Predicting the incidence of iron chlorosis in calcareous soils of southern Spain

Abstract

Iron chlorosis is a serious crop production problem in many calcareous soils of Southern Spain. The objective of this study was to determine which indigenous soil properties (i.e., those which are essentially permanent) were related to Fe chlorosis. Experiments, using two chickpea (Cicer ariethinum L.) cultivars and a sunflower (Helianthus annuuus L.) cultivar, were carried out in a growth chamber with 25 calcareous soils representing widespread Xerofluvents, Xerorthents, Xerochrepts, Haploxeralfs, Rodoxeralfs, Chromoxererts, and Pelloxererts of Southern Spain. The average chlorophyll contents for the three cultivars were significantly correlated with several properties of the carbonate and Fe oxide phases, such as calcium carbonate equivalent (r = 0.69***), “active lime”; (r = 0.58**), acid NH₄‐oxalate extractable Fe (r = 0.68***), Tiron‐extractable Fe (r = 0.61**), and DTPA‐extractable Fe (r = 0.55**). The present and other studies indicate that the soil property most consistently related to Fe chlorosis is acid NH₄‐oxalate extractable Fe (Feo). The Feo critical level separating soils with a high probability from those with a low probability of responding to Fe fertilization was 0.63 g/kg soil, a value similar to those found in other studies. This further supports the use of Feo as a key property to predicting the appearance of Fe chlorosis.     

Adaptation of some introduced eragrostis species to calcareous soil and acid mine spoil

Abstract

Plant growth is frequently limited by Fe‐related chlorosis on calcareous soils and by mineral toxicities on strongly acid soils and mine spoils. Better adapted varieties are needed for both soil situations, which are not always economically correctable. In a search for such geraplasm, 4 species (20 accessions) of Eragrostis were grown in greenhouse pots of a calcareous soil at pH 7.3. Two species were also compared on acid mine spoil at pH 3.5 and 4.7.

Species, and accessions within species, differed significantly in tolerance to the calcareous soil, as measured by susceptibility to chlorosis and yield of plant tops. The range in top yield was 11‐fold for accessions of Eragrostis capensis, 3‐fold for Eragrostis lehmanniana, and 1.7‐fold for Eragrostis superba. Eragrostis plana (P.I. 364340) was more tolerant to acid mine spoil (pH 3.5) but less tolerant to calcareous soil (pH 7.3) than Eragrostis superba (P.I. 364833).

Chlorosis and poor growth of certain accessions on calcareous soil (pH 7.3) were not explained by specific mineral deficiencies or toxicities. However, the tops of chlorosis‐susceptible accessions had lower ratios of Fe/Mn, Fe/Zn, and Fe/Cu than those of chlorosis‐resistant accessions. This imbalance is believed to interfere with Fe metabolism in plant tops.

Results suggested that superior strains of Eragrostis species can be selected for adaptation to calcareous or acid soils and that certain accessions characterized in these studies can be useful in studying the physiological mechanisms of mineral stress resistance in plants.     

Biochar application to temperate soils: Effects on soil fertility and crop growth under greenhouse conditions

