals by fescue because of increased growth. These results suggest that heavy metal contamination of ground water is not likely in heavy textured soils when sewage sludge applications are accompanied by N fertilization, at least for short periods of time. 11 references, 1 figure, 4 tables.

A field experiment was conducted to determine uptake of Cd and other heavy metals by winter wheat (Triticum aestivum L.) from three diammonium phosphate (DAP) fertilizers containing 2, 74, and 153 ..mu..g Cd/g. A phosphorus-deficient Paden silt loam (Glossic Fragiudult, pH 4.7) was limed at two rates and treated with the DAP sources at a rate of 50 kg of P/ha (100 lb of P/sub 2/O/sub 5//acre). Grain yields were higher at pH 5.9 than at pH 5.1, were significantly increased by P applications, and results were similar using all DAP sources. Concentrations of Cd in both grain and straw were significantly increased only with application of high-Cd DAP to the low-lime soil. Cadmium concentrations in grain increased from 0.028 to 0.086 ..mu..g/g, and those in straw increased from 0.067 to 0.118 ..mu..g/g (dry weight basis) with application of low-Cd DAP and high-Cd DAP, respectively, and were lower on the high-lime soil. Concentrations of Cu, Mn, Ni, and Zn in both grain and straw were not significantly affected by DAP source. Results for a second year on the same plots limed to pH 5.8 and 7.0 and treated again with the same DAP sources were similar to those for themore » first year, except that heavy metal uptake was generally lower. Results suggest that Cd concentrations in wheat products are not significantly changed by the phosphate fertilization and crop production practices in general use in the United States at this time.« less

Influence d'epandage de phosphogypse sur la croissance et l'absorption de cadmium et de radium des vegetaux (mais, ble et soja)

Many Fe sources have been tested to correct Fe chlorosis over the past 35 years, and no single Fe source has been found that is both effective and economical for most crops. Results from many Fe experiments are discussed. Variable results are reported from Fe source studies related to differences in Fe placement, rate, time of application, and weather and soil conditions. While controlling Fe chlorosis by developing more Fe‐efficient cultivars appears more promising, work to develop effective Fe fertilizers should continue. Perhaps the best method to control Fe chlorosis will be a combination of Fe‐efflcient cultivars with the best management practices, and reasonably effective Fe fertilizers.

Calcium silicate slag is a by‐product of the electric furnace reduction process for producing phosphoric acid from phosphate rock (PR). Since slag contains trace amounts of radium (Ra), and uranium (U) as contaminants from the PR, a greenhouse pot experiment was conducted to determine if plants absorb Ra from slag applied to soil. Slag at rates equivalent to 0 and 22 mt/ha was mixed with Mountview silt loam (Typic Paleudults) limed to pH 5.8 and 7.2. Three clippings each of fescue (Festuca arundinacea Schreb.), and Swiss chard (Beta vulgaris L.), and one harvest of wheat (Triticum aestivum L.) for grain and straw were grown on separate series of treated soil, and plant samples were analyzed for radioactivity due to Ra uptake. Samples of sugarcane (Saccharum officinarum L.) forage and extracted juice from field experiments in Florida testing this slag as a Si source also were analyzed for radioactivity. Dry forage yields of fescue and wheat were not affected by slag applications, but those of Swis...

This study related Cd-Zn concentrations in plants to levels of Cd and Zn in soil recovered by several extractants soon after application of Cd and Zn sources to soil. Cadmium nitrate and ZnSO/sub 4/ or Zn(C/sub 2/H/sub 3/O/sub 2/)/sub 2/ were mixed with a Zn-deficient Crowley silt loam soil, cropped with corn (Zea mays L.), and then cropped with Swiss chard (Beta vulgaris). Applied Cd significantly increased the Cd/Zn ratio in both crops, especially in Swiss chard, which accumulates heavy metals. A previously published sequential-extraction procedure was used to fractionate Cd and Zn in soil after the corn harvest. Results of statistical analyses showed the highest correlation between Cd uptake by each crop and the carbonate and sulfide fractions of Cd in soil. Including other Cd fractions resulted in only slightly higher R/sup 2/ values. Zinc uptake by each crop was best related to the organic fraction of Zn in soil, and including the other Zn fractions did not affect the relationship. These results show that Cd and Zn uptake by corn or Swiss chard was not related to similar chemical fractions of these elements in soil, and that the Cd/Zn ratio in plant tops was significantly affected by bothmore » Cd and Zn applications to soil.« less

In recent years intensive farming and the greater use of marginal lands have led to increasing incidence of deficiencies of micronutrients. This article reviews ways in which micronutrients can be made available to crops – either in the preparation of the soil or during growth – according to local circumstances. The form in which the micronutrients is applied is of critical importance for correcting the deficiency most effectively and economically.