Biochar (BC) application as a soil amendment has aroused much interest and was found to considerably improve soil nutrient status and crop yields on poor, tropical soils. However, information on the effect of BC on temperate soils is still insufficient, with effects expected to differ from tropical soils. We investigated the effects of BC on soil nutrient dynamics, crop yield, and quality in a greenhouse pot experiment. We compared three agricultural soils (Planosol, Cambisol, Chernozem), and BCs of three different feedstocks (wheat straw [WS], mixed woodchips [WC], vineyard pruning [VP]) slowly pyrolyzed at 525°C, of which the latter was also pyrolyzed at 400°C. The BCs were applied at two rates (1% and 3%, which would correspond to 30 and 90 t ha^–1 in the field). Three crops, namely mustard (Sinapis alba L.), barley (Hordeum vulgare L.), and red clover (Trifolium pretense L.) were grown successively within one year. The investigated soil properties included pH, electrical conductivity (EC), cation‐exchange capacity (CEC), calcium‐acetate‐lactate (CAL)–extractable P (P_CAL) and K (K_CAL), C, N, and nitrogen‐supplying potential (NSP). The results show a pH increase in all soils. The CEC increased only on the Planosol. The C : N ratio increased at 3% application rate. Despite improving the soil nutrient status partly, yields of the first crop (mustard) and to a lesser extent of the second crop (barley) were significantly depressed through BC application (by up to 68%); the yield of clover as third crop was not affected. Only the BC from WS maintained yields in the range of the control and even increased barley yield by 6%. The initial yield reduction was accompanied by notable decreases (Cu, Fe, Mn, Zn) and increases (Mo) in micronutrient concentrations of plant tissues while nitrogen concentrations were hardly affected. The results of the pot experiment show that despite additional mineral fertilization, short‐term growth inhibition may occur when applying BC without further treatment to temperate soils.     

Prediction of residual effects of zinc sulfate on growth and zinc uptake of corn plants using three zinc soil tests

Abstract

A significant portion of chemical zinc (Zn) fertilizers applied to calcareous soils is not absorbed by the first crop and may, therefore, affect the growth and chemical composition of the subsequent crops. This is called the residual effect of Zn. Soil tests may be used to predict such effects. The present experiment was conducted to study the residual effects of zinc sulfate (ZnSO₄) on the second crop of corn (Zea mays L.) grown on selected highly calcareous soils of Iran and to compare the suitability of three soil tests for prediction of the effects. Twenty highly calcareous soils of southern Iran (16–58% calcium carbonate equivalent; pH 7.9–8.5), previously treated with three levels of Zn (0, 10, and 20 mg Zn/kg as ZnSO₄) and under one crop of corn, was used in greenhouse to grow a second crop of corn without additional Zn fertilizer but with uniform application of nitrogen (N), phosphorus (P), and iron (Fe). Soils were sampled before the second crop and extracted with three Zn extradants, DTPA, EDTA‐(NH₄)₂CO₃, and EDTA. Dry weight of plant tops and Zn concentration and uptake after eight weeks under the greenhouse conditions were used as the plant responses to residual Zn. Statistical analyses including F‐test and multiple regression equations showed that the overall effect of previously‐applied Zn on dry matter was nonsignificant, but Zn concentration and uptake were significantly increased. The three soil tests predicted the Zn concentration and uptake equally well. Moreover, DTPA and EDTA soil tests could predict the dry matter of plants at the highest level of previuosly‐applied Zn (20 mg Zn/kg), especially when selected chemical properties of soil, namely, calcium carbonate equivalent or organic matter content, were considered in the regression equations.     

Evaluation of preflooding effects on iron extractability and phytoavailability in highly calcareous soil in containers

Previous pot cropping and laboratory incubation experiments were consistent with field observations showing that temporary flooding before cropping can increase the availability of soil Fe to plants. To study the effect of temporary flooding on changes in soil Fe phytoavailability we used 24 highly calcareous, Fe chlorosis–inducing soils to carry out a pot experiment where peanut and chickpea were successively grown after flooding for 30 d. At the end of the cropping experiment, the preflooded soil samples exhibited higher concentrations of acid oxalate‐, citrate/ascorbate‐ and diethylenetriaminepentacetic acid (DTPA)–extractable Fe (Fe_ox, Fe_ca, and Fe_DTPA, respectively) than the control (nonflooded) samples. Also, Fe_ox and Fe_ca exhibited no change by effect of reflooding of the cropped soils or three wetting–drying cycles in freeze‐dried slurries of soils previously incubated anaerobically for several weeks. Leaf chlorophyll concentration (LCC) in both peanut and chickpea was greatly increased by preflooding. The best predictor for LCC was Fe_ox, followed by Fe_ca and Fe_DTPA. The LCC–soil Fe relationships found suggest that the Fe species extracted by oxalate and citrate/ascorbate from preflooded soils were more phytoavailable than those extracted from control soils. However, the increased phytoavailability of extractable Fe forms was seemingly limited to the first crop (peanut). Flooding dramatically increased Fe_DTPA; however, high Fe_DTPA levels did not result in high LCC values, particularly in the second crop. Therefore, this test is a poor predictor of the severity of Fe chlorosis in preflooded soils.     

CHLOROSIS IN WILD PLANTS: IS IT A SIGN OF IRON DEFICIENCY?

ABSTRACT

Chlorosis in crops grown on calcareous soil is mainly due to iron (Fe) deficiency and can be alleviated by leaf application of soluble Fe²⁺ or diluted acids. Whether chlorosis in indigenous plants forced to grow on a calcareous soil is also caused by Fe deficiency has, however, not been demonstrated. Veronica officinalis, a widespread calcifuge plant in Central and Northern Europe, was cultivated in two experiments on acid and calcareous soils. As phosphorus (P) deficiency is one of the major causes of the inability of many calcifuges to grow on calcareous soil we added phosphate to half of the soils. Leaves in pots with the unfertilized and the P-fertilized soil, respectively, were either sprayed with FeSO₄ solution or left unsprayed. Total Fe, P, and manganese (Mn) in leaves and roots and N remaining in the soil after the experiment were determined. In a second experiment, no P was added. Leaves were either sprayed with FeSO₄ or with H₂SO₄ of the same pH as the FeSO₄ solution. Degree of chlorosis and Fe content in leaves were determined. Calcareous soil grown plants suffered from chlorosis, which was even more pronounced in the soils supplied with P. Newly produced leaves were green with Fe spray but leaves that were chlorotic before the onset of spraying did not totally recover. H₂SO₄ spray even increased chlorosis. This demonstrated that chlorosis was due to Fe deficiency. As total leaf Fe was similar on acid and calcareous soil, it was a physiological Fe deficiency, caused by leaf tissue immobilization in a form that was not metabolically “active”. Iron in the leaves was also extracted by 1,10-phenanthroline, an Fe chelator. In both experiments, significant differences between leaves from acid and calcareous soil were found in 1,10-phenanthroline extractable Fe but not in total leaf Fe, when calculated on a dry weight basis. Differences in 1,10-phenanthroline extractable Fe were more pronounced when calculated per unit dry weight than calculated per leaf area, whereas the opposite condition was valid for total leaf Fe.     

Stimulative effects of elemental sulfur in the presence of ammonium on chile and broccoli growth in calcareous soils

The chile pepper plant seldom responds to N and P fertilizers on fertile soils. Surplus industrial H₂SO₄ and elemental S have created interest in “mining”; calcareous soils for additional supplies of P, Ca, Mg and micronutrients. The effect of variable S, on the growth of chile and broccoli was evaluated holding other nutrients constant. Growth of chile and broccoli plants was significantly increased in the greenhouse and chile yield increased in the field. Incremental S additions increased the water extractable and desorbable Ca + Mg and P contents of soil. The total N and K content of chile plant grown in the greenhouse increased, and then decreased, P decreased, as S rates increased. Yield of dry red chile with constant N peaked at 16.5 g S m^‐2 and then decreased with increasing S in the field. Rroccoli responded more to S application than to directly applied foliar micronutrient solutions (Fe and 7n), and responded much better to (NH₄)₂SO₄ + S than to Ca(NO₃)₂ at equivalent N rates. Increased soluble Ca + Mg content of the soil in the presence of S was thought to influence plant absorption of NH₄ and/or K.     