Increased emphasis on biological fixation of N has resulted in increased soybean acreage and establishment of legumes for pastures in the Southeastern United States. Many of the soils in this region are acid and therefore may be marginal in available Mo, which is involved in symbiotic N fixation by legumes. A greenhouse experiment was conducted to determine relative response to applied Mo of two cultivars of soybeans and six species of forage legumes on acid and limed soil. Decatur sil (Rhodic Paleudult), at pH 5 and limed to pH 6 and 7, was fertilized with 0 and 2 ppm of Mo, P, K, and a micronutrient mixture minus Mo, inoculated with appropriate rhizobia, and seeded to legumes. Soybean tops were harvested after 6 weeks and three clippings of the forage legumes were made after 6, 9, and 13 weeks’ growth. Dry forage yields of all species increased with soil pH level without applied Mo. Soil application of Mo resulted in increased growth and uptake of N and Mo of most species at pH 5. Crop response...

Soil applications of inorganic Fe sources are generally not effective for crops unless rather high rates are applied. Since previous results had indicated that these sources may be effective if applied at lower rates with fluid polyphosphate fertilizers, several greenhouse pot tests were conducted. Dry matter production and Fe uptake by grain sorghum (Sorghum bicolor) were higher with FeEDDHA than with FeSO₄ or Fe₂₍SO₄)₃ applied to Epping sil (pH 7.5), a calcareous soil low in available Fe. There was very little crop response to either inorganic Fe source applied alone to the soil at rates up to 24 ppm Fe. However, both sources were somewhat effective when applied with fluid polyphosphate fertilizers at the same rates. Seed and foliar spray applications were not as effective as soil application, probably because the rate of Fe was limited by the application method. Without applied Zn, crop response to FeEDDHA was limited. However, Zn uptake by sorghum decreased while forage yields increased with FeEDDHA application rate, even though ZnSO₄ was also applied to the soil. Evidence for a marked Fe-Zn interaction in sorghum was shown on this soil. Application of ZnSO₄ to the no-Fe check treatment reduced growth and Fe uptake, while high Fe applications reduced Zn uptake in the presence or absence of applied Zn.

Zinc deficiency in rice (Oryza sativa L.) has only recently been diagnosed. Although most Zn studies have been conducted on flooded rice, comparative data concerning the availability of Zn under upland and flooded soil conditions is needed. Therefore, rice (var. ‘Bluebelle’) was grown hi greenhouse pots on flooded and moist, limed Crowley sil (pH 7.5). Phosphorus, as concentrated superphosphate, was applied at rates of 20 and 200 ppm and Zn as ZnSO₄, was applied at rates of 1, 4, and 16 ppm. Dry matter production and P uptake of immature plants were doubled and uptake of Zn was up to five times as great when the plants were grown on flooded rather than moist soil. The Zn concentration in the tissue decreased at the high P rate on moist but not on flooded soil. The percentage recovery of fertilizer Zn by the crop was greater from flooded than moist soil, especially at the high P rate. Mixed and surface applications of Zn were comparable for flooded rice, whereas middepth placement (8 cm below soil surface) was less effective. Both surface and middepth placements were less effective than mixing under nonflooded soil conditions. Results of laboratory studies indicated that the mobility of Zn applied as ZnSO₄ was comparable in flooded and moist soil. The amount of DTPA-extractable Zn tended to be higher in moist than in flooded soil. This suggests that DTPA may not provide a reliable measure of Zn availability in flooded soils and that differences in Zn uptake may be related to root distribution or some other physiological or morphological characteristic of rice roots growing in moist or flooded soil.

Fertilizers varying in level of H₂O-soluble Zn were prepared by combining fine ZnO and ZnSO₄ in various proportions and pressure-granulating the powders with ammonium nitrate, monoammonium phosphate, or ammonium polyphosphate. Forage yields and uptake of Zn by corn (Zea mays L.) grown on a P- and Zn-deficient soil in the greenhouse increased with the level of H₂O-soluble Zn in these macronutrient fertilizers. Additional crop response was slight to fertilizers with levels of H₂O-soluble Zn in excess of about 50% but this varied, depending upon the macronutrient carriers. However, the amounts of Zn extracted after cropping from untreated soils and soils initially receiving 12 mg of Zn per 3 kg of soil in a highly H₂O-soluble form were comparable. In a second experiment differential response of several corn hybrids to amount of P and Zn and level of Zn solubility in the fertilizers suggests that the test variety might influence the interpretation of the results from fertilizer evaluation studies. No relationship was found between P/Zn ratios in the corn forage and the seed. The P/Zn ratios in corn forage were not related to the response of corn to applied Zn.

Maize and pea bean plants were grown for 2 weeks in Zn-deficient soil fertilized with varying levels of Fe and Zn. The plants were then exposed to C14O2 for 2 minutes and after harvest the ethanolsoluble photosynthate was fractionated into carbohydrates, organic acids, and amino acids. There appeared to be a general increase in the CO2 assimilation rate by maize with increasing rate of Zn; rates were slightly higher with applied Fe, at each Zn level. In contrast to maize, the rate of assimilation by pea beans was unaffected by substrate Zn and was slightly lower with application of Fe, especially in the carbohydrate fraction. Increasing rates of applied Zn had little effect on concentration of Fe and Mn in the maize or pea bean tops. However, application of Fe resulted in a depression in Zn concentration of maize. Despite the relatively high levels of Zn in the tissue of both species, there was no increase in total organic acids assimilated or any indication of gross metabolic imbalances.