Evaluation of Dolomite Phosphate Rock–N‐Viro Soil Mixtures for Growth of a Horticultural Crop in an Acidic Sandy Soil

Abstract

Most agricultural soils in the Indian River area, South Florida, are sandy with minimal holding capacity for moisture and nutrients. Phosphorus (P) leaching from these soils has been suspected of contributing to the eutrophication of surface waters in this region. Dolomite phosphate rock (DPR) and N‐viro soil are promising amendments to increase crop production and reduce P loss from sandy soils. Soil incubation and greenhouse pot experiments were conducted to examine the effects of Florida DPR–N‐viro soil mixtures on the growth of a horticultural crop in an acidic sandy soil and to generate information for developing a desired formula of soil amendments. Dolomite phosphate rock and N–viro soil application increased soil pH, electrical conductivity (EC), extractable P, calcium (Ca), and magnesium (Mg). N–viro soil had greater effect on soil pH, organic matter content, and microbial biomass than the DPR. Comparatively higher nitrification rates were found in the N–viro soil treatment than the DPR treatment. A systematic decrease in soil‐extractable P was found with increasing proportions of N‐viro soil from the combined amendments. Greenhouse study demonstrated that the application of DPR and N‐viro soil significantly improved dry‐matter yield and increased plant P, Ca, and Mg concentrations of radish (Raphanus sativus L.). Based on dry‐matter yield and plant N uptake, the combined amendments that contained 30% or 20% of DPR materials appear to be optimal but remain to be confirmed by field trials.     

盆栽研究钢渣用作铁肥对玉米生长和土壤改良的作用

The feasibility of steel slag used as an iron fertilizer was studied in a pot experiment with corn. Slag alone or acidified slag was added to two Fe-deficient calcareous soils at different rates. Results showed that moderate rates （10 and 20 g kg^-1） of slag or acidified slag substantially increased corn dry matter yield and Fe uptake. Application of steel slag increased the residual concentration of ammonium bicarbonate-diethylenetriamine pentaacetic acid （AB-DTPA） extractable Fe in the soils. The increase of extractable Fe was usually proportional to the application rate, and enhanced by the acidification of slag. Steel slag appeared to be a promising and inexpensive source of Fe to alleviate crop Fe chlorosis in Fe-deficient calcareous soils.     

Soil Properties Influencing Iron Chlorosis in Grapevines Grown in the Montilla‐Moriles Area,Southern Spain

Abstract

Iron (Fe) chlorosis, an Fe deficiency commonly observed in grapevines cultivated on calcareous soils, generally inhibits plant growth and decreases yield. The objective of this research was to relate the incidence of Fe chlorosis in vines of the Montilla‐Moriles area, southern Spain, to indigenous soil properties. Thirty‐five grapevines (V. vinífera L. cv. Pedro Ximenez grafted on V. berlandieri×V. rupestris 110 Ritcher) showing different degree of Fe chlorosis were selected from 13 vineyards. The leaf chlorophyll concentration (estimated by the SPAD value measured with a Minolta meter) was positively correlated with the contents in different soil Fe forms but not with alkalinity‐related soil properties (pH, calcium carbonate equivalent, and active lime). The acid NH₄ oxalate‐extractable Fe (Fe_o) was the most useful simple variable to predict the occurrence of Fe chlorosis. A Fe_o/active lime ratio of 25×10^–4 was found to be useful to class soils into two groups according to the probability of inducing Fe chlorosis.     

Predicting Liming Needs of Soybean Using Soil pH,Aluminum, and Manganese Soil Tests

Abstract

Whether a tropical soil should be limed or not for a particular crop is strongly dependent on the levels of soil aluminum (Al) which can be determined with soil tests. Soil pH is used to predict whether lime is needed in less‐weathered soils, although some evidence indicates a soil Al test would be more accurate. The objectives of this study were to determine and to compare the accuracies of four soil tests to separate soils requiring lime from those that do not, and to determine the cause of acid‐soil injury to soybean [Glycine max (L.) Merr.]. Soybean was grown in the greenhouse on four surface soils representing the major land resource areas of Louisiana and were amended with eight rates of lime, yields determined, and soils analyzed for soil pH, extractable Al, CaCl₂‐extractable Al, CaCl₂‐extractable manganese (Mn), and Al saturation. Acid‐soil injury in soybean grown on the Litro clay and Stough fsl was probably caused by soil‐Al effects while low soil calcium (Ca) and high soil Mn was likely responsible for lower yields from the Mahan fsl. Leaf Ca from the limed Mahan‐soil treatment was 5‐fold greater and leaf‐Mn 7‐fold less than control levels. Regression analyses’ R² values were similar for all soil tests except for CaCl₂‐extractable Mn, which was lower. Soil tests were compared across soil type by selecting treatments that had the same 85% relative yield. Using this data subset, there was no difference in the soil pH among the four soils, while there were significant differences among soils for all other soil test measurements indicating the superiority of soil pH for identifying acid‐soil injury. Critical test values were 5.1 soil pH, 30 mg kg^‐1 extractable Al, 7% Al saturation, 0.7 mg‐kg^‐1 CaCl₂‐extractable Al, and 9 mg‐kg^‐1 CaCl₂‐extractable Mn.     

Calcium‐ and iron‐related phosphorus in calcareous and calcareous marsh soils: Sequential chemical fractionation and 31p nuclear magnetic resonance study

Abstract

Phosphorus (P) forms in soils determine the amount of P available for crops and the potential for this element to be released to water. Sequential chemical fractionation can provide some information about major P forms in soils, and allow one to distinguish iron (Fe)‐related phosphorus from calcium (Ca)‐bound P. The ³¹P nuclear magnetic resonance (NMR) spectroscopy has been used in the identification of organic P, precipitated Ca‐phosphates, and aluminum (Al)‐related P in acid soils. Three calcareous soils and four calcareous marsh soils were used in this study. These two types of soils differ in the nature of iron oxides, which are the main P sorbent surfaces. The ratio of low crystalline to high crystalline iron oxides is higher in marsh soils than in calcareous soils as a consequence of the special genesis and conditions of the soil (reduction‐oxidation cycles). Such a ratio is related to the proportion of occluded P in low crystalline oxides relative to that of high crystalline oxides. Citrate‐bicarbonate extractable P (CB‐P) in the fractionation schemes can be ascribed to adsorbed P and high soluble calcium phosphates. CB‐P is correlated with the sum of P fractions in all the soils, thus indicating that the amount of the P that can be easily released is related to the rate of P enrichment of the soil. The ³¹P NMR spectral data reveal that hydroxyapatite is the dominant P form in the soils studied. This is consistent with the fractionation data, where acid‐extractable P is the main P fraction. The spectra also provide some information about the amount of total inorganic P and Ca‐phosphates in calcareous soils.     

Comparison of EDTA and ammonium Bicarbonate-DTPA for the extraction of phosphorus in calcareous soils from Kerman,Iran

Developing a fast and reliable soil testing method is critical for improving soil testing efficiency and ensuring reliable fertilizer recommendation. The objectives of this study were to evaluate sodium ethylene diamine tetra acetic acid (Na₂-EDTA) as a replacement for ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA) to extract phosphorus (P) to determinate the relationships between extractable P and its uptake by crop in calcareous soils. Na₂-EDTA and AB-DTPA was compared by the amounts of extracted P by analyzing soil samples collected from agricultural production areas. There were significant correlations between Na₂-EDTA and AB-DTPA for soil test P based on soils collected from the agricultural field. Soil test P by both extractants was significantly correlated with plant P concentration. Na₂-EDTA was identified as an alternate improved extraction method instead of AB-DTPA in calcareous soils based on this study. However, more work will be needed to identify the correlation of the two extractants and crop responses under a field condition.     

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.     

Effect of sewage sludge application to amelio‐rate iron deficiency of grain sorghum

Abstract

Recent research has indicated that land application of municipal sewage sludge to calcareous soils can be used to ameliorate iron (Fe) deficiency of grain sorghum [Sorghum bicolor (L.) Monech]. A greenhouse study was conducted to determine the response of grain sorghum grown on three different soils to application of sewage sludge. Sludge applied at rates of 0, 7.5, 15.0, and 25.0 g/kg soil did not completely ameliorate grain sorghum Fe deficiency. When FeEDDHA was soil applied, sewage sludge application significantly increased plant growth due to increases in soil phosphorus (P) availability. Application of sewage sludge at rates greater than 7.5 g/kg reduced dry matter production of grain sorghum in the FeEDDHA amended Orelia SC soil, the soil with the lowest total neutralizing potential. The decreases yield was possibly due to toxic levels of soil and plant copper (Cu) and zinc (Zn), and increased soil salinity.     

Compost and P amendments for stimulating microorganisms and maize growth in a saline soil from Pakistan in comparison with a nonsaline soil from Germany

A 92 d greenhouse pot experiment with maize (Zea mays L.) was carried out with a strongly saline soil from Pakistan (P‐s) in comparison with a nonsaline soil from Germany (G‐s) similar in pH and texture. The aim was to evaluate salinity effects on the decomposition of compost and effects of compost and P amendments on (1) plant growth and (2) microbial‐biomass formation. The yield of maize shoot‐C and root‐C increased in both soils in the order nonamended control < +triple superphosphate (TSP) (A1) < +compost (A2) < +(compost + TSP) (A3) < +TSP‐enriched compost (A4). In comparison with the control, the highest yield in treatment A4 was nearly doubled on the G‐s, but was increased more than 8‐fold on the saline P‐s. Averaging the three compost treatments, 32% of the compost added was decomposed in the German soil and 36% in the Pakistani soil on the basis of the compost recovered as particulate organic matter. These data were roughly in agreement with the CO₂‐evolution data. This indicates that the decomposition of compost was not affected by salinity. Compost‐derived CO₂ was mainly evolved until day 32, the root‐derived CO₂ from day 74 until the end of the experiment. The addition of compost resulted in higher contents of microbial biomass C and biomass P, but also in that of NaHCO₃‐extractable P. These three properties were significantly interrelated (r = 0.64–0.85), but on a lower level of significance than the relationships between shoot‐C, root‐C, and NaHCO₃‐extractable P (r = 0.90–0.93). Applying compost enriched with TSP (incubation of compost and TSP for 24 h) provided considerably more P to plants and microorganisms than the separate addition of these two components. The results suggest that the role of the microbial biomass as a sink and source for available P deserves further attention.     

Economics of phosphorus fertilization of barley as influenced by concentration of extractable phosphorus in soil

Abstract

Field experiments were conducted at 60 sites in central and north‐central Alberta to determine the yield response of barley (Hordeum vulgare L.) to phosphorus (P) fertilizer and economics of P application on soils with different concentrations of extractable P in the 0–15 cm soil layer. On the unfertilized plots, barley yield increased with increasing concentration of extractable P in the soil up to 22 mg P kg^‐1, but the yield response to applied P decreased. The net present value (NPV) of returns from P fertilization increased with increasing rate of P up to approximately 51 kg P₂O₅ ha^‐1. The NPV of applied P decreased with increasing concentration of extractable P in soil. On soils with extractable P more than 22 mg P kg^‐1, P application did not result in positive NPV.     

Phosphate sorption by Egyptian,Ethiopian and German soils and P uptake by rye (Secale cereale L.) seedlings

Phosphate sorption was studied in samples (0 - 20 cm depth) of five soils from Egypt (pH 7.4 - 8.7), four soils from Ethiopia (pH 3.9 - 5.3) and six soils from Germany (pH 3.3 - 7.2). Sorption parameters were calculated according to Pagel and Van Huay (1976) and according to Langmuir (Syers et al., 1973). Phosphate sorption parameters and oxalate extractable Fe and Al (Fe_ox, Al_ox) were related to the phosphate uptake by young rye plants in Neubauer pot experiments. P sorption parameter after Pagel and Van Huay (A) correlated significantly positively with the Fe_ox and Al_ox content in acid (r = 0.73) as well as in calcareous soils (r = 0.89) if the whole equilibrium concentration range (0 - 14 mg P/L) was considered. The relations calculated after Langmuir (B) were similar. P uptake by rye in acid soils was negatively correlated with the affinity constant n (r = ?0.76, (A)). In calcareous soils, a negative correlation between P uptake and affinity constant was calculated in the lower P equilibrium range (0 - 2.8 mg P/L) only for (B). Thus, P uptake decreased with increasing strength of P bonding to soil. From these results it is concluded that phosphate sorbed to Fe/Al oxides is an important P source for plants in acid and calcareous soils.     

Availability of Organic and Inorganic Phosphorus Fractions to Wheat in Toposequences of Calcareous Soils

Abstract

Information on the availability of different soil phosphorus (P) forms is useful for crop production. Phosphorus contents of 12 Iranian calcareous soils from upper‐, mid‐, and lower‐slope positions of two arid and two semiarid toposequences were fractionated to various organic and inorganic pools, and correlations of the P fractions with wheat responses were investigated. Among the inorganic P (IP) fractions, apatite type (Ca₁₀‐P) and dicalcium phosphate equivalents (Ca₂‐P) possessed the highest and the lowest amounts of P reserve in the soils, respectively. On average, about 20% of the total P was found in organic form (OP), of which 32% was labile (LOP), 51% was moderately labile (MLOP), and 17% was nonlabile (NLOP). The amounts of the soil P fractions were considerably influenced by the positions of the soils on the landscapes. The maximum contents of soil IP, Ca₂‐P, Fe‐P (iron‐bound P), and Ca₁₀‐P were observed in the lower‐slope positions. The amount of soil available [0.5 M sodium bicarbonate (NaHCO₃) extractable] P was significantly correlated with Ca₂P (r=0.895), Fe‐P (r=0.760), and Occl‐P (iron‐occluded P) (r=0.897). Direct correlation studies, however, showed that wheat shoot dry‐matter yield (DMY) was significantly affected by the amounts of Ca₂‐P, Fe‐P, OP, LOP, and MLOP fractions both at early (4 weeks) and late (10 weeks) stages of growth. All organic and inorganic P fractions, except Al‐P (aluminum‐bound P), Ca₈‐P (octacalcium phosphate equivalents), and NLOP, also showed significant relations to the amount and/or concentration of P in wheat tissues at 4 and 10 weeks after sowing. Among the measured soil properties, the amount of organic carbon was the most affecting factor on the size of the P fractions.     

Critical Olsen P and CaCl2‐P levels as related to soil properties: results from micropot experiments

The usefulness of soil phosphorus (P) tests used in routine soil analyses is limited by the fact that a single measurement cannot encompass all P‐related factors potentially affecting plant performance. In this work, we performed micropot (15 mL) experiments to test the hypothesis that the predictive value of two common soil P tests (Olsen P and CaCl₂‐P) can be improved by considering properties commonly measured in laboratory analyses. Forty‐nine sets of soils ranging widely in properties were used for this purpose, each set consisting of samples with similar properties but differing in P status. Ryegrass and turnip were grown in a chamber for 30 days in two separate experiments and their yields at harvest recorded. The critical Olsen P and CaCl₂‐P levels, which were taken to be those corresponding to 95% asymptotic yield as calculated from data fitted to a Mitscherlich equation, were greater for turnip than for ryegrass, probably as a result of the difference in yield (49 and 160 mg dry matter/micropot on average for ryegrass and turnip, respectively) and hence in P requirements between the two species. Critical Olsen P spanned narrower ranges than critical CaCl₂‐P in both crops and is therefore seemingly the more robust of the two tests. Both critical P values exhibited moderate correlations with soil properties. Thus, critical Olsen P was (a) lower in soils with a medium pH – which is consistent with the fact that the bicarbonate solution method tends to overestimate plant‐available P in strongly acid and calcareous soils; (b) positively correlated with pH and carbonate content in calcareous soils; and (c) uncorrelated with soil properties in noncalcareous soils. On the other hand, critical CaCl₂‐P in some soil groups was negatively correlated with some properties increasing the P buffering capacity of soil (e.g. Fe oxide content). Taken together, our results suggest that routinely measured soil properties help to predict critical Olsen P better than critical CaCl₂‐P.